Welcome to YARA’s documentation!

YARA is a tool aimed at (but not limited to) helping malware researchers to identify and classify malware samples. With YARA you can create descriptions of malware families (or whatever you want to describe) based on textual or binary patterns. Each description, a.k.a rule, consists of a set of strings and a boolean expression which determine its logic. Let’s see an example:

rule silent_banker : banker
{
    meta:
        description = "This is just an example"
        thread_level = 3
        in_the_wild = true
    strings:
        $a = {6A 40 68 00 30 00 00 6A 14 8D 91}
        $b = {8D 4D B0 2B C1 83 C0 27 99 6A 4E 59 F7 F9}
        $c = "UVODFRYSIHLNWPEJXQZAKCBGMT"
    condition:
        $a or $b or $c
}

The above rule is telling YARA that any file containing one of the three strings must be reported as silent_banker. This is just a simple example, more complex and powerful rules can be created by using wild-cards, case-insensitive strings, regular expressions, special operators and many other features that you’ll find explained in this documentation.

Contents:

Getting started

YARA is a multi-platform program running on Windows, Linux and Mac OS X. You can find the latest release at https://github.com/VirusTotal/yara/releases.

Compiling and installing YARA

Download the source tarball and get prepared for compiling it:

tar -zxf yara-3.7.0.tar.gz
cd yara-3.7.0
./bootstrap.sh

Make sure you have automake, libtool, make and gcc installed in your system. Ubuntu and Debian users can use:

sudo apt-get install automake libtool make gcc

If you plan to modify YARA’s source code you may also need flex and bison for generating lexers and parsers:

sudo apt-get install flex bison

Compile and install YARA in the standard way:

./configure
make
sudo make install

Run the test cases to make sure that everything is fine:

make check

Some of YARA’s features depend on the OpenSSL library. Those features are enabled only if you have the OpenSSL library installed in your system. If not, YARA is going to work fine but you won’t be able to use the disabled features. The configure script will automatically detect if OpenSSL is installed or not. If you want to enforce the OpenSSL-dependent features you must pass --with-crypto to the configure script. Ubuntu and Debian users can use sudo apt-get install libssl-dev to install the OpenSSL library.

The following modules are not compiled into YARA by default:

  • cuckoo
  • magic
  • dotnet

If you plan to use them you must pass the corresponding --enable-<module name> arguments to the configure script.

For example:

./configure --enable-cuckoo
./configure --enable-magic
./configure --enable-dotnet
./configure --enable-cuckoo --enable-magic --enable-dotnet

Modules usually depend on external libraries, depending on the modules you choose to install you’ll need the following libraries:

  • cuckoo:
    Depends on Jansson for parsing JSON. Some Ubuntu and Debian versions already include a package named libjansson-dev, if sudo apt-get install libjansson-dev doesn’t work for you then get the source code from its repository.
  • magic:
    Depends on libmagic, a library used by the Unix standard program file. Ubuntu, Debian and CentOS include a package libmagic-dev. The source code can be found here.

Installing on Windows

Compiled binaries for Windows in both 32 and 64 bit flavors can be found in the link below. Just download the version you want, unzip the archive, and put the yara.exe and yarac.exe binaries anywhere in your disk.

To install the yara-python extension download and execute the installer corresponding to the version of Python you’re using.

Download Windows binaries

Installing on Mac OS X with Homebrew

To install YARA using Homebrew, simply type brew install yara.

Installing yara-python

If you plan to use YARA from your Python scripts you need to install the yara-python extension. Please refer to https://github.com/VirusTotal/yara-python for instructions on how to install it.

Running YARA for the first time

Now that you have installed YARA you can write a very simple rule and use the command-line tool to scan some file:

echo "rule dummy { condition: true }" > my_first_rule
yara my_first_rule my_first_rule

Don’t get confused by the repeated my_first_rule in the arguments to yara, I’m just passing the same file as both the rules and the file to be scanned. You can pass any file you want to be scanned (second argument).

If everything goes fine you should get the following output:

dummy my_first_rule

Which means that the file my_first_rule is matching the rule named dummy.

If you get an error like this:

yara: error while loading shared libraries: libyara.so.2: cannot open shared
object file: No such file or directory

It means that the loader is not finding the libyara library which is located in /usr/local/lib. In some Linux flavors the loader doesn’t look for libraries in this path by default, we must instruct it to do so by adding /usr/local/lib to the loader configuration file /etc/ld.so.conf:

sudo echo "/usr/local/lib" >> /etc/ld.so.conf
sudo ldconfig

Writing YARA rules

YARA rules are easy to write and understand, and they have a syntax that resembles the C language. Here is the simplest rule that you can write for YARA, which does absolutely nothing:

rule dummy
{
    condition:
       false
}

Each rule in YARA starts with the keyword rule followed by a rule identifier. Identifiers must follow the same lexical conventions of the C programming language, they can contain any alphanumeric character and the underscore character, but the first character can not be a digit. Rule identifiers are case sensitive and cannot exceed 128 characters. The following keywords are reserved and cannot be used as an identifier:

YARA keywords
all and any ascii at condition contains
entrypoint false filesize fullword for global in
import include int8 int16 int32 int8be int16be
int32be matches meta nocase not or of
private rule strings them true uint8 uint16
uint32 uint8be uint16be uint32be wide    

Rules are generally composed of two sections: strings definition and condition. The strings definition section can be omitted if the rule doesn’t rely on any string, but the condition section is always required. The strings definition section is where the strings that will be part of the rule are defined. Each string has an identifier consisting of a $ character followed by a sequence of alphanumeric characters and underscores, these identifiers can be used in the condition section to refer to the corresponding string. Strings can be defined in text or hexadecimal form, as shown in the following example:

rule ExampleRule
{
    strings:
        $my_text_string = "text here"
        $my_hex_string = { E2 34 A1 C8 23 FB }

    condition:
        $my_text_string or $my_hex_string
}

Text strings are enclosed in double quotes just like in the C language. Hex strings are enclosed by curly brackets, and they are composed by a sequence of hexadecimal numbers that can appear contiguously or separated by spaces. Decimal numbers are not allowed in hex strings.

The condition section is where the logic of the rule resides. This section must contain a boolean expression telling under which circumstances a file or process satisfies the rule or not. Generally, the condition will refer to previously defined strings by using their identifiers. In this context the string identifier acts as a boolean variable which evaluate to true if the string was found in the file or process memory, or false if otherwise.

Comments

You can add comments to your YARA rules just as if it was a C source file, both single-line and multi-line C-style comments are supported.

/*
    This is a multi-line comment ...
*/

rule CommentExample   // ... and this is single-line comment
{
    condition:
       false  // just a dummy rule, don't do this
}

Strings

There are three types of strings in YARA: hexadecimal strings, text strings and regular expressions. Hexadecimal strings are used for defining raw sequences of bytes, while text strings and regular expressions are useful for defining portions of legible text. However text strings and regular expressions can be also used for representing raw bytes by mean of escape sequences as will be shown below.

Hexadecimal strings

Hexadecimal strings allow three special constructions that make them more flexible: wild-cards, jumps, and alternatives. Wild-cards are just placeholders that you can put into the string indicating that some bytes are unknown and they should match anything. The placeholder character is the question mark (?). Here you have an example of a hexadecimal string with wild-cards:

rule WildcardExample
{
    strings:
       $hex_string = { E2 34 ?? C8 A? FB }

    condition:
       $hex_string
}

As shown in the example the wild-cards are nibble-wise, which means that you can define just one nibble of the byte and leave the other unknown.

Wild-cards are useful when defining strings whose content can vary but you know the length of the variable chunks, however, this is not always the case. In some circumstances you may need to define strings with chunks of variable content and length. In those situations you can use jumps instead of wild-cards:

rule JumpExample
{
        strings:
           $hex_string = { F4 23 [4-6] 62 B4 }

        condition:
           $hex_string
}

In the example above we have a pair of numbers enclosed in square brackets and separated by a hyphen, that’s a jump. This jump is indicating that any arbitrary sequence from 4 to 6 bytes can occupy the position of the jump. Any of the following strings will match the pattern:

F4 23 01 02 03 04 62 B4
F4 23 00 00 00 00 00 62 B4
F4 23 15 82 A3 04 45 22 62 B4

Any jump [X-Y] must meet the condition 0 <= X <= Y. In previous versions of YARA both X and Y must be lower than 256, but starting with YARA 2.0 there is no limit for X and Y.

These are valid jumps:

FE 39 45 [0-8] 89 00
FE 39 45 [23-45] 89 00
FE 39 45 [1000-2000] 89 00

This is invalid:

FE 39 45 [10-7] 89 00

If the lower and higher bounds are equal you can write a single number enclosed in brackets, like this:

FE 39 45 [6] 89 00

The above string is equivalent to both of these:

FE 39 45 [6-6] 89 00
FE 39 45 ?? ?? ?? ?? ?? ?? 89 00

Starting with YARA 2.0 you can also use unbounded jumps:

FE 39 45 [10-] 89 00
FE 39 45 [-] 89 00

The first one means [10-infinite], the second one means [0-infinite].

There are also situations in which you may want to provide different alternatives for a given fragment of your hex string. In those situations you can use a syntax which resembles a regular expression:

rule AlternativesExample1
{
    strings:
       $hex_string = { F4 23 ( 62 B4 | 56 ) 45 }

    condition:
       $hex_string
}

This rule will match any file containing F42362B445 or F4235645.

But more than two alternatives can be also expressed. In fact, there are no limits to the amount of alternative sequences you can provide, and neither to their lengths.

rule AlternativesExample2
{
    strings:
       $hex_string = { F4 23 ( 62 B4 | 56 | 45 ?? 67 ) 45 }

    condition:
       $hex_string
}

As can be seen also in the above example, strings containing wild-cards are allowed as part of alternative sequences.

Text strings

As shown in previous sections, text strings are generally defined like this:

rule TextExample
{
    strings:
        $text_string = "foobar"

    condition:
       $text_string
}

This is the simplest case: an ASCII-encoded, case-sensitive string. However, text strings can be accompanied by some useful modifiers that alter the way in which the string will be interpreted. Those modifiers are appended at the end of the string definition separated by spaces, as will be discussed below.

Text strings can also contain the following subset of the escape sequences available in the C language:

\" Double quote
\\ Backslash
\t Horizontal tab
\n New line
\xdd Any byte in hexadecimal notation
Case-insensitive strings

Text strings in YARA are case-sensitive by default, however you can turn your string into case-insensitive mode by appending the modifier nocase at the end of the string definition, in the same line:

rule CaseInsensitiveTextExample
{
    strings:
        $text_string = "foobar" nocase

    condition:
        $text_string
}

With the nocase modifier the string foobar will match Foobar, FOOBAR, and fOoBaR. This modifier can be used in conjunction with any other modifier.

Wide-character strings

The wide modifier can be used to search for strings encoded with two bytes per character, something typical in many executable binaries.

In the above figure, the string “Borland” appears encoded as two bytes per character, therefore the following rule will match:

rule WideCharTextExample1
{
    strings:
        $wide_string = "Borland" wide

    condition:
       $wide_string
}

However, keep in mind that this modifier just interleaves the ASCII codes of the characters in the string with zeroes, it does not support truly UTF-16 strings containing non-English characters. If you want to search for strings in both ASCII and wide form, you can use the ascii modifier in conjunction with wide , no matter the order in which they appear.

rule WideCharTextExample2
{
    strings:
        $wide_and_ascii_string = "Borland" wide ascii

    condition:
       $wide_and_ascii_string
}

The ascii modifier can appear alone, without an accompanying wide modifier, but it’s not necessary to write it because in absence of wide the string is assumed to be ASCII by default.

Searching for full words

Another modifier that can be applied to text strings is fullword. This modifier guarantees that the string will match only if it appears in the file delimited by non-alphanumeric characters. For example the string domain, if defined as fullword, doesn’t match www.mydomain.com but it matches www.my-domain.com and www.domain.com.

Regular expressions

Regular expressions are one of the most powerful features of YARA. They are defined in the same way as text strings, but enclosed in forward slashes instead of double-quotes, like in the Perl programming language.

rule RegExpExample1
{
    strings:
        $re1 = /md5: [0-9a-fA-F]{32}/
        $re2 = /state: (on|off)/

    condition:
        $re1 and $re2
}

Regular expressions can be also followed by nocase, ascii, wide, and fullword modifiers just like in text strings. The semantics of these modifiers are the same in both cases.

In previous versions of YARA, external libraries like PCRE and RE2 were used to perform regular expression matching, but starting with version 2.0 YARA uses its own regular expression engine. This new engine implements most features found in PCRE, except a few of them like capture groups, POSIX character classes and backreferences.

YARA’s regular expressions recognise the following metacharacters:

\ Quote the next metacharacter
^ Match the beginning of the file
$ Match the end of the file
| Alternation
() Grouping
[] Bracketed character class

The following quantifiers are recognised as well:

* Match 0 or more times
+ Match 1 or more times
? Match 0 or 1 times
{n} Match exactly n times
{n,} Match at least n times
{,m} Match at most m times
{n,m} Match n to m times

All these quantifiers have a non-greedy variant, followed by a question mark (?):

*? Match 0 or more times, non-greedy
+? Match 1 or more times, non-greedy
?? Match 0 or 1 times, non-greedy
{n}? Match exactly n times, non-greedy
{n,}? Match at least n times, non-greedy
{,m}? Match at most m times, non-greedy
{n,m}? Match n to m times, non-greedy

The following escape sequences are recognised:

\t Tab (HT, TAB)
\n New line (LF, NL)
\r Return (CR)
\f Form feed (FF)
\a Alarm bell
\xNN Character whose ordinal number is the given hexadecimal number

These are the recognised character classes:

\w Match a word character (alphanumeric plus “_”)
\W Match a non-word character
\s Match a whitespace character
\S Match a non-whitespace character
\d Match a decimal digit character
\D Match a non-digit character

Starting with version 3.3.0 these zero-width assertions are also recognized:

\b Match a word boundary
\B Match except at a word boundary

Conditions

Conditions are nothing more than Boolean expressions as those that can be found in all programming languages, for example in an if statement. They can contain the typical Boolean operators and, or, and not, and relational operators >=, <=, <, >, == and !=. Also, the arithmetic operators (+, -, *, \, %) and bitwise operators (&, |, <<, >>, ~, ^) can be used on numerical expressions.

