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YARA Rule Authoring

packages/skills/skills/yara-authoring/SKILL.md

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YARA Rule Authoring

When to Use

  • Writing YARA rules to detect malware samples or families
  • Creating detection signatures for indicators of compromise (IOCs)
  • Scanning files or directories for known threat patterns
  • Building threat hunting rules from intelligence reports
  • Classifying unknown samples based on behavioral or structural patterns

When NOT to Use

  • Dynamic malware analysis (use sandbox environments)
  • Network traffic analysis (use Suricata/Snort rules)
  • Static analysis of source code (use Semgrep/CodeQL)

Rule Template

yara
rule MalwareFamily_Variant : tag1 tag2 {
    meta:
        author = "analyst"
        description = "Detects MalwareFamily variant based on unique strings"
        date = "2024-01-01"
        reference = "https://example.com/report"
        hash = "abc123..."
        severity = "high"

    strings:
        $s1 = "unique_malware_string" ascii
        $s2 = { 4D 5A 90 00 03 00 }  // hex pattern
        $s3 = /https?:\/\/[a-z0-9]+\.evil\.com/ nocase  // regex

    condition:
        uint16(0) == 0x5A4D and  // MZ header (PE file)
        filesize < 5MB and
        (2 of ($s*))
}

String Types

TypeSyntaxUse Case
Text"string"ASCII strings
Hex{ AA BB CC }Byte patterns, shellcode
Regex/pattern/Flexible text matching

Modifiers

  • ascii / wide — encoding
  • nocase — case insensitive
  • fullword — word boundary matching
  • xor — XOR-encoded strings
  • base64 — base64-encoded strings

Condition Operators

yara
condition:
    all of them           // All strings match
    any of ($a*)          // Any string starting with $a
    2 of ($s1, $s2, $s3)  // At least 2 of listed strings
    #s1 > 3               // String $s1 appears more than 3 times
    @s1 < 0x100           // String $s1 found before offset 0x100
    filesize < 1MB        // File size constraint
    uint16(0) == 0x5A4D   // Magic bytes at offset

Scanning

bash
# Scan a file
yara rule.yar target_file

# Scan directory recursively
yara -r rules/ /path/to/scan/

# Scan with metadata output
yara -m -s rule.yar target_file

# Compile rules for faster repeated scanning
yarac rules/ compiled.yarc
yara -C compiled.yarc /path/to/scan/

Best Practices

  1. Always include meta with author, description, date, and reference
  2. Use filesize and magic byte checks to limit scope
  3. Prefer multiple weak indicators over one strong indicator
  4. Test against known samples AND clean files for false positives
  5. Use private rules for helper conditions
  6. Avoid overly broad regex patterns that cause performance issues
  7. Version control your rules and track detection rates

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