Yara Writing

YARA Rule Writing Guide

YARA rules identify and classify malware by matching byte patterns, strings, and conditions in files.

Rule Structure

rule RuleName : tag1 tag2 {
    meta:
        author = "CTI Platform"
        date = "YYYY-MM-DD"
        description = "Description of what this rule detects"
        reference = "https://source-reference.com"
        tlp = "GREEN"
        mitre_attack = "T1566"
        hash = "sha256_of_sample"
        confidence = "high"

    strings:
        $text1 = "suspicious string" ascii wide
        $hex1 = { 4D 5A 90 00 03 00 00 00 }
        $regex1 = /https?:\/\/[a-z0-9\-\.]+\.onion/

    condition:
        uint16(0) == 0x5A4D and
        filesize < 5MB and
        (2 of ($text*) or $hex1) and
        $regex1
}

String Types

Text strings

$s1 = "CreateRemoteThread"          # Exact ASCII match
$s2 = "kernel32.dll" ascii wide     # Match both ASCII and UTF-16
$s3 = "password" nocase             # Case-insensitive
$s4 = "cmd /c" ascii wide nocase    # Combined modifiers
$s5 = "C:\\Windows\\Temp\\"         # Escaped backslashes

Hex strings

$h1 = { 4D 5A 90 00 }              # Exact bytes (PE header)
$h2 = { 4D 5A ?? ?? 03 00 }        # Wildcards (?? = any byte)
$h3 = { 4D 5A [2-4] 03 00 }        # Jump (2-4 bytes between)
$h4 = { 4D 5A ( 90 00 | 00 00 ) }  # Alternation (OR)

Regular expressions

$r1 = /https?:\/\/[\w\-\.]+/       # URL pattern
$r2 = /[A-Za-z0-9+\/]{50,}={0,2}/  # Base64 encoded content
$r3 = /(\d{1,3}\.){3}\d{1,3}/      # IP address pattern

Conditions

File properties

uint16(0) == 0x5A4D                 # PE file (MZ header)
uint32(0) == 0x464C457F             # ELF file
uint32(0) == 0xBEBAFECA             # Mach-O fat binary
filesize < 10MB                     # File size limit

String matching

all of them                         # All strings must match
any of them                         # At least one string
2 of ($text*)                       # At least 2 of $text group
3 of ($s1, $s2, $s3, $s4)          # 3 of these 4 strings
$s1 at 0                            # $s1 at offset 0
$s1 in (0..1024)                    # $s1 in first 1KB
#s1 > 3                             # $s1 occurs more than 3 times

PE module (Windows executables)

import "pe"

pe.number_of_sections > 5
pe.imports("kernel32.dll", "VirtualAlloc")
pe.exports("DllRegisterServer")
pe.is_dll()
pe.timestamp > 1704067200           # Compiled after 2024-01-01

Best Practices

Do

• Start with a PE/ELF/file type check to limit scope
• Add a filesize constraint to avoid scanning large files
• Use unique strings — prefer byte sequences or specific API combinations over generic strings
• Combine multiple indicators (string + structure + behavior)
• Include metadata with reference to source intelligence
• Test against both malicious samples AND clean files (false positive testing)

Don't

• Match on single, common strings (e.g., just "http://" or "cmd.exe")
• Create rules without file type constraints (performance killer)
• Use overly broad regex patterns
• Forget to escape special characters in hex patterns
• Write rules that match on packer/crypter artifacts only (will match unrelated packed software)

Common Malware Patterns

Encoded PowerShell

rule Encoded_PowerShell_Execution {
    meta:
        description = "Detects base64 encoded PowerShell commands"
        mitre_attack = "T1059.001"
    strings:
        $enc1 = "-EncodedCommand" ascii nocase
        $enc2 = "-enc " ascii nocase
        $enc3 = "FromBase64String" ascii nocase
        $b64 = /[A-Za-z0-9+\/]{100,}={0,2}/ ascii
    condition:
        any of ($enc*) and $b64
}

Suspicious PE imports

import "pe"
rule Suspicious_Injection_Imports {
    meta:
        description = "PE with process injection API imports"
        mitre_attack = "T1055"
    condition:
        pe.imports("kernel32.dll", "VirtualAllocEx") and
        pe.imports("kernel32.dll", "WriteProcessMemory") and
        pe.imports("kernel32.dll", "CreateRemoteThread")
}

Naming Convention

• APT_<GroupName>_<MalwareName>.yar — For attributed malware
• MAL_<MalwareName>_<Variant>.yar — For named malware families
• SUSP_<Behavior>_<Context>.yar — For suspicious behavior patterns
• HUNT_<Target>_<Technique>.yar — For threat hunting queries

Output Location

Write YARA rules to: data/detection-rules/yara/<malware-name>.yar

Validation against a real corpus

Before deploying a rule beyond your test set, validate it against a broader sample collection. When ReversingLabs Spectra Analyze is available:

• /lookup-reversinglabs yara-matches <ruleset_name> — for a rule you have already deployed to RL, list the samples it has matched in the continuous-scanning corpus. Useful to confirm the rule isn't firing on goodware and to discover new variants caught by the same logic.
• /lookup-reversinglabs search 'threatname:<family> classification:malicious' --max-results 100 — before writing a family rule, retrieve a candidate sample set to test against. Then pull each sample's hashes for inclusion in your test suite.
• /lookup-reversinglabs containers <hash> and extracted <hash> — find parents/children of a known sample to ensure the rule fires across the dropper-payload chain, not just on one layer.

Without RL access, fall back to retro-hunting via /lookup-virustotal (premium) and /lookup-otx pulse traversal.

Related skills

• /hash-investigation, /malware-analysis — produce the structural and behavioural intel that informs the rule
• /lookup-reversinglabs — corpus validation, family pivots, parent/child relationships
• /lookup-virustotal, /lookup-otx — community detection signals and pulse-driven sample collection
• /sigma-writing — behavioural-detection counterpart for log telemetry