String identifiers can be also used within a condition, acting as Boolean variables whose value depends on the presence or not of the associated string in the file.

rule Example
{
    strings:
        $a = "text1"
        $b = "text2"
        $c = "text3"
        $d = "text4"

    condition:
        ($a or $b) and ($c or $d)
}

Counting strings

Sometimes we need to know not only if a certain string is present or not, but how many times the string appears in the file or process memory. The number of occurrences of each string is represented by a variable whose name is the string identifier but with a # character in place of the $ character. For example:

rule CountExample
{
    strings:
        $a = "dummy1"
        $b = "dummy2"

    condition:
        #a == 6 and #b > 10
}

This rule matches any file or process containing the string $a exactly six times, and more than ten occurrences of string $b.

String offsets or virtual addresses

In the majority of cases, when a string identifier is used in a condition, we are willing to know if the associated string is anywhere within the file or process memory, but sometimes we need to know if the string is at some specific offset on the file or at some virtual address within the process address space. In such situations the operator at is what we need. This operator is used as shown in the following example:

rule AtExample
{
    strings:
        $a = "dummy1"
        $b = "dummy2"

    condition:
        $a at 100 and $b at 200
}

The expression $a at 100 in the above example is true only if string $a is found at offset 100 within the file (or at virtual address 100 if applied to a running process). The string $b should appear at offset 200. Please note that both offsets are decimal, however hexadecimal numbers can be written by adding the prefix 0x before the number as in the C language, which comes very handy when writing virtual addresses. Also note the higher precedence of the operator at over the and.

While the at operator allows to search for a string at some fixed offset in the file or virtual address in a process memory space, the in operator allows to search for the string within a range of offsets or addresses.

rule InExample
{
    strings:
        $a = "dummy1"
        $b = "dummy2"

    condition:
        $a in (0..100) and $b in (100..filesize)
}

In the example above the string $a must be found at an offset between 0 and 100, while string $b must be at an offset between 100 and the end of the file. Again, numbers are decimal by default.

You can also get the offset or virtual address of the i-th occurrence of string $a by using @a[i]. The indexes are one-based, so the first occurrence would be @a[1] the second one @a[2] and so on. If you provide an index greater then the number of occurrences of the string, the result will be a NaN (Not A Number) value.

Match length

For many regular expressions and hex strings containing jumps, the length of the match is variable. If you have the regular expression /fo*/ the strings “fo”, “foo” and “fooo” can be matches, all of them with a different length.

You can use the length of the matches as part of your condition by using the character ! in front of the string identifier, in a similar way you use the @ character for the offset. !a[1] is the length for the first match of $a, !a[2] is the length for the second match, and so on. !a is a abbreviated form of !a[1].

File size

String identifiers are not the only variables that can appear in a condition (in fact, rules can be defined without any string definition as will be shown below), there are other special variables that can be used as well. One of these special variables is filesize, which holds, as its name indicates, the size of the file being scanned. The size is expressed in bytes.

rule FileSizeExample
{
    condition:
       filesize > 200KB
}

The previous example also demonstrates the use of the KB postfix. This postfix, when attached to a numerical constant, automatically multiplies the value of the constant by 1024. The MB postfix can be used to multiply the value by 2^20. Both postfixes can be used only with decimal constants.

The use of filesize only makes sense when the rule is applied to a file. If the rule is applied to a running process it won’t ever match because filesize doesn’t make sense in this context.

Executable entry point

Another special variable than can be used in a rule is entrypoint. If the file is a Portable Executable (PE) or Executable and Linkable Format (ELF), this variable holds the raw offset of the executable’s entry point in case we are scanning a file. If we are scanning a running process, the entrypoint will hold the virtual address of the main executable’s entry point. A typical use of this variable is to look for some pattern at the entry point to detect packers or simple file infectors.

rule EntryPointExample1
{
    strings:
        $a = { E8 00 00 00 00 }

    condition:
       $a at entrypoint
}

rule EntryPointExample2
{
    strings:
        $a = { 9C 50 66 A1 ?? ?? ?? 00 66 A9 ?? ?? 58 0F 85 }

    condition:
       $a in (entrypoint..entrypoint + 10)
}

The presence of the entrypoint variable in a rule implies that only PE or ELF files can satisfy that rule. If the file is not a PE or ELF, any rule using this variable evaluates to false.

Warning

The entrypoint variable is deprecated, you should use the equivalent pe.entry_point from the PE module instead. Starting with YARA 3.0 you’ll get a warning if you use entrypoint and it will be completely removed in future versions.

Accessing data at a given position

There are many situations in which you may want to write conditions that depend on data stored at a certain file offset or virtual memory address, depending on if we are scanning a file or a running process. In those situations you can use one of the following functions to read data from the file at the given offset:

int8(<offset or virtual address>)
int16(<offset or virtual address>)
int32(<offset or virtual address>)

uint8(<offset or virtual address>)
uint16(<offset or virtual address>)
uint32(<offset or virtual address>)

int8be(<offset or virtual address>)
int16be(<offset or virtual address>)
int32be(<offset or virtual address>)

uint8be(<offset or virtual address>)
uint16be(<offset or virtual address>)
uint32be(<offset or virtual address>)

The intXX functions read 8, 16, and 32 bits signed integers from <offset or virtual address>, while functions uintXX read unsigned integers. Both 16 and 32 bit integers are considered to be little-endian. If you want to read a big-endian integer use the corresponding function ending in be. The <offset or virtual address> parameter can be any expression returning an unsigned integer, including the return value of one the uintXX functions itself. As an example let’s see a rule to distinguish PE files:

rule IsPE
{
  condition:
     // MZ signature at offset 0 and ...
     uint16(0) == 0x5A4D and
     // ... PE signature at offset stored in MZ header at 0x3C
     uint32(uint32(0x3C)) == 0x00004550
}

Sets of strings

There are circumstances in which it is necessary to express that the file should contain a certain number strings from a given set. None of the strings in the set are required to be present, but at least some of them should be. In these situations the of operator can be used.

rule OfExample1
{
    strings:
        $a = "dummy1"
        $b = "dummy2"
        $c = "dummy3"

    condition:
        2 of ($a,$b,$c)
}

This rule requires that at least two of the strings in the set ($a,$b,$c) must be present in the file, but it does not matter which two. Of course, when using this operator, the number before the of keyword must be less than or equal to the number of strings in the set.

The elements of the set can be explicitly enumerated like in the previous example, or can be specified by using wild cards. For example:

rule OfExample2
{
    strings:
        $foo1 = "foo1"
        $foo2 = "foo2"
        $foo3 = "foo3"

    condition:
        2 of ($foo*)  // equivalent to 2 of ($foo1,$foo2,$foo3)
}

rule OfExample3
{
    strings:
        $foo1 = "foo1"
        $foo2 = "foo2"

        $bar1 = "bar1"
        $bar2 = "bar2"

    condition:
        3 of ($foo*,$bar1,$bar2)
}

You can even use ($*) to refer to all the strings in your rule, or write the equivalent keyword them for more legibility.

rule OfExample4
{
    strings:
        $a = "dummy1"
        $b = "dummy2"
        $c = "dummy3"

    condition:
        1 of them // equivalent to 1 of ($*)
}

In all the examples above, the number of strings have been specified by a numeric constant, but any expression returning a numeric value can be used. The keywords any and all can be used as well.

all of them       // all strings in the rule
any of them       // any string in the rule
all of ($a*)      // all strings whose identifier starts by $a
any of ($a,$b,$c) // any of $a, $b or $c
1 of ($*)         // same that "any of them"

Applying the same condition to many strings

There is another operator very similar to of but even more powerful, the for..of operator. The syntax is:

for expression of string_set : ( boolean_expression )

And its meaning is: from those strings in string_set at least expression of them must satisfy boolean_expression.

In other words: boolean_expression is evaluated for every string in string_set and there must be at least expression of them returning True.

Of course, boolean_expression can be any boolean expression accepted in the condition section of a rule, except for one important detail: here you can (and should) use a dollar sign ($) as a place-holder for the string being evaluated. Take a look at the following expression:

for any of ($a,$b,$c) : ( $ at entrypoint  )

The $ symbol in the boolean expression is not tied to any particular string, it will be $a, and then $b, and then $c in the three successive evaluations of the expression.

Maybe you already realised that the of operator is an special case of for..of. The following expressions are the same:

any of ($a,$b,$c)
for any of ($a,$b,$c) : ( $ )

You can also employ the symbols # and @ to make reference to the number of occurrences and the first offset of each string respectively.

for all of them : ( # > 3 )
for all of ($a*) : ( @ > @b )

Using anonymous strings with of and for..of

When using the of and for..of operators followed by them, the identifier assigned to each string of the rule is usually superfluous. As we are not referencing any string individually we do not need to provide a unique identifier for each of them. In those situations you can declare anonymous strings with identifiers consisting only of the $ character, as in the following example:

rule AnonymousStrings
{
    strings:
        $ = "dummy1"
        $ = "dummy2"

    condition:
        1 of them
}

Iterating over string occurrences

As seen in String offsets or virtual addresses, the offsets or virtual addresses where a given string appears within a file or process address space can be accessed by using the syntax: @a[i], where i is an index indicating which occurrence of the string $a you are referring to. (@a[1], @a[2],...).

Sometimes you will need to iterate over some of these offsets and guarantee they satisfy a given condition. For example:

rule Occurrences
{
    strings:
        $a = "dummy1"
        $b = "dummy2"

    condition:
        for all i in (1,2,3) : ( @a[i] + 10 == @b[i] )
}

The previous rule says that the first three occurrences of $b should be 10 bytes away from the first three occurrences of $a.

The same condition could be written also as:

for all i in (1..3) : ( @a[i] + 10 == @b[i] )

Notice that we’re using a range (1..3) instead of enumerating the index values (1,2,3). Of course, we’re not forced to use constants to specify range boundaries, we can use expressions as well like in the following example:

for all i in (1..#a) : ( @a[i] < 100 )

In this case we’re iterating over every occurrence of $a (remember that #a represents the number of occurrences of $a). This rule is specifying that every occurrence of $a should be within the first 100 bytes of the file.

In case you want to express that only some occurrences of the string should satisfy your condition, the same logic seen in the for..of operator applies here:

for any i in (1..#a) : ( @a[i] < 100 )
for 2 i in (1..#a) : ( @a[i] < 100 )

In summary, the syntax of this operator is:

for expression identifier in indexes : ( boolean_expression )

Referencing other rules

When writing the condition for a rule you can also make reference to a previously defined rule in a manner that resembles a function invocation of traditional programming languages. In this way you can create rules that depend on others. Let’s see an example:

rule Rule1
{
    strings:
        $a = "dummy1"

    condition:
        $a
}

rule Rule2
{
    strings:
        $a = "dummy2"

    condition:
        $a and Rule1
}

As can be seen in the example, a file will satisfy Rule2 only if it contains the string “dummy2” and satisfies Rule1. Note that it is strictly necessary to define the rule being invoked before the one that will make the invocation.

More about rules

There are some aspects of YARA rules that have not been covered yet, but are still very important. These are: global rules, private rules, tags and metadata.

Global rules

Global rules give you the possibility of imposing restrictions in all your rules at once. For example, suppose that you want all your rules ignoring those files that exceed a certain size limit, you could go rule by rule making the required modifications to their conditions, or just write a global rule like this one:

global rule SizeLimit
{
    condition:
        filesize < 2MB
}

You can define as many global rules as you want, they will be evaluated before the rest of the rules, which in turn will be evaluated only if all global rules are satisfied.

Private rules

Private rules are a very simple concept. They are just rules that are not reported by YARA when they match on a given file. Rules that are not reported at all may seem sterile at first glance, but when mixed with the possibility offered by YARA of referencing one rule from another (see Referencing other rules) they become useful. Private rules can serve as building blocks for other rules, and at the same time prevent cluttering YARA’s output with irrelevant information. To delcare a rule as private just add the keyword private before the rule declaration.

private rule PrivateRuleExample
{
    ...
}

You can apply both private and global modifiers to a rule, resulting in a global rule that does not get reported by YARA but must be satisfied.

Rule tags

Another useful feature of YARA is the possibility of adding tags to rules. Those tags can be used later to filter YARA’s output and show only the rules that you are interested in. You can add as many tags as you want to a rule, they are declared after the rule identifier as shown below:

rule TagsExample1 : Foo Bar Baz
{
    ...
}

rule TagsExample2 : Bar
{
    ...
}

Tags must follow the same lexical convention of rule identifiers, therefore only alphanumeric characters and underscores are allowed, and the tag cannot start with a digit. They are also case sensitive.

When using YARA you can output only those rules which are tagged with the tag or tags that you provide.

Metadata

Besides the string definition and condition sections, rules can also have a metadata section where you can put additional information about your rule. The metadata section is defined with the keyword meta and contains identifier/value pairs like in the following example:

rule MetadataExample
{
    meta:
        my_identifier_1 = "Some string data"
        my_identifier_2 = 24
        my_identifier_3 = true

    strings:
        $my_text_string = "text here"
        $my_hex_string = { E2 34 A1 C8 23 FB }

    condition:
        $my_text_string or $my_hex_string
}

As can be seen in the example, metadata identifiers are always followed by an equals sign and the value assigned to them. The assigned values can be strings, integers, or one of the boolean values true or false. Note that identifier/value pairs defined in the metadata section can not be used in the condition section, their only purpose is to store additional information about the rule.

Using modules

Modules are extensions to YARA’s core functionality. Some modules like the PE module and the Cuckoo module are officially distributed with YARA and additional ones can be created by third-parties or even yourself as described in Writing your own modules.

The first step to using a module is importing it with the import statement. These statements must be placed outside any rule definition and followed by the module name enclosed in double-quotes. Like this:

import "pe"
import "cuckoo"

After importing the module you can make use of its features, always using <module name>. as a prefix to any variable or function exported by the module. For example:

pe.entry_point == 0x1000
cuckoo.http_request(/someregexp/)

Undefined values

Modules often leave variables in an undefined state, for example when the variable doesn’t make sense in the current context (think of pe.entry_point while scanning a non-PE file). YARA handles undefined values in way that allows the rule to keep its meaningfulness. Take a look at this rule:

import "pe"

rule Test
{
  strings:
      $a = "some string"

  condition:
      $a and pe.entry_point == 0x1000
}

If the scanned file is not a PE you wouldn’t expect this rule to match the file, even if it contains the string, because both conditions (the presence of the string and the right value for the entry point) must be satisfied. However, if the condition is changed to:

$a or pe.entry_point == 0x1000

You would expect the rule to match in this case if the file contains the string, even if it isn’t a PE file. That’s exactly how YARA behaves. The logic is simple: any arithmetic, comparison, or boolean operation will result in an undefined value if one of its operands is undefined, except for OR operations where an undefined operand is interpreted as a False.

