The YARA Rules Guide: How to Use This Malware Research Tool

The YARA Rules Guide: How to Use This Malware Research Tool
The YARA Rules Guide: How to Use This Malware Research Tool

This post was most recently updated on February 24th, 2023

By constructing descriptions of malware families based on textual or binary patterns, YARA rules are used to classify and detect malware samples. In this article, you will learn what is yara rules

The Purpose of YARA Regulations

YARA rules function like programming languages since they set up a number of variables that include patterns found in a sample of malware. Depending on the rule, it can be used to successfully identify malware if any or all of the circumstances are met.

In order to determine the threat group and malware family the sample belongs to, researchers analyze the source code of malware for identifiable patterns and terms. 

It is feasible to detect numerous samples that are all linked to the same campaign or threat actor by building a YARA rule from multiple samples that are members of the same malware family.

While analyzing a piece of malware, an analyst may develop a YARA rule for a fresh sample. They could then look for samples that were similar using this criterion by searching their personal malware database or other databases like VirusTotal.

If the malware analyst works for an organization that uses an IPS or another malware prevention technology that is supported by YARA, they may be able to use YARA rules as an incident response tool to locate potentially hazardous binaries inside the organization.

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Use case study

YARA has gained a lot of traction in the information security world since it can be used in a variety of situations:

  • Malware characterization and classification
  • Find new samples by using family-specific patterns.
  • Responders to incidents can identify contaminated samples and equipment by using YARA rules.
  • Custom YARA rules that are applied proactively can strengthen a company’s defenses.

Understanding the Fundamentals of YARA

To design a meaningful YARA rule, you must first understand the various components that can be used to build your own special YARA rule.


Metadata, on the other hand, gives crucial information about the rule and does not impact the search criteria of the YARA rule.

  • Name, email, and Twitter handle of the author.
  • Date – The law’s creation date.
  • Version – The YARA regulation’s revision number to keep track of changes.
  • Reference: A URL connecting to text or a sample download that provides important information about the malware sample the rule is designed to identify.
  • An overview of the rule’s objective and the malware it seeks to find.
  • Hash: A collection of prototype hashes used to create the YARA rule.


Strings Malware samples frequently contain amusing and unique strings, which are great starting points for YARA rule creation. Before a string can be declared inside of a rule, it must first be specified as a variable.

Sample malware string is $a. In addition to specifying a string, we can also fine-tune the search by putting modifiers after the supplied string.

  • The $a=”malwarestring” full word modifier matches a specified keyword. In contrast to “www.abcmalwarestring.com,” “www.malwarestring.com” would produce a match.
  • This recognizes unicode strings like “w.w.w…m.a.l.w.a.r.e.s.t.r.i.n.g…c.o.m,” which are separated by null bytes. Wide $a=”malwarestring”
  • Due to this, the rule can match both ascii and Unicode characters. Wide $a with malwarestring ascii
  • $a=”MalwareString” nocase – The string will always be matched by the rule.
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A YARA rule’s strings part outlines the search criteria, whereas the conditions section outlines the conditions under which the rule will provide a successful match. I’ll go over some other potential usage conditions.

  • The value of uint16(0) is 0x5A4D. – Checking a file’s header is a fantastic condition to include in your YARA rules. Since the hex numbers 4D 5A is always present at the beginning of an executable file header, this requirement requires that the file be a Windows executable. In YARA, this is inverted due to endianness.
  • uint32(0)==0x464c457f, or (uint32(0) == 0xfeedfacf, or (uint32(0) == 0xcffaedfe, or (uint32) (0) – Checks the file header for Linux binaries.
  • There are six strings (#a == 6).
  • (#a > 6) – There are more than six strings.

I’ll describe how to use the PE import in this post, which is a terrific approach to expand the criteria that apply to your YARA rules.

PE library

If you are unable to locate any unique strings, you can use YARA’s PE capabilities by adding the syntax “import pe” at the beginning of a YARA rule.

Exports are fantastic additions to a YARA rule; use their separate names since they are features that the malware author designed.

Introduction of the Rule

The word “rule” is followed by the name of the rule in the YARA rule. ” is used after this to denote the YARA regulation’s content.

The command “import pe,” which is utilized in the rule’s condition section, was imported directly above this.


The example rule contains the following information: the author, the malware’s file type, the rule’s creation date, its version, a remark about where I acquired the sample, and a hash of the infection. 

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For anyone else who might apply the rule or who might even be useful to the author when they later analyze the rule, this provides some background information.

String Declaration

Following that, I included a few strings that were discovered in the malware sample; these strings are defined as variables within the rule and can be used to search for files with content that is comparable.

I combined strange Windows API strings with strings that I believe are unique to this family of malware that I found using PEStudio.


The circumstances that must be met in order for the YARA rule to be in effect are described in the requirements section of the regulation. The first condition is that the file header must be Windows Executable.


Armed with this knowledge, you can begin developing your own YARA criteria to find fresh samples for analysis or to implement proactive detections within your firm.

Adil Husnain

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