YARA Rules 101: Classify Malware Like a Pro

YARA lets you describe malware by patterns - strings and byte sequences - and then scan files, memory or processes for matches. It's the workhorse of malware classification and IOC hunting.

Anatomy of a rule

rule Suspicious_Downloader {
  meta:
    author = "NexurAI"
    description = "Example: certutil download cradle"
  strings:
    $a = "certutil" nocase
    $b = "-urlcache" nocase
    $c = { 68 74 74 70 3A 2F 2F }   // "http://" in hex
  condition:
    $a and $b and $c
}

• strings - text ($a = "..."), hex byte patterns ({ 4D 5A } = "MZ"), or regex.
• condition - the boolean logic: $a and $b, 2 of them, all of ($s*), plus file properties (filesize, uint16(0) == 0x5A4D for PE files).

Where it's used

• Malware analysis - classify samples into families.
• Threat hunting - scan hosts/memory for known-bad patterns (Loki, THOR, Velociraptor all run YARA).
• Detection pipelines - flag files at the gateway or endpoint.

Write good rules

Anchor on stable, meaningful strings (unique code, C2 markers) - not generic ones that cause false positives. Combine several weak signals with N of them for balance, and always test against a clean corpus so precision stays high.