External variables

External variables allow you to define rules which depends on values provided from the outside. For example you can write the following rule:

rule ExternalVariableExample1
{
    condition:
       ext_var == 10
}

In this case ext_var is an external variable whose value is assigned at run-time (see -d option of command-line tool, and externals parameter of compile and match methods in yara-python). External variables could be of types: integer, string or boolean; their type depends on the value assigned to them. An integer variable can substitute any integer constant in the condition and boolean variables can occupy the place of boolean expressions. For example:

rule ExternalVariableExample2
{
    condition:
       bool_ext_var or filesize < int_ext_var
}

External variables of type string can be used with the operators: contains and matches. The contains operator returns true if the string contains the specified substring. The matches operator returns true if the string matches the given regular expression.

rule ExternalVariableExample3
{
    condition:
        string_ext_var contains "text"
}

rule ExternalVariableExample4
{
    condition:
        string_ext_var matches /[a-z]+/
}

You can use regular expression modifiers along with the matches operator, for example, if you want the regular expression from the previous example to be case insensitive you can use /[a-z]+/i. Notice the i following the regular expression in a Perl-like manner. You can also use the s modifier for single-line mode, in this mode the dot matches all characters including line breaks. Of course both modifiers can be used simultaneously, like in the following example:

rule ExternalVariableExample5
{
    condition:
        /* case insensitive single-line mode */
        string_ext_var matches /[a-z]+/is
}

Keep in mind that every external variable used in your rules must be defined at run-time, either by using the -d option of the command-line tool, or by providing the externals parameter to the appropriate method in yara-python.

Including files

In order to allow for more flexible organization of your rules files, YARA provides the include directive. This directive works in a similar way to the #include pre-processor directive in C programs, which inserts the content of the specified source file into the current file during compilation. The following example will include the content of other.yar into the current file:

include "other.yar"

The base path when searching for a file in an include directive will be the directory where the current file resides. For this reason, the file other.yar in the previous example should be located in the same directory of the current file. However, you can also specify relative paths like these:

include "./includes/other.yar"
include "../includes/other.yar"

Or use absolute paths:

include "/home/plusvic/yara/includes/other.yar"

In Windows, both forward and back slashes are accepted, but don’t forget to write the drive letter:

include "c:/yara/includes/other.yar"
include "c:\\yara\\includes\\other.yar"

Modules

Modules are the method YARA provides for extending its features. They allow you to define data structures and functions which can be used in your rules to express more complex conditions. Here you’ll find described some modules officially distributed with YARA, but you can also learn how to write your own modules in the Writing your own modules section.

PE module

The PE module allows you to create more fine-grained rules for PE files by using attributes and features of the PE file format. This module exposes most of the fields present in a PE header and provides functions which can be used to write more expressive and targeted rules. Let’s see some examples:

import "pe"

rule single_section
{
    condition:
        pe.number_of_sections == 1
}

rule control_panel_applet
{
    condition:
        pe.exports("CPlApplet")
}

rule is_dll
{
    condition:
        pe.characteristics & pe.DLL
}

Reference

machine

Changed in version 3.3.0.

Integer with one of the following values:

MACHINE_UNKNOWN
MACHINE_AM33
MACHINE_AMD64
MACHINE_ARM
MACHINE_ARMNT
MACHINE_ARM64
MACHINE_EBC
MACHINE_I386
MACHINE_IA64
MACHINE_M32R
MACHINE_MIPS16
MACHINE_MIPSFPU
MACHINE_MIPSFPU16
MACHINE_POWERPC
MACHINE_POWERPCFP
MACHINE_R4000
MACHINE_SH3
MACHINE_SH3DSP
MACHINE_SH4
MACHINE_SH5
MACHINE_THUMB
MACHINE_WCEMIPSV2

Example: pe.machine == pe.MACHINE_AMD64

checksum

New in version 3.6.0.

Integer with the “PE checksum” as stored in the OptionalHeader

calculate_checksum

New in version 3.6.0.

Function that calculates the “PE checksum”

Example: pe.checksum == pe.calculate_checksum()

subsystem

Integer with one of the following values:

SUBSYSTEM_UNKNOWN
SUBSYSTEM_NATIVE
SUBSYSTEM_WINDOWS_GUI
SUBSYSTEM_WINDOWS_CUI
SUBSYSTEM_OS2_CUI
SUBSYSTEM_POSIX_CUI
SUBSYSTEM_NATIVE_WINDOWS
SUBSYSTEM_WINDOWS_CE_GUI
SUBSYSTEM_EFI_APPLICATION
SUBSYSTEM_EFI_BOOT_SERVICE_DRIVER
SUBSYSTEM_EFI_RUNTIME_DRIVER
SUBSYSTEM_XBOX
SUBSYSTEM_WINDOWS_BOOT_APPLICATION

Example: pe.subsystem == pe.SUBSYSTEM_NATIVE

timestamp

PE timestamp.

entry_point

Entry point raw offset or virtual address depending on whether YARA is scanning a file or process memory respectively. This is equivalent to the deprecated entrypoint keyword.

image_base

Image base relative virtual address.

characteristics

Bitmap with PE FileHeader characteristics. Individual characteristics can be inspected by performing a bitwise AND operation with the following constants:

RELOCS_STRIPPED
EXECUTABLE_IMAGE
LINE_NUMS_STRIPPED
LOCAL_SYMS_STRIPPED
AGGRESIVE_WS_TRIM
LARGE_ADDRESS_AWARE
BYTES_REVERSED_LO
MACHINE_32BIT
DEBUG_STRIPPED
REMOVABLE_RUN_FROM_SWAP
NET_RUN_FROM_SWAP
SYSTEM
DLL
UP_SYSTEM_ONLY
BYTES_REVERSED_HI

Example: pe.characteristics & pe.DLL

linker_version

An object with two integer attributes, one for each major and minor linker version.

major

Major linker version.

minor

Minor linker version.

os_version

An object with two integer attributes, one for each major and minor OS version.

major

Major OS version.

minor

Minor OS version.

image_version

An object with two integer attributes, one for each major and minor image version.

major

Major image version.

minor

Minor image version.

subsystem_version

An object with two integer attributes, one for each major and minor subsystem version.

major

Major subsystem version.

minor

Minor subsystem version.

dll_characteristics

Bitmap with PE OptionalHeader DllCharacteristics. Do not confuse these flags with the PE FileHeader Characteristics. Individual characteristics can be inspected by performing a bitwise AND operation with the following constants:

DYNAMIC_BASE

File can be relocated - also marks the file as ASLR compatible

FORCE_INTEGRITY
NX_COMPAT

Marks the file as DEP compatible

NO_ISOLATION
NO_SEH

The file does not contain structured exception handlers, this must be set to use SafeSEH

NO_BIND
WDM_DRIVER

Marks the file as a Windows Driver Model (WDM) device driver.

TERMINAL_SERVER_AWARE

Marks the file as terminal server compatible

number_of_sections

Number of sections in the PE.

sections

New in version 3.3.0.

A zero-based array of section objects, one for each section the PE has. Individual sections can be accessed by using the [] operator. Each section object has the following attributes:

name

Section name.

characteristics

Section characteristics.

virtual_address

Section virtual address.

virtual_size

Section virtual size.

raw_data_offset

Section raw offset.

raw_data_size

Section raw size.

Example: pe.sections[0].name == ”.text”

Individual section characteristics can be inspected using a bitwise AND operation with the following constants:

SECTION_CNT_CODE
SECTION_CNT_INITIALIZED_DATA
SECTION_CNT_UNINITIALIZED_DATA
SECTION_GPREL
SECTION_MEM_16BIT
SECTION_LNK_NRELOC_OVFL
SECTION_MEM_DISCARDABLE
SECTION_MEM_NOT_CACHED
SECTION_MEM_NOT_PAGED
SECTION_MEM_SHARED
SECTION_MEM_EXECUTE
SECTION_MEM_READ
SECTION_MEM_WRITE

Example: pe.sections[1].characteristics & SECTION_CNT_CODE

overlay

New in version 3.6.0.

A structure containing the following integer members:

offset

Overlay section offset.

size

Overlay section size.

Example: uint8(0x0d) at pe.overlay.offset and pe.overlay.size > 1024

number_of_resources

Number of resources in the PE.

resource_timestamp

Resource timestamp. This is stored as an integer.

resource_version

An object with two integer attributes, major and minor versions.

major

Major resource version.

minor

Minor resource version.

resources

Changed in version 3.3.0.

A zero-based array of resource objects, one for each resource the PE has. Individual resources can be accessed by using the [] operator. Each resource object has the following attributes:

offset

Offset for the resource data.

length

Length of the resource data.

type

Type of the resource (integer).

id

ID of the resource (integer).

language

Language of the resource (integer).

type_string

Type of the resource as a string, if specified.

name_string

Name of the resource as a string, if specified.

language_string

Language of the resource as a string, if specified.

All resources must have a type, id (name), and language specified. They can be either an integer or string, but never both, for any given level.

Example: pe.resources[0].type == pe.RESOURCE_TYPE_RCDATA

Example: pe.resources[0].name_string == “F\x00I\x00L\x00E\x00”

Resource types can be inspected using the following constants:

RESOURCE_TYPE_CURSOR
RESOURCE_TYPE_BITMAP
RESOURCE_TYPE_ICON
RESOURCE_TYPE_MENU
RESOURCE_TYPE_DIALOG
RESOURCE_TYPE_STRING
RESOURCE_TYPE_FONTDIR
RESOURCE_TYPE_FONT
RESOURCE_TYPE_ACCELERATOR
RESOURCE_TYPE_RCDATA
RESOURCE_TYPE_MESSAGETABLE
RESOURCE_TYPE_GROUP_CURSOR
RESOURCE_TYPE_GROUP_ICON
RESOURCE_TYPE_VERSION
RESOURCE_TYPE_DLGINCLUDE
RESOURCE_TYPE_PLUGPLAY
RESOURCE_TYPE_VXD
RESOURCE_TYPE_ANICURSOR
RESOURCE_TYPE_ANIICON
RESOURCE_TYPE_HTML
RESOURCE_TYPE_MANIFEST

For more information refer to:

http://msdn.microsoft.com/en-us/library/ms648009(v=vs.85).aspx

version_info

New in version 3.2.0.

Dictionary containing the PE’s version information. Typical keys are:

Comments CompanyName FileDescription FileVersion InternalName LegalCopyright LegalTrademarks OriginalFilename ProductName ProductVersion

For more information refer to:

http://msdn.microsoft.com/en-us/library/windows/desktop/ms646987(v=vs.85).aspx

Example: pe.version_info[“CompanyName”] contains “Microsoft”

number_of_signatures

Number of authenticode signatures in the PE.

signatures

A zero-based array of signature objects, one for each authenticode signature in the PE file. Usually PE files have a single signature.

issuer

A string containing information about the issuer. These are some examples:

"/C=US/ST=Washington/L=Redmond/O=Microsoft Corporation/CN=Microsoft Code Signing PCA"

"/C=US/O=VeriSign, Inc./OU=VeriSign Trust Network/OU=Terms of use at https://www.verisign.com/rpa (c)10/CN=VeriSign Class 3 Code Signing 2010 CA"

"/C=GB/ST=Greater Manchester/L=Salford/O=COMODO CA Limited/CN=COMODO Code Signing CA 2"
subject

A string containing information about the subject.

version

Version number.

algorithm

Algorithm used for this signature. Usually “sha1WithRSAEncryption”.

serial

A string containing the serial number. This is an example:

"52:00:e5:aa:25:56:fc:1a:86:ed:96:c9:d4:4b:33:c7"
not_before

Unix timestamp on which the validity period for this signature begins.

not_after

Unix timestamp on which the validity period for this signature ends.

valid_on(timestamp)

Function returning true if the signature was valid on the date indicated by timestamp. The following sentence:

pe.signatures[n].valid_on(timestamp)

Is equivalent to:

timestamp >= pe.signatures[n].not_before and timestamp <= pe.signatures[n].not_after
rich_signature

Structure containing information about the PE’s rich signature as documented here.

offset

Offset where the rich signature starts. It will be undefined if the file doesn’t have a rich signature.

length

Length of the rich signature, not including the final “Rich” marker.

key

Key used to encrypt the data with XOR.

raw_data

Raw data as it appears in the file.

clear_data

Data after being decrypted by XORing it with the key.

version(version, [toolid])

New in version 3.5.0: Function returning true if the PE has the specified version in the PE’s rich signature. Provide the optional toolid argument to only match when both match for one entry. More information can be found here:

http://www.ntcore.com/files/richsign.htm

Example: pe.rich_signature.version(21005)

toolid(toolid, [version])

New in version 3.5.0: Function returning true if the PE has the specified id in the PE’s rich signature. Provide the optional version argument to only match when both match for one entry. More information can be found here:

http://www.ntcore.com/files/richsign.htm

Example: pe.rich_signature.toolid(222)

exports(function_name)

Function returning true if the PE exports function_name or false otherwise.

Example: pe.exports(“CPlApplet”)

exports(ordinal)

New in version 3.6.0.

Function returning true if the PE exports ordinal or false otherwise.

Example: pe.exports(72)

number_of_exports

New in version 3.6.0.

Number of exports in the PE.

number_of_imports

New in version 3.6.0.

Number of imports in the PE.

imports(dll_name, function_name)

Function returning true if the PE imports function_name from dll_name, or false otherwise. dll_name is case insensitive.

Example: pe.imports(“kernel32.dll”, “WriteProcessMemory”)

imports(dll_name)

New in version 3.5.0.

Function returning true if the PE imports anything from dll_name, or false otherwise. dll_name is case insensitive.

Example: pe.imports(“kernel32.dll”)

imports(dll_name, ordinal)

New in version 3.5.0.

Function returning true if the PE imports ordinal from dll_name, or false otherwise. dll_name is case insensitive.

Example: pe.imports(“WS2_32.DLL”, 3)

locale(locale_identifier)

New in version 3.2.0.

