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17. Building Reversing Signatures with YARA Rules

YARA is a tool for declaratively matching patterns in binaries and memory dumps. During reversing, it is useful for quickly tagging which version a function belongs to or which family a sample comes from.

Key points​

  • Mix byte sequences and wildcards in strings to reduce noise.
  • Add context conditions such as filesize, pe.is_pe, and uint16(0)==0x5A4D in condition.
  • To distinguish versions, combine a short shared pattern with one or two unique patterns.

Minimal rule example​

rule fast_tag {
  strings:
    $s1 = "XOR_KEY" ascii
    $b1 = { 55 48 89 E5 ?? ?? 5D }
  condition:
    uint16(0)==0x5A4D and 1 of ($s*) and $b1
}

Practical tip​

Split rules into many small units, log their false-positive rates, and tune them. This sharply accelerates both CTF and incident-response work.