By Charles Joseph | Cybersecurity Researcher

 Published on November 26th, 2022

This post was updated on November 25th, 2023

A YARA rule is a tool used primarily in cybersecurity to identify and classify malware samples based on textual or binary patterns. Developed by Victor M. Alvarez, YARA allows researchers and incident responders to hunt for specific patterns in a collection of files or memory dumps.

A YARA rule comprises a series of strings and a Boolean expression to determine its occurrence. If the conditions specified in the rule are met when scanning a file or a block of memory, then the YARA rule is said to have been triggered, suggesting that the target data contains the described patterns.

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Here’s a basic structure of a YARA rule:

rule RuleName
{
    meta:
        author = "Author Name"
        description = "Description of what the rule detects"
        date = "Creation or modification date"

    strings:
        $string1 = "specific string or pattern"
        $string2 = { B? [2-4] ( 01 | 02 ) ?? }

    condition:
        $string1 and $string2
}

• meta: Contains metadata about the rule. This could be the author, date, description of what the rule is meant to detect, or any other relevant data. This section is optional and for informational purposes only.
• strings: Defines the patterns that you’re looking for. Patterns can be in text, hexadecimal, or regular expression format.
• condition: Specifies the condition under which the rule should trigger. It can be as simple as looking for the existence of one string or as complex as combining multiple strings using Boolean logic.

YARA rules are used in various security tools and platforms, especially in the realm of threat intelligence and digital forensics. They’re beneficial for automating the detection of known threats and discovering new variations of existing threats based on patterns and behaviors.

Simple Examples of How You Might Use Yara Rules

1. Detecting a specific string in files

This rule looks for files containing the specific string “MaliciousCode123”.

rule Detect_Malicious_String
{
    meta:
        author = "ChatGPT"
        description = "Detect files containing the string 'MaliciousCode123'"
        date = "2023-08-10"

    strings:
        $malicious_string = "MaliciousCode123"

    condition:
        $malicious_string
}

2. Detecting a specific file header

This rule detects PNG image files based on their header bytes.

rule Detect_PNG_Files
{
    meta:
        author = "ChatGPT"
        description = "Detect PNG image files based on their header"
        date = "2023-08-10"

    strings:
        $png_header = { 89 50 4E 47 0D 0A 1A 0A }  // The typical PNG file header in hexadecimal

    condition:
        $png_header at 0  // The header should be at the start of the file
}

5 Reasons Why You Would Use YARA Rules

1. To detect and classify malware
2. To determine the purpose of a given piece of code
3. To find code that performs a specific function
4. To map out the relationships between different pieces of code
5. To optimize code for performance or security

Related Questions

1. What are the main components of a YARA rule?

A YARA rule consists of:

rule: A unique name identifying the rule.

meta: A section containing metadata about the rule, such as the author, description, and date.

strings: Defines the patterns or strings the rule will search for within target files or memory.

condition: Specifies the criteria to be met for the rule to be considered a match.

2. Can YARA rules detect malicious behavior or only static patterns?

YARA is primarily designed for static pattern matching. It’s used to identify specific sequences of bytes or strings in files or memory. However, it doesn’t evaluate the behavior of a program. For dynamic analysis (observing program behavior), tools like sandboxes or system monitoring utilities are more appropriate.

3. How can YARA rules be optimized for performance?

To optimize YARA rules:

Use fewer wildcards and jumps in patterns.

Prioritize searching for unique and infrequent strings.

Use external variables to filter out non-relevant samples before scanning.

Group commonly used strings under a single condition to reduce computational effort.

4. Are there any repositories where I can find pre-written YARA rules?

Yes, there are several repositories and community-driven platforms where you can find pre-written YARA rules. One notable example is the “Yara-Rules” project on GitHub, which contains many community-generated rules.

5. Can YARA scan compressed or encrypted files?

YARA itself is designed to scan raw bytes, so it won’t natively unpack compressed or decrypt encrypted files. However, if you have the ability to decompress or decrypt the files before scanning them, YARA can be used to scan the resulting data. Some integrations or platforms might include pre-processing steps that handle certain types of compression or encryption before YARA scanning.