Function returning true if the PE has a resource with the specified locale identifier. Locale identifiers are 16-bit integers and can be found here:

http://msdn.microsoft.com/en-us/library/windows/desktop/dd318693(v=vs.85).aspx

Example: pe.locale(0x0419) // Russian (RU)

language(language_identifier)

New in version 3.2.0.

Function returning true if the PE has a resource with the specified language identifier. Language identifiers are 8-bit integers and can be found here:

http://msdn.microsoft.com/en-us/library/windows/desktop/dd318693(v=vs.85).aspx

Example: pe.language(0x0A) // Spanish

imphash()

New in version 3.2.0.

Function returning the import hash or imphash for the PE. The imphash is a MD5 hash of the PE’s import table after some normalization. The imphash for a PE can be also computed with pefile and you can find more information in Mandiant’s blog.

Example: pe.imphash() == “b8bb385806b89680e13fc0cf24f4431e”

section_index(name)

Function returning the index into the sections array for the section that has name. name is case sensitive.

Example: pe.section_index(”.TEXT”)

section_index(addr)

New in version 3.3.0: Function returning the index into the sections array for the section that has addr. addr can be an offset into the file or a memory address.

Example: pe.section_index(pe.entry_point)

is_dll()

New in version 3.5.0.

Function returning true if the PE is a DLL.

Example: pe.is_dll()

is_32bit()

New in version 3.5.0.

Function returning true if the PE is 32bits.

Example: pe.is_32bit()

is_64bit()

New in version 3.5.0.

Function returning true if the PE is 64bits.

Example: pe.is_64bit()

rva_to_offset(addr)

New in version 3.6.0: Function returning the file offset for RVA addr.

Example: pe.rva_to_offset(pe.entry_point)

ELF module

New in version 3.2.0.

The ELF module is very similar to the PE module, but for ELF files. This module exposes most of the fields present in an ELF header. Let’s see some examples:

import "elf"

rule single_section
{
    condition:
        elf.number_of_sections == 1
}

rule elf_64
{
    condition:
        elf.machine == elf.EM_X86_64
}

Reference

type

Integer with one of the following values:

ET_NONE

No file type.

ET_REL

Relocatable file.

ET_EXEC

Executable file.

ET_DYN

Shared object file.

ET_CORE

Core file.

Example: elf.type == elf.ET_EXEC

machine

Integer with one of the following values:

EM_M32
EM_SPARC
EM_386
EM_68K
EM_88K
EM_860
EM_MIPS
EM_MIPS_RS3_LE
EM_PPC
EM_PPC64
EM_ARM
EM_X86_64
EM_AARCH64

Example: elf.machine == elf.EM_X86_64

entry_point

Entry point raw offset or virtual address depending on whether YARA is scanning a file or process memory respectively. This is equivalent to the deprecated entrypoint keyword.

number_of_sections

Number of sections in the ELF file.

sections

A zero-based array of section objects, one for each section the ELF has. Individual sections can be accessed by using the [] operator. Each section object has the following attributes:

name

Section’s name.

Example: elf.sections[3].name == ”.bss”

size

Section’s size in bytes. Unless the section type is SHT_NOBITS, the section occupies sh_size bytes in the file. A section of SHT_NOBITS may have a non-zero size, but it occupies no space in the file.

offset

Offset from the beginning of the file to the first byte in the section. One section type, SHT_NOBITS described below, occupies no space in the file, and its offset member locates the conceptual placement in the file.

type

Integer with one of the following values:

SHT_NULL

This value marks the section as inactive; it does not have an associated section. Other members of the section header have undefined values.

SHT_PROGBITS

The section holds information defined by the program, whose format and meaning are determined solely by the program.

SHT_SYMTAB

The section holds a symbol table.

SHT_STRTAB

The section holds a string table. An object file may have multiple string table sections.

SHT_RELA

The section holds relocation entries.

SHT_HASH

The section holds a symbol hash table.

SHT_DYNAMIC

The section holds information for dynamic linking.

SHT_NOTE

The section holds information that marks the file in some way.

SHT_NOBITS

A section of this type occupies no space in the file but otherwise resembles SHT_PROGBITS.

SHT_REL

The section holds relocation entries.

SHT_SHLIB

This section type is reserved but has unspecified semantics.

SHT_DYNSYM

This section holds dynamic linking symbols.

flags

Integer with section’s flags as defined below:

SHF_WRITE

The section contains data that should be writable during process execution.

SHF_ALLOC

The section occupies memory during process execution. Some control sections do not reside in the memory image of an object file; this attribute is off for those sections.

SHF_EXECINSTR

The section contains executable machine instructions.

Example: elf.sections[2].flags & elf.SHF_WRITE

address

New in version 3.6.0.

The virtual address the section starts at.

number_of_segments

New in version 3.4.0.

Number of segments in the ELF file.

segments

New in version 3.4.0.

A zero-based array of segment objects, one for each segment the ELF has. Individual segments can be accessed by using the [] operator. Each segment object has the following attributes:

alignment

Value to which the segments are aligned in memory and in the file.

file_size

Number of bytes in the file image of the segment. It may be zero.

flags

A combination of the following segment flags:

PF_R

The segment is readable.

PF_W

The segment is writable.

PF_X

The segment is executable.

memory_size

In-memory segment size.

offset

Offset from the beginning of the file where the segment resides.

physical_address

On systems for which physical addressing is relevant, contains the segment’s physical address.

type

Type of segment indicated by one of the following values:

PT_NULL
PT_LOAD
PT_DYNAMIC
PT_INTERP
PT_NOTE
PT_SHLIB
PT_PHDR
PT_LOPROC
PT_HIPROC
PT_GNU_STACK
virtual_address

Virtual address at which the segment resides in memory.

dynamic_section_entries

New in version 3.6.0.

Number of entries in the dynamic section in the ELF file.

dynamic

New in version 3.6.0.

A zero-based array of dynamic objects, one for each entry in found in the ELF’s dynamic section. Individual dynamic objects can be accessed by using the [] operator. Each dynamic object has the following attributes:

type

Value that describes the type of dynamic section. Builtin values are:

DT_NULL
DT_NEEDED
DT_PLTRELSZ
DT_PLTGOT
DT_HASH
DT_STRTAB
DT_SYMTAB
DT_RELA
DT_RELASZ
DT_RELAENT
DT_STRSZ
DT_SYMENT
DT_INIT
DT_FINI
DT_SONAME
DT_RPATH
DT_SYMBOLIC
DT_REL
DT_RELSZ
DT_RELENT
DT_PLTREL
DT_DEBUG
DT_TEXTREL
DT_JMPREL
DT_BIND_NOW
DT_INIT_ARRAY
DT_FINI_ARRAY
DT_INIT_ARRAYSZ
DT_FINI_ARRAYSZ
DT_RUNPATH
DT_FLAGS
DT_ENCODING
value

A value associated with the given type. The type of value (address, size, etc.) is dependant on the type of dynamic entry.

symtab_entries

New in version 3.6.0.

Number of entries in the symbol table found in the ELF file.

symtab

New in version 3.6.0.

A zero-based array of symbol objects, one for each entry in found in the ELF’s SYMBTAB. Individual symbol objects can be accessed by using the [] operator. Each symbol object has the following attributes:

name

The symbol’s name.

value

A value associated with the symbol. Generally a virtual address.

size

The symbol’s size.

type

The type of symbol. Built values are:

STT_NOTYPE
STT_OBJECT
STT_FUNC
STT_SECTION
STT_FILE
STT_COMMON
STT_TLS
bind

The binding of the symbol. Builtin values are:

STB_LOCAL
STB_GLOBAL
STB_WEAK
shndx

The section index which the symbol is associated with.

Cuckoo module

The Cuckoo module enables you to create YARA rules based on behavioral information generated by Cuckoo sandbox. While scanning a PE file with YARA, you can pass additional information about its behavior to the cuckoo module and create rules based not only on what it contains, but also on what it does.

Important

This module is not built into YARA by default, to learn how to include it refer to Compiling and installing YARA. Good news for Windows users: this module is already included in the official Windows binaries.

Suppose that you’re interested in executable files sending a HTTP request to http://someone.doingevil.com. In previous versions of YARA you had to settle with:

rule evil_doer
{
    strings:
        $evil_domain = "http://someone.doingevil.com"

    condition:
        $evil_domain
}

The problem with this rule is that the domain name could be contained in the file for perfectly valid reasons not related with sending HTTP requests to http://someone.doingevil.com. Furthermore, the malicious executable could contain the domain name ciphered or obfuscated, in which case your rule would be completely useless.

But now with the cuckoo module you can take the behavior report generated for the executable file by your Cuckoo sandbox, pass it alongside the executable file to YARA, and write a rule like this:

import "cuckoo"

rule evil_doer
{
    condition:
        cuckoo.network.http_request(/http:\/\/someone\.doingevil\.com/)
}

Of course you can mix your behavior-related conditions with good old string-based conditions:

import "cuckoo"

rule evil_doer
{
    strings:
        $some_string = { 01 02 03 04 05 06 }

    condition:
        $some_string and
        cuckoo.network.http_request(/http:\/\/someone\.doingevil\.com/)
}

But how do we pass the behavior information to the cuckoo module? Well, in the case of the command-line tool you must use the -x option in this way:

$yara -x cuckoo=behavior_report_file rules_file pe_file

behavior_report_file is the path to a file containing the behavior file generated by the Cuckoo sandbox in JSON format.

If you are using yara-python then you must pass the behavior report in the modules_data argument for the match method:

import yara
rules = yara.compile('./rules_file')
report_file = open('./behavior_report_file')
report_data = report_file.read()
rules.match(pe_file, modules_data={'cuckoo': bytes(report_data)})

Reference

network
http_request(regexp)

Function returning true if the program sent a HTTP request to a URL matching the provided regular expression.

Example: cuckoo.network.http_request(/evil\.com/)

http_get(regexp)

Similar to http_request(), but only takes into account GET requests.

http_post(regexp)

Similar to http_request(), but only takes into account POST requests.

dns_lookup(regexp)

Function returning true if the program sent a domain name resolution request for a domain matching the provided regular expression.

Example: cuckoo.network.dns_lookup(/evil\.com/)

registry
key_access(regexp)

Function returning true if the program accessed a registry entry matching the provided regular expression.

Example: cuckoo.registry.key_access(/\\Software\\Microsoft\\Windows\\CurrentVersion\\Run/)

filesystem
file_access(regexp)

Function returning true if the program accessed a file matching the provided regular expression.

Example: cuckoo.filesystem.file_access(/autoexec\.bat/)

sync
mutex(regexp)

Function returning true if the program opens or creates a mutex matching the provided regular expression.

Example: cuckoo.sync.mutex(/EvilMutexName/)

Magic module

New in version 3.1.0.

The Magic module allows you to identify the type of the file based on the output of file, the standard Unix command.

Important

This module is not built into YARA by default, to learn how to include it refer to Compiling and installing YARA. Bad news for Windows users: this module is not supported on Windows.

There are two functions in this module: type() and mime_type(). The first one returns the descriptive string returned by file, for example, if you run file against some PDF document you’ll get something like this:

$file some.pdf
some.pdf: PDF document, version 1.5

The type() function would return “PDF document, version 1.5” in this case. Using the mime_type() function is similar to passing the --mime argument to file.:

$file --mime some.pdf
some.pdf: application/pdf; charset=binary

mime_type() would return “application/pdf”, without the charset part.

By experimenting a little with the file command you can learn which output to expect for different file types. These are a few examples:

  • JPEG image data, JFIF standard 1.01
  • PE32 executable for MS Windows (GUI) Intel 80386 32-bit
  • PNG image data, 1240 x 1753, 8-bit/color RGBA, non-interlaced
  • ASCII text, with no line terminators
  • Zip archive data, at least v2.0 to extract
type()

Function returning a string with the type of the file.

Example: magic.type() contains “PDF”

mime_type()

Function returning a string with the MIME type of the file.

Example: magic.mime_type() == “application/pdf”

Hash module

New in version 3.2.0.

The Hash module allows you to calculate hashes (MD5, SHA1, SHA256) from portions of your file and create signatures based on those hashes.

Important

This module depends on the OpenSSL library. Please refer to Compiling and installing YARA for information about how to build OpenSSL-dependant features into YARA.

Good news for Windows users: this module is already included in the official Windows binaries.

md5(offset, size)

Returns the MD5 hash for size bytes starting at offset. When scanning a running process the offset argument should be a virtual address within the process address space. The returned string is always in lowercase.

Example: hash.md5(0, filesize) == “feba6c919e3797e7778e8f2e85fa033d”

md5(string)

Returns the MD5 hash for the given string.

Example: hash.md5(“dummy”) == “275876e34cf609db118f3d84b799a790”

sha1(offset, size)

Returns the SHA1 hash for the size bytes starting at offset. When scanning a running process the offset argument should be a virtual address within the process address space. The returned string is always in lowercase.

sha1(string)

Returns the SHA1 hash for the given string.

sha256(offset, size)

Returns the SHA256 hash for the size bytes starting at offset. When scanning a running process the offset argument should be a virtual address within the process address space. The returned string is always in lowercase.

sha256(string)

Returns the SHA256 hash for the given string.

checksum32(offset, size)

Returns a 32-bit checksum for the size bytes starting at offset. The checksum is just the sum of all the bytes (unsigned).

checksum32(string)

Returns a 32-bit checksum for the given string. The checksum is just the sum of all the bytes in the string (unsigned).

Math module

New in version 3.3.0.

The Math module allows you to calculate certain values from portions of your file and create signatures based on those results.

Important

Where noted these functions return floating point numbers. YARA is able to convert integers to floating point numbers during most operations. For example this will convert 7 to 7.0 automatically, because the return type of the entropy function is a floating point value:

math.entropy(0, filesize) >= 7

The one exception to this is when a function requires a floating point number as an argument. For example, this will cause a syntax error because the arguments must be floating point numbers:

math.in_range(2, 1, 3)

entropy(offset, size)

Returns the entropy for size bytes starting at offset. When scanning a running process the offset argument should be a virtual address within the process address space. The returned value is a float.

Example: math.entropy(0, filesize) >= 7

entropy(string)

Returns the entropy for the given string.

Example: math.entropy(“dummy”) > 7

monte_carlo_pi(offset, size)

Returns the percentage away from Pi for the size bytes starting at offset when run through the Monte Carlo from Pi test. When scanning a running process the offset argument should be a virtual address within the process address space. The returned value is a float.

