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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