In short

How do you write an effective YARA rule?

A good YARA rule pairs distinctive strings — text, hex, or regex — with a condition specific enough to avoid false positives and cheap enough to scan at volume, anchored on file structure rather than trivially mutable bytes. This cheat sheet covers rule anatomy, condition logic, and performance tuning.

YARA is the de-facto pattern-matching language for malware analysts. The syntax is small, the conditions are flexible, and the rules run anywhere — endpoint EDR, mail gateway, SIEM, retro-hunting buckets. This cheat sheet covers the syntax that matters in production plus the mistakes that quietly turn good intel into noise.

1. Rule anatomy

A YARA rule has three sections: meta (free-form attributes), strings (the patterns you'll match), and condition (the boolean expression that decides hits). Strings are referenced by their $variable name in the condition.

2. String types

Type	Syntax	Notes
Text	$a = "GET /api/v1"	Quoted, case-sensitive by default
Text (nocase)	$a = "admin" nocase	Case-insensitive matching
Text (wide)	$a = "admin" wide	Match UTF-16LE strings (Windows PE common)
Hex	$h = { 4D 5A ?? ?? 50 45 }	Bytes, with ?? wildcards and [n-m] ranges
Regex	$r = /admin[0-9]{1,3}/	Use sparingly — perf hit

3. Condition shortcuts

any of them — at least one $string hits
all of them — every $string must hit
N of them — at least N hits
$a at 0 — anchored at file offset
filesize < 10MB — file-size gating to skip big files cheaply
uint16(0) == 0x5A4D — PE magic gate; runs only on Windows EXEs

4. Performance — the only thing that matters at scale

Add a filesize gate first; nothing is cheaper than skipping the file
Gate by magic bytes (uint16(0) == 0x5A4D for PE, 0x7F454C46 for ELF)
Avoid regex strings unless you've proved a text/hex alternative won't work
Use 4+ byte hex anchors; 2-byte hex matches everything
Test on a benign-file corpus (Govt-clean-set, Windows ISO, Linux distro) before deploy

5. The five mistakes that ruin rules in production

1. Matching strings the malware borrowed from a public library (curl UA, OpenSSL banner) — false-positives on every benign binary that imports the same library.
2. Single 4-byte hex pattern as the only signal — collides with random data in compressed archives.
3. Using $string at offset against a packed sample — the unpacked image moves the offset.
4. Forgetting `wide` for Windows malware that uses UTF-16LE — half the malware family is missed.
5. Author / threat-actor names in meta without verification — survives the binary update; rule becomes wrong after the attacker refactors.

Two-string minimum rule.

Every production rule should require ≥ 2 distinct strings to hit. Single-string rules survive one malware update, then they're noise. Two-or-three string rules tied by `and` survive the next variant.

6. Template you can paste

rule Suspicious_Loader_Macksofy_2026 { meta: author = "Macksofy DFIR" date = "2026-05" hash1 = "a1b2c3..." reference = "INT-2026-042" strings: $s1 = "HelloKitty.dll" wide nocase $s2 = { 48 8B ?? ?? 48 89 ?? ?? E8 } $s3 = /\/cmd\/[a-f0-9]{16}/ condition: filesize < 5MB and uint16(0) == 0x5A4D and 2 of them }

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