Example: math.monte_carlo_pi(0, filesize) < 0.07

monte_carlo_pi(string)

Return the percentage away from Pi for the given string.

serial_correlation(offset, size)

Returns the serial correlation for the size bytes starting at offset. When scanning a running process the offset argument should be a virtual address within the process address space. The returned value is a float between 0.0 and 1.0.

Example: math.serial_correlation(0, filesize) < 0.2

serial_correlation(string)

Return the serial correlation for the given string.

mean(offset, size)

Returns the mean for the size bytes starting at offset. When scanning a running process the offset argument should be a virtual address within the process address space. The returned value is a float.

Example: math.mean(0, filesize) < 72.0

mean(string)

Return the mean for the given string.

deviation(offset, size, mean)

Returns the deviation from the mean for the size bytes starting at offset. When scanning a running process the offset argument should be a virtual address within the process address space. The returned value is a float.

The mean of an equally distributed random sample of bytes is 127.5, which is available as the constant math.MEAN_BYTES.

Example: math.deviation(0, filesize, math.MEAN_BYTES) == 64.0

deviation(string, mean)

Return the deviation from the mean for the given string.

in_range(test, lower, upper)

Returns true if the test value is between lower and upper values. The comparisons are inclusive.

Example: math.in_range(math.deviation(0, filesize, math.MEAN_BYTES), 63.9, 64,1)

dotnet module

New in version 3.6.0.

The dotnet module allows you to create more fine-grained rules for .NET files by using attributes and features of the .NET file format. Let’s see some examples:

import "dotnet"

rule not_exactly_five_streams
{
    condition:
        dotnet.number_of_streams != 5
}

rule blop_stream
{
    condition:
        for any i in (0..dotnet.number_of_streams - 1):
            (dotnet.streams[i].name == "#Blop")
}

Reference

version

The version string contained in the metadata root.

Example: dotnet.version == “v2.0.50727”

module_name

The name of the module.

Example: dotnet.module_name == “axs”

number_of_streams

The number of streams in the file.

streams

A zero-based array of stream objects, one for each stream contained in the file. Individual streams can be accessed by using the [] operator. Each stream object has the following attributes:

name

Stream name.

offset

Stream offset.

size

Stream size.

Example: dotnet.streams[0].name == “#~”

number_of_guids

The number of GUIDs in the guids array.

guids

A zero-based array of strings, one for each GUID. Individual guids can be accessed by using the [] operator.

Example: dotnet.guids[0] == “99c08ffd-f378-a891-10ab-c02fe11be6ef”

number_of_resources

The number of resources in the .NET file. These are different from normal PE resources.

resources

A zero-based array of resource objects, one for each resource the .NET file has. Individual resources can be accessed by using the [] operator. Each resource object has the following attributes:

offset

Offset for the resource data.

length

Length of the resource data.

name

Name of the resource (string).

Example: uint16be(dotnet.resources[0].offset) == 0x4d5a

assembly

Object for .NET assembly information.

version

An object with integer values representing version information for this assembly. Attributes are:

major minor build_number revision_number

name

String containing the assembly name.

culture

String containing the culture (language/country/region) for this assembly.

Example: dotnet.assembly.name == “Keylogger”

Example: dotnet.assembly.version.major == 7 and dotnet.assembly.version.minor == 0

number_of_modulerefs

The number of module references in the .NET file.

modulerefs

A zero-based array of strings, one for each module reference the .NET file has. Individual module references can be accessed by using the [] operator.

Example: dotnet.modulerefs[0] == “kernel32”

typelib

The typelib of the file.

assembly_refs

Object for .NET assembly reference information.

version

An object with integer values representing version information for this assembly. Attributes are:

major minor build_number revision_number

name

String containing the assembly name.

public_key_or_token

String containing the public key or token which identifies the author of this assembly. assembly.

number_of_user_strings

The number of user strings in the file.

user_strings

An zero-based array of user strings, one for each stream contained in the file. Individual strings can be accessed by using the [] operator.

Time module

New in version 3.7.0.

The Time module allows you to use temporal conditions in your YARA rules.

now()

Function returning an integer which is the number of seconds since January 1, 1970.

Example: pe.timestamp > time.now()

Writing your own modules

For the first time ever, in YARA 3.0 you can extend its features to express more complex and refined conditions. YARA 3.0 does this by employing modules, which you can use to define data structures and functions, which can be later used from within your rules. You can see some examples of what a module can do in the Using modules section.

The purpose of the following sections is to teach you how to create your own modules for giving YARA that cool feature you always dreamed of.

The “Hello World!” module

Modules are written in C and built into YARA as part of the compiling process. In order to create your own modules you must be familiar with the C programming language and how to configure and build YARA from source code. You don’t need to understand how YARA does its magic; YARA exposes a simple API for modules, which is all you need to know.

The source code for your module must reside in the libyara/modules directory of the source tree. It’s recommended to use the module name as the file name for the source file, if your module’s name is foo its source file should be foo.c.

In the libyara/modules directory you’ll find a demo.c file we’ll use as our starting point. The file looks like this:

#include <yara/modules.h>

#define MODULE_NAME demo

begin_declarations;

  declare_string("greeting");

end_declarations;

int module_initialize(
    YR_MODULE* module)
{
  return ERROR_SUCCESS;
}

int module_finalize(
    YR_MODULE* module)
{
  return ERROR_SUCCESS;
}

int module_load(
    YR_SCAN_CONTEXT* context,
    YR_OBJECT* module_object,
    void* module_data,
    size_t module_data_size)
{
  set_string("Hello World!", module_object, "greeting");
  return ERROR_SUCCESS;
}

int module_unload(
    YR_OBJECT* module_object)
{
  return ERROR_SUCCESS;
}

#undef MODULE_NAME

Let’s start dissecting the source code so you can understand every detail. The first line in the code is:

#include <yara/modules.h>

The modules.h header file is where the definitions for YARA’s module API reside, therefore this include directive is required in all your modules. The second line is:

#define MODULE_NAME demo

This is how you define the name of your module and is also required. Every module must define its name at the start of the source code. Module names must be unique among the modules built into YARA.

Then follows the declaration section:

begin_declarations;

  declare_string("greeting");

end_declarations;

Here is where the module declares the functions and data structures that will be available for your YARA rules. In this case we are declaring just a string variable named greeting. We are going to discuss these concepts in greater detail in the The declaration section.

After the declaration section you’ll find a pair of functions:

int module_initialize(
    YR_MODULE* module)
{
  return ERROR_SUCCESS;
}

int module_finalize(
    YR_MODULE* module)
{
  return ERROR_SUCCESS;
}

The module_initialize function is called during YARA’s initialization while its counterpart module_finalize is called while finalizing YARA. These functions allow you to initialize and finalize any global data structure you may need to use in your module.

Then comes the module_load function:

int module_load(
    YR_SCAN_CONTEXT* context,
    YR_OBJECT* module_object,
    void* module_data,
    size_t module_data_size)
{
  set_string("Hello World!", module_object, "greeting");
  return ERROR_SUCCESS;
}

This function is invoked once for each scanned file, but only if the module is imported by some rule with the import directive. The module_load function is where your module has the opportunity to inspect the file being scanned, parse or analyze it in the way preferred, and then populate the data structures defined in the declarations section.

In this example the module_load function doesn’t inspect the file content at all, it just assigns the string, “Hello World!” to the variable greeting declared before.

And finally, we have the module_unload function:

int module_unload(
    YR_OBJECT* module_object)
{
  return ERROR_SUCCESS;
}

For each call to module_load there is a corresponding call to module_unload. This function allows your module to free any resource allocated during module_load. There’s nothing to free in this case, so the function just returns ERROR_SUCCESS. Both module_load and module_unload should return ERROR_SUCCESS to indicate that everything went fine. If a different value is returned the scanning will be aborted and an error reported to the user.

Building our “Hello World!”

Modules are not magically built into YARA just by dropping their source code into the libyara/modules directory, you must follow two further steps in order to get them to work. The first step is adding your module to the module_list file also found in the libyara/modules directory.

The module_list file looks like this:

MODULE(tests)
MODULE(pe)

#ifdef CUCKOO_MODULE
MODULE(cuckoo)
#endif

You must add a line MODULE(<name>) with the name of your module to this file. In our case the resulting module_list is:

MODULE(tests)
MODULE(pe)

#ifdef CUCKOO_MODULE
MODULE(cuckoo)
#endif

MODULE(demo)

The second step is modifying the Makefile.am to tell the make program that the source code for your module most be compiled and linked into YARA. At the very beginning of libyara/Makefile.am you’ll find this:

MODULES =  modules/tests.c
MODULES += modules/pe.c

if CUCKOO_MODULE
MODULES += modules/cuckoo.c
endif

Just add a new line for your module:

MODULES =  modules/tests.c
MODULES += modules/pe.c

if CUCKOO_MODULE
MODULES += modules/cuckoo.c
endif

MODULES += modules/demo.c

And that’s all! Now you’re ready to build YARA with your brand-new module included. Just go to the source tree root directory and type as always:

make
sudo make install

Now you should be able to create a rule like this:

import "demo"

rule HelloWorld
{
    condition:
        demo.greeting == "Hello World!"
}

Any file scanned with this rule will match the HelloWord because demo.greeting == "Hello World!" is always true.

The declaration section

The declaration section is where you declare the variables, structures and functions that will be available for your YARA rules. Every module must contain a declaration section like this:

begin_declarations;

    <your declarations here>

end_declarations;

Basic types

Within the declaration section you can use declare_string(<variable name>), declare_integer(<variable name>) and declare_float(<variable name>) to declare string, integer, or float variables respectively. For example:

begin_declarations;

    declare_integer("foo");
    declare_string("bar");
    declare_float("baz");

end_declarations;

Note

Floating-point variables require YARA version 3.3.0 or later.

Variable names can’t contain characters other than letters, numbers and underscores. These variables can be used later in your rules at any place where an integer or string is expected. Supposing your module name is “mymodule”, they can be used like this:

mymodule.foo > 5

mymodule.bar matches /someregexp/

Structures

Your declarations can be organized in a more structured way:

begin_declarations;

    declare_integer("foo");
    declare_string("bar");
    declare_float("baz");

    begin_struct("some_structure");

        declare_integer("foo");

        begin_struct("nested_structure");

            declare_integer("bar");

        end_struct("nested_structure");

    end_struct("some_structure");

    begin_struct("another_structure");

        declare_integer("foo");
        declare_string("bar");
        declare_string("baz");
        declare_float("tux");

    end_struct("another_structure");

end_declarations;

In this example we’re using begin_struct(<structure name>) and end_struct(<structure name>) to delimit two structures named some_structure and another_structure. Within the structure delimiters you can put any other declarations you want, including another structure declaration. Also notice that members of different structures can have the same name, but members within the same structure must have unique names.

When referring to these variables from your rules it would be like this:

mymodule.foo
mymodule.some_structure.foo
mymodule.some_structure.nested_structure.bar
mymodule.another_structure.baz

Arrays

In the same way you declare individual strings, integers, floats or structures, you can declare arrays of them:

begin_declarations;

    declare_integer_array("foo");
    declare_string_array("bar");
    declare_float_array("baz");

    begin_struct_array("struct_array");

        declare_integer("foo");
        declare_string("bar");

    end_struct_array("struct_array");

end_declarations;

Individual values in the array are referenced like in most programming languages:

foo[0]
bar[1]
baz[3]
struct_array[4].foo
struct_array[1].bar

Arrays are zero-based and don’t have a fixed size, they will grow as needed when you start initializing its values.

Dictionaries

New in version 3.2.0.

You can also declare dictionaries of integers, floats, strings, or structures:

begin_declarations;

    declare_integer_dictionary("foo");
    declare_string_dictionary("bar");
    declare_float_dictionary("baz")

    begin_struct_dictionary("struct_dict");

        declare_integer("foo");
        declare_string("bar");

    end_struct_dictionary("struct_dict");

end_declarations;

Individual values in the dictionary are accessed by using a string key:

foo["somekey"]
bar["anotherkey"]
baz["yetanotherkey"]
struct_dict["k1"].foo
struct_dict["k1"].bar

Functions

One of the more powerful features of YARA modules is the possibility of declaring functions that can be later invoked from your rules. Functions must appear in the declaration section in this way:

declare_function(<function name>, <argument types>, <return tuype>, <C function>);

<function name> is the name that will be used in your YARA rules to invoke the function.

<argument types> is a string containing one character per function argument, where the character indicates the type of the argument. Functions can receive four different types of arguments: string, integer, float and regular expression, denoted by characters: s, i, f and r respectively. If your function receives two integers <argument types> must be “ii”, if it receives an integer as the first argument and a string as the second one <argument types> must be “is”, if it receives three strings and a float <argument types> must be “sssf”.

<return type> is a string with a single character indicating the return type. Possible return types are string (“s”) integer (“i”) and float (“f”).

<C function> is the identifier for the actual implementation of your function.

Here you have a full example:

define_function(isum)
{
  int64_t a = integer_argument(1);
  int64_t b = integer_argument(2);

  return_integer(a + b);
}

define_function(fsum)
{
  double a = float_argument(1);
  double b = float_argument(2);

  return_integer(a + b);
}

begin_declarations;

    declare_function("sum", "ii", "i", sum);

end_declarations;

As you can see in the example above, your function code must be defined before the declaration section, like this:

define_function(<function identifier>)
{
  ...your code here
}

Functions can be overloaded as in C++ and other programming languages. You can declare two functions with the same name as long as they differ in the type or number of arguments. One example of overloaded functions can be found in the Hash module, it has two functions for calculating MD5 hashes, one receiving an offset and length within the file and another one receiving a string:

begin_declarations;

    declare_function("md5", "ii", "s", data_md5);
    declare_function("md5", "s", "s", string_md5);

end_declarations;

We are going to discuss function implementation more in depth in the More about functions section.

Initialization and finalization

Every module must implement two functions for initialization and finalization: module_initialize and module_finalize. The former is called during YARA’s initialization by yr_initialize() while the latter is called during finalization by yr_finalize(). Both functions are invoked whether or not the module is being imported by some rule.

These functions give your module an opportunity to initialize any global data structure it may need, but most of the time they are just empty functions:

int module_initialize(
    YR_MODULE* module)
{
  return ERROR_SUCCESS;
}

int module_finalize(
    YR_MODULE* module)
{
  return ERROR_SUCCESS;
}

Any returned value different from ERROR_SUCCESS will abort YARA’s execution.

