S³ YARA Rule Authoring

Write high-quality YARA-X detection rules for malware identification and threat hunting. Covers naming conventions, string selection, performance optimization, and false positive reduction. Use when writing, reviewing, or optimizing YARA rules, converting IOCs to signatures, or debugging detection issues.

YARA-X Rule Authoring

Write detection rules that catch malware without drowning in false positives. Based on Trail of Bits methodology.

Core Principles

1. Strings must generate good atoms — YARA extracts 4-byte subsequences for fast matching. Strings with repeated bytes, common sequences, or under 4 bytes force slow bytecode scans.
2. Target specific families, not categories — "Detects ransomware" is useless. "Detects LockBit 3.0 config extraction routine" is useful.
3. Test against goodware — Validate against clean file sets before deployment.
4. Short-circuit with cheap checks first — filesize < 10MB and uint16(0) == 0x5A4D before expensive string searches.
5. Metadata is documentation — Future you needs to know what this catches and why.

YARA-X Basics

YARA-X is the Rust successor to legacy YARA: 5-10x faster, better errors, built-in formatter, stricter validation, new modules (crx, dex).

Install: brew install yara-x / cargo install yara-x Commands: yr scan, yr check, yr fmt, yr dump

Rule Template

yara

import "pe"

rule FamilyName_Variant_Technique : tag1 tag2 {
    meta:
        author      = "Solomon Neas"
        date        = "2026-02-14"
        description = "Detects [specific behavior] in [malware family]"
        reference   = "https://..."
        tlp         = "TLP:WHITE"
        hash        = ""
        score       = 75  // 0-100 confidence

    strings:
        // Unique strings from the sample
        $api1 = "VirtualAllocEx" ascii
        $api2 = "WriteProcessMemory" ascii
        $str1 = { 48 8B 05 ?? ?? ?? ?? 48 85 C0 }  // hex with wildcards
        $pdb  = /[A-Z]:\\.*\\Release\\.*\.pdb/ nocase

    condition:
        uint16(0) == 0x5A4D and
        filesize < 5MB and
        (2 of ($api*) and $str1) or
        $pdb
}

Naming Convention

Family_Variant_Technique — examples:

• Emotet_Loader_DocumentMacro
• CobaltStrike_Beacon_x64
• Generic_Cryptominer_XMRig

String Selection

Good strings (unique, specific):

• Mutex names, PDB paths, C2 URLs
• Unique byte sequences from disassembly
• Custom encryption constants
• Uncommon API call sequences

Bad strings (too common, high FP):

• http://, https://, common API names alone
• Single common words, short strings (<4 bytes)
• Strings found in Windows system files

Condition Patterns

yara

// Performance-ordered (cheap → expensive)
condition:
    uint16(0) == 0x5A4D and     // Magic bytes (instant)
    filesize < 10MB and          // Size filter (instant)
    2 of ($unique*) and          // String matching (fast)
    pe.imports("kernel32.dll")   // Module check (slower)

Common magic bytes:

Platform	Check
PE (Windows)	uint16(0) == 0x5A4D
ELF (Linux)	uint32(0) == 0x464C457F
Mach-O 64-bit	uint32(0) == 0xFEEDFACF
PDF	uint32(0) == 0x25504446
Office/ZIP	uint32(0) == 0x504B0304

Performance Rules

1. Put filesize and magic byte checks FIRST in condition
2. Never use unbounded regex like /.*/
3. Avoid for all with complex conditions on large files
4. Use ascii or wide, not both unless needed
5. Hex strings with specific bytes > wildcards > regex
6. Use at for fixed offsets instead of scanning entire file

Testing

bash

# Validate syntax
yr check rules/

# Scan a sample
yr scan rules/my_rule.yar suspicious_file.exe

# Scan directory
yr scan rules/ samples/ --threads 4

# Format rules consistently
yr fmt rules/my_rule.yar

False Positive Reduction

• Add filesize constraints (malware has typical size ranges)
• Require multiple string matches (2 of ($str*) not any of)
• Exclude known good paths/publishers via not conditions
• Score-based approach: assign confidence scores in metadata, triage by threshold
• Test against goodware corpus before deployment

Reference

Full methodology, module docs (pe, elf, crx, dex), and migration guide from legacy YARA: https://github.com/trailofbits/skills/tree/main/plugins/yara-authoring