What is YARA?

YARA is an open-source tool (and rule language) that helps malware researchers and threat hunters identify and classify malware (or any files) based on patterns you define.

Think of it as “grep on steroids” for binary files. You write rules that describe what a malicious file (or family of malware) looks like, then you scan files or memory dumps with YARA, and it tells you which rules match.

Official tagline: “The pattern matching swiss knife for malware researchers.”

YARA is primarily used for static analysis (analyzing files without executing them), but it can also scan running processes/memory with tools like yarGen, Volatility plugins, Loki, Thor, etc.

Core Concepts of a YARA Rule

A basic YARA rule has three sections:

rule RuleName : optional_tag1 tag2
{
    meta:
        author = "Your Name"
        description = "Detects something bad"
        date = "2025-11-23"
        hash = "abc123..."

    strings:
        $string1 = "BadString"          // plain text (case-sensitive by default)
        $string2 = /evil[0-9]{3}\.exe/ // regular expression
        $hex1 = { 4D 5A 90 00 }         // hex bytes (MZ header of PE files)
        $wide1 = "BadString" wide      // UTF-16 (common in Windows malware)

    condition:
        all of them          // all strings must be found
        or any of ($string*) // any of the $stringX
        or $hex1 at 0        // MZ at offset 0 (PE file)
        or uint16(0) == 0x5A4D
}

The “strings” Section Explained

The strings: section is where you define the patterns you’re looking for.

Types of strings:

The “condition” Section

This is the boolean logic that decides if the rule triggers.

Common useful conditions:

uint16(0) == 0x5A4D                  // starts with "MZ" (PE file)
uint32(0) == 0x464c457f              // ELF file
filesize < 2MB
pe.number_of_sections > 10
pe.entry_point == 0x401000
for all of ($mz*) : ( @mz[i] < 1000 ) // all MZ strings found in first 1KB

Simple Real-World Example Rule

rule SuspiciousCalcSpawn
{
    meta:
        author = "Grok"
        description = "Detects common process injection that spawns calc.exe"

    strings:
        $s1 = "calc.exe" wide nocase
        $s2 = "CreateRemoteThread"
        $s3 = "VirtualAllocEx"

    condition:
        all of them
}

How to Actually Use YARA Rules

1. Install YARA
   • Windows/Linux/macOS: pip install yara-python or download binaries from https://github.com/VirusTotal/yara
   • Kali already has it.
2. Write your rule → save as myrules.yar or folder rules/
3. Scan something:

yara myrules.yar suspicious.exe
yara -r rules/ /malware_collection/

4. Popular public rule sets (great for learning):

• https://github.com/InQuest/yara-rules
• https://github.com/Yara-Rules/rules
• https://github.com/Neo23x0/signature-base (Florian Roth’s excellent set)

Common Beginner Workflow

1. Get a malware sample.
2. Open it in PEStudio, Detect-It-Easy, or strings.exe → look for interesting strings.
3. Look at imported DLLs/functions (VirtualAllocEx, WriteProcessMemory, etc.).
4. Write a rule with the unique strings + maybe some hex patterns or PE characteristics.
5. Test it against the sample and clean files (to avoid false positives).
6. Refine until it only hits the family you want.

Quick Tips for Better Rules

• Use private keyword if you don’t want the rule name shown in scans.
• Use tags: rule EvilRAT : rat malware
• Use for any of ($a1,$a2,$a3) : ( $ in (0x1000..0x5000) ) → any of these strings in that section.
• Combine with pe. module for rich PE header checks.

Tools That Use YARA Heavily

• Loki (free scanner)
• Thor (Honeynet Project, now commercial Lite version)
• Volexity Surge Collect
• Many EDRs (CrowdStrike, SentinelOne, etc.) accept custom YARA
• VirusTotal (you can upload private rules)

That’s the foundation. Once you’re comfortable with the basics, the next level is writing high-quality, low-FP rules for entire malware families using fuzzy strings, good meta, and clever conditions.