Implementing the module’s logic

Besides module_initialize and module_finalize every module must implement two other functions which are called by YARA during the scanning of a file or process memory space: module_load and module_unload. Both functions are called once for each scanned file or process, but only if the module was imported by means of the import directive. If the module is not imported by some rule neither module_load nor module_unload will be called.

The module_load function has the following prototype:

int module_load(
    YR_SCAN_CONTEXT* context,
    YR_OBJECT* module_object,
    void* module_data,
    size_t module_data_size)

The context argument contains information relative to the current scan, including the data being scanned. The module_object argument is a pointer to a YR_OBJECT structure associated with the module. Each structure, variable or function declared in a YARA module is represented by a YR_OBJECT structure. These structures form a tree whose root is the module’s YR_OBJECT structure. If you have the following declarations in a module named mymodule:

begin_declarations;

    declare_integer("foo");

    begin_struct("bar");

        declare_string("baz");

    end_struct("bar");

end_declarations;

Then the tree will look like this:

YR_OBJECT(type=OBJECT_TYPE_STRUCT, name="mymodule")
 |
 |_ YR_OBJECT(type=OBJECT_TYPE_INTEGER, name="foo")
 |
 |_ YR_OBJECT(type=OBJECT_TYPE_STRUCT, name="bar")
     |
     |_ YR_OBJECT(type=OBJECT_TYPE_STRING, name="baz")

Notice that both bar and mymodule are of the same type OBJECT_TYPE_STRUCT, which means that the YR_OBJECT associated with the module is just another structure like bar. In fact, when you write in your rules something like mymodule.foo you’re performing a field lookup in a structure in the same way that bar.baz does.

In summary, the module_object argument allows you to access every variable, structure or function declared by the module by providing a pointer to the root of the objects tree.

The module_data argument is a pointer to any additional data passed to the module, and module_data_size is the size of that data. Not all modules require additional data, most of them rely on the data being scanned alone, but a few of them require more information as input. The Cuckoo module is a good example of this, it receives a behavior report associated with PE files being scanned which is passed in the module_data and module_data_size arguments.

For more information on how to pass additional data to your module take a look at the -x argument in Running YARA from the command-line.

Accessing the scanned data

Most YARA modules need to access the file or process memory being scanned to extract information from it. The data being scanned is sent to the module in the YR_SCAN_CONTEXT structure passed to the module_load function. The data is sometimes sliced in blocks, therefore your module needs to iterate over the blocks by using the foreach_memory_block macro:

int module_load(
    YR_SCAN_CONTEXT* context,
    YR_OBJECT* module_object,
    void* module_data,
    size_t module_data_size)
{
    YR_MEMORY_BLOCK* block;

    foreach_memory_block(context, block)
    {
        ..do something with the current memory block
    }
}

Each memory block is represented by a YR_MEMORY_BLOCK structure with the following attributes:

YR_MEMORY_BLOCK_FETCH_DATA_FUNC fetch_data

Pointer to a function returning a pointer to the block’s data.

size_t size

Size of the data block.

size_t base

Base offset/address for this block. If a file is being scanned this field contains the offset within the file where the block begins, if a process memory space is being scanned this contains the virtual address where the block begins.

The blocks are always iterated in the same order as they appear in the file or process memory. In the case of files the first block will contain the beginning of the file. Actually, a single block will contain the whole file’s content in most cases, but you can’t rely on that while writing your code. For very big files YARA could eventually split the file into two or more blocks, and your module should be prepared to handle that.

The story is very different for processes. While scanning a process memory space your module will definitely receive a large number of blocks, one for each committed memory region in the process address space.

However, there are some cases where you don’t actually need to iterate over the blocks. If your module just parses the header of some file format you can safely assume that the whole header is contained within the first block (put some checks in your code nevertheless). In those cases you can use the first_memory_block macro:

int module_load(
    YR_SCAN_CONTEXT* context,
    YR_OBJECT* module_object,
    void* module_data,
    size_t module_data_size)
{
    YR_MEMORY_BLOCK* block;
    const uint8_t* block_data;

    block = first_memory_block(context);
    block_data = block->fetch_data(block)

    if (block_data != NULL)
    {
      ..do something with the memory block
    }
}

In the previous example you can also see how to use the fetch_data function. This function, which is a member of the YR_MEMORY_BLOCK structure, receives a pointer to the same block (as a self or this pointer) and returns a pointer to the block’s data. Your module doesn’t own the memory pointed to by this pointer, freeing that memory is not your responsibility. However keep in mind that the pointer is valid only until you ask for the next memory block. As long as you use the pointer within the scope of a foreach_memory_block you are on the safe side. Also take into account that fetch_data can return a NULL pointer, your code must be prepared for that case.

const uint8_t* block_data;

foreach_memory_block(context, block)
{
  block_data = block->fetch_data(block);

  if (block_data != NULL)
  {
    // using block_data is safe here.
  }
}

// the memory pointed to by block_data can be already freed here.

Setting variable’s values

The module_load function is where you assign values to the variables declared in the declarations section, once you’ve parsed or analyzed the scanned data and/or any additional module’s data. This is done by using the set_integer and set_string functions:

void set_integer(int64_t value, YR_OBJECT* object, const char* field, ...)
void set_string(const char* value, YR_OBJECT* object, const char* field, ...)

Both functions receive a value to be assigned to the variable, a pointer to a YR_OBJECT representing the variable itself or some ancestor of that variable, a field descriptor, and additional arguments as defined by the field descriptor.

If we are assigning the value to the variable represented by object itself, then the field descriptor must be NULL. For example, assuming that object points to a YR_OBJECT structure corresponding to some integer variable, we can set the value for that integer variable with:

set_integer(<value>, object, NULL);

The field descriptor is used when you want to assign the value to some descendant of object. For example, consider the following declarations:

begin_declarations;

    begin_struct("foo");

        declare_string("bar");

        begin_struct("baz");

            declare_integer("qux");

        end_struct("baz");

    end_struct("foo");

end_declarations;

If object points to the YR_OBJECT associated with the foo structure you can set the value for the bar string like this:

set_string(<value>, object, "bar");

And the value for qux like this:

set_integer(<value>, object, "baz.qux");

Do you remember that the module_object argument for module_load was a pointer to a YR_OBJECT? Do you remember that this YR_OBJECT is a structure just like bar is? Well, you could also set the values for bar and qux like this:

set_string(<value>, module_object, "foo.bar");
set_integer(<value>, module_object, "foo.baz.qux");

But what happens with arrays? How can I set the value for array items? If you have the following declarations:

begin_declarations;

    declare_integer_array("foo");

    begin_struct_array("bar")

        declare_string("baz");
        declare_integer_array("qux");

    end_struct_array("bar");

end_declarations;

Then the following statements are all valid:

set_integer(<value>, module, "foo[0]");
set_integer(<value>, module, "foo[%i]", 2);
set_string(<value>, module, "bar[%i].baz", 5);
set_string(<value>, module, "bar[0].qux[0]");
set_string(<value>, module, "bar[0].qux[%i]", 0);
set_string(<value>, module, "bar[%i].qux[%i]", 100, 200);

Those %i in the field descriptor are replaced by the additional integer arguments passed to the function. This works in the same way as printf in C programs, but the only format specifiers accepted are %i and %s, for integer and string arguments respectively.

The %s format specifier is used for assigning values to a certain key in a dictionary:

set_integer(<value>, module, "foo[\"key\"]");
set_integer(<value>, module, "foo[%s]", "key");
set_string(<value>, module, "bar[%s].baz", "another_key");

If you don’t explicitly assign a value to a declared variable, array or dictionary item it will remain in an undefined state. That’s not a problem at all, and is even useful in many cases. For example, if your module parses files from a certain format and it receives one from a different format, you can safely leave all your variables undefined instead of assigning them bogus values that don’t make sense. YARA will handle undefined values in rule conditions as described in Using modules.

In addition to set_integer and set_string functions you have their get_integer and get_string counterparts. As the names suggest they are used for getting the value of a variable, which can be useful in the implementation of your functions to retrieve values previously stored by module_load.

int64_t get_integer(YR_OBJECT* object, const char* field, ...)
char* get_string(YR_OBJECT* object, const char* field, ...)

There’s also a function to get any YR_OBJECT in the objects tree:

YR_OBJECT* get_object(YR_OBJECT* object, const char* field, ...)

Here is a little exam...

Are the following two lines equivalent? Why?

set_integer(1, get_object(module_object, "foo.bar"), NULL);
set_integer(1, module_object, "foo.bar");

Storing data for later use

Sometimes the information stored directly in your variables by means of set_integer and set_string is not enough. You may need to store more complex data structures or information that doesn’t need to be exposed to YARA rules.

Storing information is essential when your module exports functions to be used in YARA rules. The implementation of these functions usually require to access information generated by module_load which must kept somewhere. You may be tempted to define global variables to store the required information, but this would make your code non-thread-safe. The correct approach is using the data field of the YR_OBJECT structures.

Each YR_OBJECT has a void* data field which can be safely used by your code to store a pointer to any data you may need. A typical pattern is using the data field of the module’s YR_OBJECT, like in the following example:

typedef struct _MY_DATA
{
   int some_integer;

} MY_DATA;

int module_load(
    YR_SCAN_CONTEXT* context,
    YR_OBJECT* module_object,
    void* module_data,
    size_t module_data_size)
{
    module->data = yr_malloc(sizeof(MY_DATA));
    ((MY_DATA*) module_object->data)->some_integer = 0;

    return ERROR_SUCCESS;
}

Don’t forget to release the allocated memory in the module_unload function:

int module_unload(
    YR_OBJECT* module_object)
{
    yr_free(module_object->data);

    return ERROR_SUCCESS;
}

Warning

Don’t use global variables for storing data. Functions in a module can be invoked from different threads at the same time and data corruption or misbehavior can occur.

More about functions

We already showed how to declare a function in The declaration section. Here we are going to discuss how to provide an implementation for them.

Function arguments

Within the function’s code you get its arguments by using integer_argument(n), float_argument(n), regexp_argument(n), string_argument(n) or sized_string_argument(n) depending on the type of the argument, where n is the 1-based argument’s number.

string_argument(n) can be used when your function expects to receive a NULL-terminated C string, if your function can receive arbitrary binary data possibly containing NULL characters you must use sized_string_argument(n).

Here you have some examples:

int64_t arg_1 = integer_argument(1);
RE* arg_2 = regexp_argument(2);
char* arg_3 = string_argument(3);
SIZED_STRING* arg_4 = sized_string_argument(4);
double arg_5 = float_argument(1);

The C type for integer arguments is int64_t, for float arguments is double, for regular expressions is RE*, for NULL-terminated strings is char* and for strings possibly containing NULL characters is SIZED_STRING*. SIZED_STRING structures have the following attributes:

SIZED_STRING
length

String’s length.

c_string

char* pointing to the string content.

Return values

Functions can return three types of values: strings, integers and floats. Instead of using the C return statement you must use return_string(x), return_integer(x) or return_float(x) to return from a function, depending on the function’s return type. In all cases x is a constant, variable, or expression evaluating to char*, int64_t or double respectively.

You can use return_string(UNDEFINED), return_float(UNDEFINED) and return_integer(UNDEFINED) to return undefined values from the function. This is useful in many situations, for example if the arguments passed to the functions don’t make sense, or if your module expects a particular file format and the scanned file is from another format, or in any other case where your function can’t a return a valid value.

Warning

Don’t use the C return statement for returning from a function. The returned value will be interpreted as an error code.

Accessing objects

While writing a function we sometimes need to access values previously assigned to the module’s variables, or additional data stored in the data field of YR_OBJECT structures as discussed earlier in Storing data for later use. But for that we need a way to get access to the corresponding YR_OBJECT first. There are two functions to do that: module() and parent(). The module() function returns a pointer to the top-level YR_OBJECT corresponding to the module, the same one passed to the module_load function. The parent() function returns a pointer to the YR_OBJECT corresponding to the structure where the function is contained. For example, consider the following code snippet:

define_function(f1)
{
    YR_OBJECT* module = module();
    YR_OBJECT* parent = parent();

    // parent == module;
}

define_function(f2)
{
    YR_OBJECT* module = module();
    YR_OBJECT* parent = parent();

    // parent != module;
}

begin_declarations;

    declare_function("f1", "i", "i", f1);

    begin_struct("foo");

        declare_function("f2", "i", "i", f2);

    end_struct("foo");

end_declarations;

In f1 the module variable points to the top-level YR_OBJECT as well as the parent variable, because the parent for f1 is the module itself. In f2 however the parent variable points to the YR_OBJECT corresponding to the foo structure while module points to the top-level YR_OBJECT as before.

Scan context

From within a function you can also access the YR_SCAN_CONTEXT structure discussed earlier in Accessing the scanned data. This is useful for functions which needs to inspect the file or process memory being scanned. This is how you get a pointer to the YR_SCAN_CONTEXT structure:

YR_SCAN_CONTEXT* context = scan_context();

Running YARA from the command-line

In order to invoke YARA you’ll need two things: a file with the rules you want to use (either in source code or compiled form) and the target to be scanned. The target can be a file, a folder, or a process.

yara [OPTIONS] RULES_FILE TARGET

RULES_FILE can be passed directly in source code form, or can be previously compiled with the yarac tool. You may prefer to use your rules in compiled form if you are going to invoke YARA multiple times with the same rules. This way you’ll save time, because for YARA it is faster to load compiled rules than compiling the same rules over and over again.

You can also pass multiple source files to yara like in the following example:

yara [OPTIONS] RULES_FILE_1 RULES_FILE_2 RULES_FILE_3 TARGET

Notice however that this only works for rules in source form. When invoking YARA with compiled rules a single file is accepted.

In the example above all rules share the same “default” namespace, which means that rule identifiers must be unique among all files. However you can specify a namespace for individual files. For example

yara [OPTIONS] namespace1:RULES_FILE_1 RULES_FILE_2 RULES_FILE_3 TARGET

In this case RULE_FILE_1 uses namespace1 while RULES_FILE_2 and RULES_FILE_3 share the default namespace.

In all cases rules will be applied to the target specified as the last argument to YARA, if it’s a path to a directory all the files contained in it will be scanned. By default YARA does not attempt to scan directories recursively, but you can use the -r option for that.

