Yara rules are a mechanism for determining whether a file is malicious (or another type of file) by defining rules that look for specific characteristics. YARA is a tool that was created by Virustotal’s Victor Alvarez.
Its principal application is in malware investigation and detection. It was created with the intention of describing patterns that detect both single malware strains and large malware families.
Syntax
Every rule must begin with the word “rule,” followed by the name or identifier. Any alphanumeric character, as well as the underscore character, may be used in the identification; however, the first character cannot be a number.
A list of Yara keywords that should not be used as ids exists since each one has a specific connotation that cannot be modified.
The Condition Rules are divided into several sections. The only section that must be completed is the conditions section.
This section specifies the conditions that must be met in order for the rule result to be regarded legitimate for the object (file) under investigation. It is made up of a Boolean expression that is utilized to determine the outcome.
Boolean expressions are the standard format for conditions and can contain any logical, relational, or mathematical operator. You are able to add one more rule to your criteria if you choose.
Strings You will also need a strings section to give meaning to the condition section. The strings section of the file allows you to declare the strings that will be searched for throughout the file.
The rule just showed is called vendor, and it looks for the strings “Vendor name” and “Alias name.” If any of the strings is detected, the conclusion reached from applying the rule is validated.
There are other additional types of strings you could look for, including the following:
- Hops, alternatives, and wild cards are optional additions to Hexadecimal.
- Text strings and their related modifiers, such as no case, full word, wide, and ASCII.
- Regular expressions that are editable in the same way that text strings are.
- There are many more intricate criteria you might employ, but discussing them all would be beyond the scope of this article. If you want to learn more, you can consult the documentation that comes with yara labs.
Metadata
It is possible to incorporate information to aid in the identification of files discovered by a certain rule. The metadata ids are always followed by the equal sign and the value that is being set.
The values that are assigned could be strings, numbers, or a Boolean value. It is critical to remember that the identifier/value pairs defined in the metadata section cannot be used in the condition section because its primary purpose is to carry additional information about the rule.
Summary
YARA is a tool that may be used to identify files that meet a predefined set of criteria. This malware classification scheme is mostly used by security researchers.
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