Yara Rules

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Index

• Overview
• Keywords and sections
• Import overview
• Meta overview
• Strings overview
• Conditions overview

Yara rule overview

┌───────────────────────────── Rule Header ───────────────────────────────┐
│    ┌──────────────── Imports ───────────────┐                          
│    │  import "pe"                             <- Import modules        
│    └─────────────────────────────────────────┘                          
│                                                                         
│    rule <rule_name> : <tag1> <tag2> ...                                 
│    { [ Start the rule with a open bracket ]                             
│     ┌──────────────── Section: Meta ────────────┐                      
│     │ meta:                                                            
│     │   description = "..."                       <- Freeform comments 
│     │   author = "..."                                                 
│     │   date = "YYYY-MM-DD"                                            
│     └────────────────────────────────────────────┘                      
│                                                                         
│     ┌──────────────── Section: Strings ───────┐                       
│     │ strings:                                                         
│     │   $text1 = "malicious string"              <- Plaintext          
│     │   $hex1  = { 6D 61 6C 77 61 72 65 }        <- Hex pattern        
│     │   $re1   = /evil[\d]+/                     <- Regex pattern      
│     └───────────────────────────────────────────┘                       
│                                                                         
│     ┌───────────── Section: Condition ────────┐                       
│     │ condition:                                                       
│     │   any of them                              <- Trigger logic      
│     │   // other examples:                                             
│     │   all of ($text1, $hex1)                                         
│     │   $re1 and filesize < 1MB                                        
│     └───────────────────────────────────────────┘                       
│   } [ End the rule with a close bracket ]                               
└─────────────────────────────────────────────────────────────────────────┘

---

Keywords and sections

• import

  • Use to import modules

• meta:

  • Used for comments and metadata, this section is not used in the logic of the rule

• strings:

  • Where you define the context you're looking for

• condition:

  • Where the logic is defined, you set your condition here to determine matches

---

Imports overview

Imports allow you to access modules within the Yara framework. To use imports all you have to do is import "module" at the start of the rule.

Module	Use case examples
pe	Analyze PE headers, entry point, etc.
math	Use math functions in conditions
dotnet	Gather attributes out of .NET files
cuckoo	Use dynamic analysis from Cuckoo sandbox
lnk	Get lnk file attributes
console	Provides the ability to log to the console

You can find an exhaustive breakdown here

---

Meta overview

Meta sections contain the metadata of the rule and are not used within the logic of the rule. It is filled with key/value pairs and can contain pretty much anything.

...
  meta:
   author = "Some Author" 
   copyright = "PCEF"
   something = "something
...

---

Strings overview

This is the section where you create your strings to determine if it is the same file or not. All strings must start with a $ character. These are used in the conditions section to refer to the string. The strings section is not actually needed to make a complete rule as long as the conditions section doesn't rely on any strings.

...
  strings:
    # This is a regular string
    $s1 = "This is a test string"
    
    # This is a hexidecimal string
    # ??            == Any bytes match
    # [1-50         == Indicate a jump from 1 to 5 match
    # ~             == This is a not operator, any value but the defined
    # ~?            == Not operator with an any bytes to indicate any but 
    #                  defined
    # [1-]          == Indicates and unbound jump, there is not an end
    # (90 89 | 12)  == This provides alternatives for the bytes either the 
    #                  first two or the second one
    $s2 = { ?? 90 90 [1-5] ~00 ~?0 [1-] (90 89 | 12) } 
    
    # This is the regular expression. All regular expressions are in the 
    # Perl programming language format.
    $s3 = /some.magic\.regex\"/
    
    # Strings are able to take "modifiers" to make them match more 
    # types of the string. This specific one means that there is 
    # no character case constraint. For example this will match
    # bot 'test string' and 'TEST STRING' and any variation 
    $s4 = "test string" nocase 
...

Text strings

Text strings are able to contain subsets of the C language escape sequences such as:

Sequence	Description
\"	Double quote
\\	Backslash
	Carriage return
	Tab space
	New line
\xdd	Hexadecimal bytes

Modifiers

You can also use modifiers for text string as seen in the above example. Modifiers include:

Modifier	Description
nocase	No character case constraints
wide	Matches strings encoded with two up to bytes per character
ascii	Standard ascii characters
xor	Single byte Xor applied to the string
base64	Look for the string in a base64 encoding (base64wide exists as well, you can also set the charset of the encoding)
fullword	Guarantees full word match
private	Will never be included in the output of Yara

Regular expressions

Regular expressions can take the following metacharacters:

Meta	Description
\	Quote the next character
^	Match the beginning
.	Matches any single character except new lines
`	Alteration
()	Group
[]	Set of characters
*	Match 0 or more times
+	Match 1 or more times
?	Match 0 or 1 times
{n}	Match exactly n amount times
{n,}	Match at least n amount times
{,n}	Match at most n amount times
{n,n}	Match at least n amount times and at most n amount times
\w	Alphanumeric word character
\W	Any non-word character
\s	Whitespace character
\S	Non-whitespace character
\d	Digit character
\D	Non-digit character
\b	Word boundaries
\B	Except at a word boundary

Conditions overview

Conditions are basically just boolean expressions used to establish if the rule matches your comparison.

...
  condition:
   # Three of any of the strings that start with $s
   3 of ($s*) 
...

Operators

Meta	Description
[]	Quote the next character
~	Bitwise not
-	Subtraction
.	Structure member access
*	Multiplication
/	Division
%	Remainder
+	Addition
<<	Bitwise left shift
>>	Bitwise right shift
&	Bitwise AND
^	Bitwise XOR
|	Bitwise OR (minus \)
<	Less than
>	Greater than
<=	Less than or equal to
>=	Greater than or equal to
==	Equal to
!=	Not equal to
[i]contains	String contains substring, adding i makes it non-case sensitive
[i]startswith	String startswith substring, adding i makes it non-case sensitive
[i]endswith	String ends with substring, adding i makes it non-case sensitive
iequals	Non-case sensitive string comparison
matches	String matches regular expression
not defined	Logical NOT check for non-defined expression
and	Logical AND
or	Logical OR
filesize	Checks the file size
at	String offset or virtual address search
entrypoint	Special variable to check the Pe or ELF entrypoint (deprecated)

Accessing data at specific locations

You may need to access data at a certain locations and read 16, 32, or 64bit integers using an offset, use one of the following to read data from them:

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>)