Available options are:

-t <tag> --tag=<tag>

Print rules tagged as <tag> and ignore the rest.

-i <identifier> --identifier=<identifier>

Print rules named <identifier> and ignore the rest.

-c --count

Print only number of matches.

-n

Print not satisfied rules only (negate).

-D --print-module-data

Print module data.

-g --print-tags

Print tags.

-m --print-meta

Print metadata.

-s --print-strings

Print matching strings.

-L --print-string-length

Print length of matching strings.

-e --print-namespace

Print rules’ namespace.

-p <number> --threads=<number>

Use the specified <number> of threads to scan a directory.

-l <number> --max-rules=<number>

Abort scanning after matching a number of rules.

-a <seconds> --timeout=<seconds>

Abort scanning after a number of seconds has elapsed.

-k <slots> --stack-size=<slots>

Allocate a stack size of “slots” number of slots. Default: 16384. This will allow you to use larger rules, albeit with more memory overhead.

New in version 3.5.0.

--max-strings-per-rule=<number>

Set maximum number of strings per rule (default=10000). If a rule has more then the specified number of strings an error will occur.

New in version 3.7.0.

-d <identifier>=<value>

Define external variable.

-x <module>=<file>

Pass file’s content as extra data to module.

-r --recursive

Recursively search for directories.

-f --fast-scan

Fast matching mode.

-w --no-warnings

Disable warnings.

--fail-on-warnings

Treat warnings as errors. Has no effect if used with –no-warnings.

-v --version

Show version information.

-h --help

Show help.

Here you have some examples:

  • Apply rule in /foo/bar/rules to all files in the current directory. Subdirectories are not scanned:

    yara /foo/bar/rules  .
    
  • Apply rules in /foo/bar/rules to bazfile. Only reports rules tagged as Packer or Compiler:

    yara -t Packer -t Compiler /foo/bar/rules bazfile
    
  • Scan all files in the /foo directory and its subdirectories:

    yara -r /foo
    
  • Defines three external variables mybool, myint and mystring:

    yara -d mybool=true -d myint=5 -d mystring="my string" /foo/bar/rules bazfile
    
  • Apply rules in /foo/bar/rules to bazfile while passing the content of cuckoo_json_report to the cuckoo module:

    yara -x cuckoo=cuckoo_json_report /foo/bar/rules bazfile
    

Using YARA from Python

YARA can be also used from Python through the yara-python library. Once the library is built and installed as described in Compiling and installing YARA you’ll have access to the full potential of YARA from your Python scripts.

The first step is importing the YARA library:

import yara

Then you will need to compile your YARA rules before applying them to your data, the rules can be compiled from a file path:

rules = yara.compile(filepath='/foo/bar/myrules')

The default argument is filepath, so you don’t need to explicitly specify its name:

rules = yara.compile('/foo/bar/myrules')

You can also compile your rules from a file object:

fh = open('/foo/bar/myrules')
rules = yara.compile(file=fh)
fh.close()

Or you can compile them directly from a Python string:

rules = yara.compile(source='rule dummy { condition: true }')

If you want to compile a group of files or strings at the same time you can do it by using the filepaths or sources named arguments:

rules = yara.compile(filepaths={

  'namespace1':'/my/path/rules1',
  'namespace2':'/my/path/rules2'
})

rules = yara.compile(sources={

  'namespace1':'rule dummy { condition: true }',
  'namespace2':'rule dummy { condition: false }'
})

Notice that both filepaths and sources must be dictionaries with keys of string type. The dictionary keys are used as a namespace identifier, allowing to differentiate between rules with the same name in different sources, as occurs in the second example with the dummy name.

The compile method also has an optional boolean parameter named includes which allows you to control whether or not the include directive should be accepted in the source files, for example:

rules = yara.compile('/foo/bar/my_rules', includes=False)

If the source file contains include directives the previous line would raise an exception.

If includes are used, a python callback can be set to define a custom source for the imported files (by default they are read from disk). This callback function is set through the include_callback optional parameter. It receives the following parameters:

*requested_filename: file requested with ‘include’ *filename: file containing the ‘include’ directive if applicable, else None *namespace: namespace

And returns the requested rules sources as a single string.

If you are using external variables in your rules you must define those external variables either while compiling the rules, or while applying the rules to some file. To define your variables at the moment of compilation you should pass the externals parameter to the compile method. For example:

rules = yara.compile('/foo/bar/my_rules’,
  externals= {'var1': 'some string’, 'var2': 4, 'var3': True})

The externals parameter must be a dictionary with the names of the variables as keys and an associated value of either string, integer or boolean type.

The compile method also accepts the optional boolean argument error_on_warning. This arguments tells YARA to raise an exception when a warning is issued during compilation. Such warnings are typically issued when your rules contains some construct that could be slowing down the scanning. The default value for the error_on_warning argument is False.

In all cases compile returns an instance of the class yara.Rules Rules. This class has a save method that can be used to save the compiled rules to a file:

rules.save('/foo/bar/my_compiled_rules')

The compiled rules can be loaded later by using the load method:

rules = yara.load('/foo/bar/my_compiled_rules')

Starting with YARA 3.4 both save and load accept file objects. For example, you can save your rules to a memory buffer with this code:

import StringIO

buff = StringIO.StringIO()
rules.save(file=buff)

The saved rules can be loaded from the memory buffer:

buff.seek(0)
rule = yara.load(file=buff)

The result of load is also an instance of the class yara.Rules.

Instances of Rules also have a match method, which allows you to apply the rules to a file:

matches = rules.match('/foo/bar/my_file')

But you can also apply the rules to a Python string:

with open('/foo/bar/my_file', 'rb') as f:
  matches = rules.match(data=f.read())

Or to a running process:

matches = rules.match(pid=1234)

As in the case of compile, the match method can receive definitions for external variables in the externals argument.

matches = rules.match('/foo/bar/my_file',
  externals= {'var1': 'some other string', 'var2': 100})

External variables defined during compile-time don’t need to be defined again in subsequent calls to the match method. However you can redefine any variable as needed, or provide additional definitions that weren’t provided during compilation.

In some situations involving a very large set of rules or huge files the match method can take too much time to run. In those situations you may find useful the timeout argument:

matches = rules.match('/foo/bar/my_huge_file', timeout=60)

If the match function does not finish before the specified number of seconds elapsed, a TimeoutError exception is raised.

You can also specify a callback function when invoking the match method. By default, the provided function will be called for every rule, no matter if matching or not. You can choose when your callback function is called by setting the which_callbacks parameter to one of yara.CALLBACK_MATCHES, yara.CALLBACK_NON_MATCHES or yara.CALLBACK_ALL. The default is to use yara.CALLBACK_ALL. Your callback function should expect a single parameter of dictionary type, and should return CALLBACK_CONTINUE to proceed to the next rule or CALLBACK_ABORT to stop applying rules to your data.

Here is an example:

import yara

def mycallback(data):
  print data
  return yara.CALLBACK_CONTINUE

matches = rules.match('/foo/bar/my_file', callback=mycallback, which_callbacks=yara.CALLBACK_MATCHES)

The passed dictionary will be something like this:

{
  'tags': ['foo', 'bar'],
  'matches': True,
  'namespace': 'default',
  'rule': 'my_rule',
  'meta': {},
  'strings': [(81L, '$a', 'abc'), (141L, '$b', 'def')]
}

The matches field indicates if the rule matches the data or not. The strings fields is a list of matching strings, with vectors of the form:

(<offset>, <string identifier>, <string data>)

The match method returns a list of instances of the class Match. Instances of this class have the same attributes as the dictionary passed to the callback function.

You can also specify a module callback function when invoking the match method. The provided function will be called for every imported module that scanned a file. Your callback function should expect a single parameter of dictionary type, and should return CALLBACK_CONTINUE to proceed to the next rule or CALLBACK_ABORT to stop applying rules to your data.

Here is an example:

import yara

def modules_callback(data):
  print data
  return yara.CALLBACK_CONTINUE

matches = rules.match('/foo/bar/my_file', modules_callback=modules_callback)

The passed dictionary will contain the information from the module.

Reference

yara.compile(...)

Compile YARA sources.

Either filepath, source, file, filepaths or sources must be provided. The remaining arguments are optional.

Parameters:
  • filepath (str) – Path to the source file.
  • source (str) – String containing the rules code.
  • file (file-object) – Source file as a file object.
  • filepaths (dict) – Dictionary where keys are namespaces and values are paths to source files.
  • sources (dict) – Dictionary where keys are namespaces and values are strings containing rules code.
  • externals (dict) – Dictionary with external variables. Keys are variable names and values are variable values.
  • includes (boolean) – True if include directives are allowed or False otherwise. Default value: True.
  • error_on_warning (boolean) – If true warnings are treated as errors, raising an exception.
Returns:

Compiled rules object.

Return type:

yara.Rules

Raises:
  • YaraSyntaxError – If a syntax error was found.
  • YaraError – If an error occurred.
yara.load(...)

Changed in version 3.4.0.

Load compiled rules from a path or file object. Either filepath or file must be provided.

Parameters:
  • filepath (str) – Path to a compiled rules file
  • file (file-object) – A file object supporting the read method.
Returns:

Compiled rules object.

Return type:

yara.Rules

Raises:

YaraError: If an error occurred while loading the file.

class yara.Rules

Instances of this class are returned by yara.compile() and represents a set of compiled rules.

match(filepath, pid, data, externals=None, callback=None, fast=False, timeout=None, modules_data=None, modules_callback=None)

Scan a file, process memory or data string.

Either filepath, pid or data must be provided. The remaining arguments are optional.

Parameters:
  • filepath (str) – Path to the file to be scanned.
  • pid (int) – Process id to be scanned.
  • data (str) – Data to be scanned.
  • externals (dict) – Dictionary with external variables. Keys are variable names and values are variable values.
  • callback (function) – Callback function invoked for each rule.
  • fast (bool) – If true performs a fast mode scan.
  • timeout (int) – Aborts the scanning when the number of specified seconds have elapsed.
  • modules_data (dict) – Dictionary with additional data to modules. Keys are module names and values are bytes objects containing the additional data.
  • modules_callback (function) – Callback function invoked for each module.
Raises:
  • YaraTimeoutError – If the timeout was reached.
  • YaraError – If an error occurred during the scan.
save(...)

Changed in version 3.4.0.

Save compiled rules to a file. Either filepath or file must be provided.

Parameters:
  • filepath (str) – Path to the file.
  • file (file-object) – A file object supporting the write method.
Raises:

YaraError: If an error occurred while saving the file.

The C API

You can integrate YARA into your C/C++ project by using the API provided by the libyara library. This API gives you access to every YARA feature and it’s the same API used by the command-line tools yara and yarac.

Initializing and finalizing libyara

The first thing your program must do when using libyara is initializing the library. This is done by calling the yr_initialize() function. This function allocates any resources needed by the library and initializes internal data structures. Its counterpart is yr_finalize(), which must be called when you are finished using the library.

In a multi-threaded program only the main thread must call yr_initialize() and yr_finalize(), but any additional thread using the library must call yr_finalize_thread() before exiting.

Compiling rules

Before using your rules to scan any data you need to compile them into binary form. For that purpose you’ll need a YARA compiler, which can be created with yr_compiler_create(). After being used, the compiler must be destroyed with yr_compiler_destroy().

You can use yr_compiler_add_file(), yr_compiler_add_fd(), or yr_compiler_add_string() to add one or more input sources to be compiled. Both of these functions receive an optional namespace. Rules added under the same namespace behave as if they were contained within the same source file or string, so, rule identifiers must be unique among all the sources sharing a namespace. If the namespace argument is NULL the rules are put in the default namespace.

The yr_compiler_add_file(), yr_compiler_add_fd(), and yr_compiler_add_string() functions return the number of errors found in the source code. If the rules are correct they will return 0. If any of these functions return an error the compiler can’t used anymore, neither for adding more rules nor getting the compiled rules.

For obtaining detailed error information you must set a callback function by using yr_compiler_set_callback() before calling any of the compiling functions. The callback function has the following prototype:

void callback_function(
    int error_level,
    const char* file_name,
    int line_number,
    const char* message,
    void* user_data)

Changed in version 3.3.0.

Possible values for error_level are YARA_ERROR_LEVEL_ERROR and YARA_ERROR_LEVEL_WARNING. The arguments file_name and line_number contains the file name and line number where the error or warning occurs. file_name is the one passed to yr_compiler_add_file() or yr_compiler_add_fd(). It can be NULL if you passed NULL or if you’re using yr_compiler_add_string(). The user_data pointer is the same you passed to yr_compiler_set_callback().

By default, for rules containing references to other files (include "filename.yara"), yara will try to find those files on disk. However, if you want to fetch the imported rules from another source (eg: from a database or remote service), a callback function can be set with yr_compiler_set_include_callback(). The callback receives the following parameters:

  • include_name: name of the requested file.
  • calling_rule_filename: the requesting file name (NULL if not a file).
  • calling_rule_namespace: namespace (NULL if undefined).
  • user_data pointer is the same you passed to yr_compiler_set_include_callback().

It should return the requested file’s content as a null-terminated string. The memory for this string should be allocated by the callback function. Once it is safe to free the memory used to return the callback’s result, the include_free function passed to yr_compiler_set_include_callback() will be called. If the memory does not need to be freed, NULL can be passed as include_free instead. You can completely disable support for includes by setting a NULL callback function with yr_compiler_set_include_callback().

The callback function has the following prototype:

const char* include_callback(
    const char* include_name,
    const char* calling_rule_filename,
    const char* calling_rule_namespace,
    void* user_data);

The free function has the following prototype:

void include_free(
    const char* callback_result_ptr,
    void* user_data);

After you successfully added some sources you can get the compiled rules using the yr_compiler_get_rules() function. You’ll get a pointer to a YR_RULES structure which can be used to scan your data as described in Scanning data. Once yr_compiler_get_rules() is invoked you can not add more sources to the compiler, but you can get multiple instances of the compiled rules by calling yr_compiler_get_rules() multiple times.

Each instance of YR_RULES must be destroyed with yr_rules_destroy().

Saving and retrieving compiled rules

Compiled rules can be saved to a file and retrieved later by using yr_rules_save() and yr_rules_load(). Rules compiled and saved in one machine can be loaded in another machine as long as they have the same endianness, no matter the operating system or if they are 32-bit or 64-bit systems. However files saved with older versions of YARA may not work with newer versions due to changes in the file layout.

