yara-rule-writing-malware

YARA Rule Writing for Malware Detection

When to Use

• When performing incident response and you need to scan the entire environment for indicators of compromise (IoCs) related to a specific malware family.
• After extracting unique string patterns, mutexes, paths, or code blocks from a malware sample during static/dynamic analysis.

Prerequisites

• Authorized scope and rules of engagement for the target environment
• Appropriate tools installed on the attack/analysis platform
• Understanding of the target technology stack and architecture
• Documentation template ready for findings and evidence capture

Workflow

Phase 1: Understanding Basic YARA Structure

# Concept: Rule syntax rule Basic_Ransomware_Detection {
    meta:
        description = "Detects generic ransomware strings"
        author = "CyberSkills"
        date = "2024-05-10"
    
    strings:
        $s1 = "Your files have been encrypted" ascii wide nocase
        $s2 = "比特币" // Bitcoin in Chinese (UTF-8)
        $s3 = "vssadmin.exe Delete Shadows /All /Quiet" ascii wide
        
    condition:
        2 of them
}

Phase 2: Utilizing Hexadecimal Signatures

# rule Emotet_Hex_Pattern {
    meta:
        description = "Detects Emotet unpacking loop pattern"

    strings:
        // 8B 45 ?? 03 45 ?? 50 FF 15
        $hex_pattern = { 8B 45 ?? 03 45 ?? 50 FF 15 [4] }
        
    condition:
        $hex_pattern
}

Phase 3: Leveraging the PE Module (Windows Executables)

# import module import "pe"

rule Suspicious_Document_Icon {
    meta:
        description = "Executable disguised as PDF/Word doc"
        
    condition:
        uint16(0) == 0x5a4d and // MZ header
        pe.number_of_resources > 0 and 
        (
            pe.version_info["OriginalFilename"] contains ".pdf" or
            pe.version_info["OriginalFilename"] contains ".docx"
        )
}

Phase 4: Validating and Executing the Scan

# yara -r my_rules.yar /path/to/suspicious/files/