You can also save and retrieve your rules to and from generic data streams by using functions yr_rules_save_stream() and yr_rules_load_stream(). These functions receive a pointer to a YR_STREAM structure, defined as:

typedef struct _YR_STREAM
{
  void* user_data;

  YR_STREAM_READ_FUNC read;
  YR_STREAM_WRITE_FUNC write;

} YR_STREAM;

You must provide your own implementation for read and write functions. The read function is used by yr_rules_load_stream() to read data from your stream and the write function is used by yr_rules_save_stream() to write data into your stream.

Your read and write functions must respond to these prototypes:

size_t read(
    void* ptr,
    size_t size,
    size_t count,
    void* user_data);

size_t write(
    const void* ptr,
    size_t size,
    size_t count,
    void* user_data);

The ptr argument is a pointer to the buffer where the read function should put the read data, or where the write function will find the data that needs to be written to the stream. In both cases size is the size of each element being read or written and count the number of elements. The total size of the data being read or written is size * count. Both functions must return the total size of the data read/written.

The user_data pointer is the same you specified in the YR_STREAM structure. You can use it to pass arbitrary data to your read and write functions.

Scanning data

Once you have an instance of YR_RULES you can use it with either yr_rules_scan_file(), yr_rules_scan_fd() or yr_rules_scan_mem(). The results from the scan are returned to your program via a callback function. The callback has the following prototype:

int callback_function(
    int message,
    void* message_data,
    void* user_data);

Possible values for message are:

CALLBACK_MSG_RULE_MATCHING
CALLBACK_MSG_RULE_NOT_MATCHING
CALLBACK_MSG_SCAN_FINISHED
CALLBACK_MSG_IMPORT_MODULE
CALLBACK_MSG_MODULE_IMPORTED

Your callback function will be called once for each rule with either a CALLBACK_MSG_RULE_MATCHING or CALLBACK_MSG_RULE_NOT_MATCHING message, depending if the rule is matching or not. In both cases a pointer to the YR_RULE structure associated with the rule is passed in the message_data argument. You just need to perform a typecast from void* to YR_RULE* to access the structure.

This callback is also called with the CALLBACK_MSG_IMPORT_MODULE message. All modules referenced by an import statement in the rules are imported once for every file being scanned. In this case message_data points to a YR_MODULE_IMPORT structure. This structure contains a module_name field pointing to a null terminated string with the name of the module being imported and two other fields module_data and module_data_size. These fields are initially set to NULL and 0, but your program can assign a pointer to some arbitrary data to module_data while setting module_data_size to the size of the data. This way you can pass additional data to those modules requiring it, like the Cuckoo module for example.

Once a module is imported the callback is called again with the CALLBACK_MSG_MODULE_IMPORTED. When this happens message_data points to a YR_OBJECT_STRUCTURE structure. This structure contains all the information provided by the module about the currently scanned file.

Lastly, the callback function is also called with the CALLBACK_MSG_SCAN_FINISHED message when the scan is finished. In this case message_data is NULL.

Your callback function must return one of the following values:

CALLBACK_CONTINUE
CALLBACK_ABORT
CALLBACK_ERROR

If it returns CALLBACK_CONTINUE YARA will continue normally, CALLBACK_ABORT will abort the scan but the result from the yr_rules_scan_XXXX function will be ERROR_SUCCESS. On the other hand CALLBACK_ERROR will abort the scanning too, but the result from yr_rules_scan_XXXX will be ERROR_CALLBACK_ERROR.

The user_data argument passed to your callback function is the same you passed yr_rules_scan_XXXX. This pointer is not touched by YARA, it’s just a way for your program to pass arbitrary data to the callback function.

All yr_rules_scan_XXXX functions receive a flags argument and a timeout argument. The only flag defined at this time is SCAN_FLAGS_FAST_MODE, so you must pass either this flag or a zero value. The timeout argument forces the function to return after the specified number of seconds approximately, with a zero meaning no timeout at all.

The SCAN_FLAGS_FAST_MODE flag makes the scanning a little faster by avoiding multiple matches of the same string when not necessary. Once the string was found in the file it’s subsequently ignored, implying that you’ll have a single match for the string, even if it appears multiple times in the scanned data. This flag has the same effect of the -f command-line option described in Running YARA from the command-line.

API reference

Data structures

YR_COMPILER

Data structure representing a YARA compiler.

YR_MATCH

Data structure representing a string match.

int64_t base

Base offset/address for the match. While scanning a file this field is usually zero, while scanning a process memory space this field is the virtual address of the memory block where the match was found.

int64_t offset

Offset of the match relative to base.

int32_t match_length

Length of the matching string

const uint8_t* data

Pointer to a buffer containing a portion of the matching string.

int32_t data_length

Length of data buffer. data_length is the minimum of match_length and MAX_MATCH_DATA.

Changed in version 3.5.0.

YR_META

Data structure representing a metadata value.

const char* identifier

Meta identifier.

int32_t type

One of the following metadata types:

META_TYPE_NULL META_TYPE_INTEGER META_TYPE_STRING META_TYPE_BOOLEAN
YR_MODULE_IMPORT
const char* module_name

Name of the module being imported.

void* module_data

Pointer to additional data passed to the module. Initially set to NULL, your program is responsible for setting this pointer while handling the CALLBACK_MSG_IMPORT_MODULE message.

size_t module_data_size

Size of additional data passed to module. Your program must set the appropriate value if module_data is modified.

YR_RULE

Data structure representing a single rule.

const char* identifier

Rule identifier.

const char* tags

Pointer to a sequence of null terminated strings with tag names. An additional null character marks the end of the sequence. Example: tag1\0tag2\0tag3\0\0. To iterate over the tags you can use yr_rule_tags_foreach().

YR_META* metas

Pointer to a sequence of YR_META structures. To iterate over the structures use yr_rule_metas_foreach().

YR_STRING* strings

Pointer to a sequence of YR_STRING structures. To iterate over the structures use yr_rule_strings_foreach().

YR_NAMESPACE* ns

Pointer to a YR_NAMESPACE structure.

YR_RULES

Data structure representing a set of compiled rules.

YR_STREAM

New in version 3.4.0.

Data structure representing a stream used with functions yr_rules_load_stream() and yr_rules_save_stream().

void* user_data

A user-defined pointer.

YR_STREAM_READ_FUNC read

A pointer to the stream’s read function provided by the user.

YR_STREAM_WRITE_FUNC write

A pointer to the stream’s write function provided by the user.

YR_STRING

Data structure representing a string declared in a rule.

const char* identifier

String identifier.

YR_NAMESPACE

Data structure representing a rule namespace.

const char* name

Rule namespace.

Functions

int yr_initialize(void)

Initialize the library. Must be called by the main thread before using any other function. Return ERROR_SUCCESS on success another error code in case of error. The list of possible return codes vary according to the modules compiled into YARA.

int yr_finalize(void)

Finalize the library. Must be called by the main free to release any resource allocated by the library. Return ERROR_SUCCESS on success another error code in case of error. The list of possible return codes vary according to the modules compiled into YARA.

void yr_finalize_thread(void)

Any thread using the library, except the main thread, must call this function when it finishes using the library.

int yr_compiler_create(YR_COMPILER** compiler)

Create a YARA compiler. You must pass the address of a pointer to a YR_COMPILER, the function will set the pointer to the newly allocated compiler. Returns one of the following error codes:

void yr_compiler_destroy(YR_COMPILER* compiler)

Destroy a YARA compiler.

void yr_compiler_set_callback(YR_COMPILER* compiler, YR_COMPILER_CALLBACK_FUNC callback, void* user_data)

Changed in version 3.3.0.

Set a callback for receiving error and warning information. The user_data pointer is passed to the callback function.

void yr_compiler_set_include_callback(YR_COMPILER* compiler, YR_COMPILER_INCLUDE_CALLBACK_FUNC callback, YR_COMPILER_INCLUDE_FREE_FUNC include_free, void* user_data)

New in version 3.7.0: Set a callback to provide rules from a custom source when include directive is invoked. The user_data pointer is untouched and passed back to the callback function and to the free function. Once the callback’s result is no longer needed, the include_free function will be called. If the memory does not need to be freed, include_free can be set to NULL. If callback is set to NULL support for include directives is disabled.

int yr_compiler_add_file(YR_COMPILER* compiler, FILE* file, const char* namespace, const char* file_name)

Compile rules from a file. Rules are put into the specified namespace, if namespace is NULL they will be put into the default namespace. file_name is the name of the file for error reporting purposes and can be set to NULL. Returns the number of errors found during compilation.

int yr_compiler_add_fd(YR_COMPILER* compiler, YR_FILE_DESCRIPTOR rules_fd, const char* namespace, const char* file_name)

New in version 3.6.0.

Compile rules from a file descriptor. Rules are put into the specified namespace, if namespace is NULL they will be put into the default namespace. file_name is the name of the file for error reporting purposes and can be set to NULL. Returns the number of errors found during compilation.

int yr_compiler_add_string(YR_COMPILER* compiler, const char* string, const char* namespace_)

Compile rules from a string. Rules are put into the specified namespace, if namespace is NULL they will be put into the default namespace. Returns the number of errors found during compilation.

int yr_compiler_get_rules(YR_COMPILER* compiler, YR_RULES** rules)

Get the compiled rules from the compiler. Returns one of the following error codes:

int yr_compiler_define_integer_variable(YR_COMPILER* compiler, const char* identifier, int64_t value)

Defines an integer external variable.

int yr_compiler_define_float_variable(YR_COMPILER* compiler, const char* identifier, double value)

Defines a float external variable.

int yr_compiler_define_boolean_variable(YR_COMPILER* compiler, const char* identifier, int value)

Defines a boolean external variable.

int yr_compiler_define_string_variable(YR_COMPILER* compiler, const char* identifier, const char* value)

Defines a string external variable.

void yr_rules_destroy(YR_RULES* rules)

Destroy compiled rules.

int yr_rules_save(YR_RULES* rules, const char* filename)

Save compiled rules into the file specified by filename. Returns one of the following error codes:

int yr_rules_save_stream(YR_RULES* rules, YR_STREAM* stream)

New in version 3.4.0.

Save compiled rules into stream. Returns one of the following error codes:

int yr_rules_load(const char* filename, YR_RULES** rules)

Load compiled rules from the file specified by filename. Returns one of the following error codes:

int yr_rules_load_stream(YR_STREAM* stream, YR_RULES** rules)

New in version 3.4.0.

Load compiled rules from stream. Returns one of the following error codes:

int yr_rules_scan_mem(YR_RULES* rules, const uint8_t* buffer, size_t buffer_size, int flags, YR_CALLBACK_FUNC callback, void* user_data, int timeout)

Scan a memory buffer. Returns one of the following error codes:

int yr_rules_scan_file(YR_RULES* rules, const char* filename, int flags, YR_CALLBACK_FUNC callback, void* user_data, int timeout)

Scan a file. Returns one of the following error codes:

int yr_rules_scan_fd(YR_RULES* rules, YR_FILE_DESCRIPTOR fd, int flags, YR_CALLBACK_FUNC callback, void* user_data, int timeout)

Scan a file descriptor. In POSIX systems YR_FILE_DESCRIPTOR is an int, as returned by the open() function. In Windows YR_FILE_DESCRIPTOR is a HANDLE as returned by CreateFile().

Returns one of the following error codes:

yr_rule_tags_foreach(rule, tag)

Iterate over the tags of a given rule running the block of code that follows each time with a different value for tag of type const char*. Example:

const char* tag;

/* rule is a YR_RULE object */

yr_rule_tags_foreach(rule, tag)
{
  ..do something with tag
}
yr_rule_metas_foreach(rule, meta)

Iterate over the YR_META structures associated with a given rule running the block of code that follows each time with a different value for meta. Example:

YR_META* meta;

/* rule is a YR_RULE object */

yr_rule_metas_foreach(rule, meta)
{
  ..do something with meta
}
yr_rule_strings_foreach(rule, string)

Iterate over the YR_STRING structures associated with a given rule running the block of code that follows each time with a different value for string. Example:

YR_STRING* string;

/* rule is a YR_RULE object */

yr_rule_strings_foreach(rule, string)
{
  ..do something with string
}
yr_string_matches_foreach(string, match)

Iterate over the YR_MATCH structures associated with a given string running the block of code that follows each time with a different value for match. Example:

YR_MATCH* match;

/* string is a YR_STRING object */

yr_string_matches_foreach(string, match)
{
  ..do something with match
}
yr_rules_foreach(rules, rule)

Iterate over each YR_RULE in a YR_RULES object running the block of code that follows each time with a different value for rule. Example:

YR_RULE* rule;

/* rules is a YR_RULES object */

yr_rules_foreach(rules, rule)
{
  ..do something with rule
}
yr_rule_disable(rule)

New in version 3.7.0.

Disable the specified rule. Disabled rules are completely ignored during the scanning process and they won’t match. If the disabled rule is used in the condition of some other rule the value for the disabled rule is neither true nor false but undefined. For more information about undefined values see Undefined values.

yr_rule_enable(rule)

New in version 3.7.0.

Enables the specified rule. After being disabled with yr_rule_disable() a rule can be enabled again by using this function.

Error codes

ERROR_SUCCESS

Everything went fine.

ERROR_INSUFFICIENT_MEMORY

Insufficient memory to complete the operation.

ERROR_COULD_NOT_OPEN_FILE

File could not be opened.

ERROR_COULD_NOT_MAP_FILE

File could not be mapped into memory.

ERROR_ZERO_LENGTH_FILE

File length is zero.

ERROR_INVALID_FILE

File is not a valid rules file.

ERROR_CORRUPT_FILE

Rules file is corrupt.

ERROR_UNSUPPORTED_FILE_VERSION

File was generated by a different YARA and can’t be loaded by this version.

ERROR_TOO_MANY_SCAN_THREADS

Too many threads trying to use the same YR_RULES object simultaneously. The limit is defined by MAX_THREADS in ./include/yara/limits.h

ERROR_SCAN_TIMEOUT

Scan timed out.

ERROR_CALLBACK_ERROR

Callback returned an error.

ERROR_TOO_MANY_MATCHES

Too many matches for some string in your rules. This usually happens when your rules contains very short or very common strings like 01 02 or FF FF FF FF. The limit is defined by MAX_STRING_MATCHES in ./include/yara/limits.h