Reverser Yara Hunting

YARA Rule Authoring + Hunting

Anatomy of a good rule

import "pe"
import "hash"

rule MalFamily_DropperVariant_Q4_2024 {
    meta:
        author = "you"
        date = "2024-11-20"
        description = "MalFamily dropper, observed Q4 2024 campaign"
        hash = "deadbeefdeadbeefdeadbeefdeadbeef"
        tlp = "amber"
        confidence = "high"
        false_positives = "low (test against AV vendor samples)"
        mitre = "T1055"

    strings:
        $magic = { 4D 5A 90 00 03 }            // MZ + DOS header
        $config_marker = { 7A 89 ?? ?? 4F 8C } // 6-byte signature, 2 wildcards
        $string1 = "loader_stage_2" wide
        $string2 = "cmd.exe /c " ascii
        $api1   = "VirtualAllocEx"
        $api2   = "WriteProcessMemory"
        $api3   = "CreateRemoteThread"
        // Avoid generic strings — they cause FPs.

    condition:
        uint16(0) == 0x5A4D and                    // PE header
        filesize < 5MB and                          // bounded — avoids huge-file scans
        $config_marker and                          // unique marker
        2 of ($api1, $api2, $api3) and             // at least 2 of the 3 process-inject APIs
        any of ($string*) and
        pe.imports("ws2_32.dll", "WSAStartup") and  // network capability
        not pe.is_signed                            // unsigned
}

The condition tools you need

// Filesize / offset reads (no module)
filesize > 100KB and filesize < 5MB
uint32(0x3c) > 0 and uint32(uint32(0x3c)) == 0x4550  // valid PE

// "for" loops over offsets / sections
for any i in (0..pe.number_of_sections - 1):
    (pe.sections[i].name == ".text" and pe.sections[i].entropy > 7.0)

// "of"
1 of them                              // any one string
all of ($a*)                           // all $a*-prefix strings
3 of ($s1, $s2, $s3, $s4, $s5)         // at least 3 of 5

// Hash module
hash.sha256(0, filesize) == "..."      // exact-match rule
hash.imphash() == "..."                // PE import hash (fragile but high-signal)

High-leverage patterns

Family-stable byte patterns

Look for unique byte sequences that survive obfuscation:

• Crypto constants (SHA-256 IVs, AES sbox, RC4 init pattern)
• Hard-coded SID strings
• C2 packet headers (magic bytes, fixed offsets)
• Reflective loader entry stub bytes (Donut, Cobalt Strike's PE loader)

Capability rules

Don't fingerprint a sample — fingerprint a TECHNIQUE:

rule Suspicious_ProcessInjection {
    meta: description = "VirtualAlloc + WriteProcessMemory + CreateRemoteThread = injection"
    condition:
        pe.imports("kernel32.dll", "VirtualAllocEx") and
        pe.imports("kernel32.dll", "WriteProcessMemory") and
        pe.imports("kernel32.dll", "CreateRemoteThread")
}

This catches every injector regardless of family. False-positive: legitimate tools (Procmon, IDA, debuggers). Reduce via additional conditions (unsigned, in temp dir, etc.).

Anti-analysis fingerprinting

rule AntiVm_VMware_Strings {
    strings:
        $s1 = "VMware" ascii nocase
        $s2 = "VBoxService" ascii nocase
        $s3 = "qemu" ascii nocase
        $s4 = { 56 4D 58 68 } // "VMXh" — VMware backdoor port magic
    condition: 2 of them
}

Hunting at scale

VirusTotal Retrohunt (paid)

// Upload rule to VT — runs against the past 90 days of submissions
// Outputs new hashes matching the signature

MalwareBazaar (free)

# yarafs.io / abuse.ch — runs YARA against MB corpus
curl https://mb-api.abuse.ch/api/v1/ -X POST -d 'query=get_yara&yara_rule=YourRule'

Local corpus scan

# Recursive scan
yara -r rules/ /samples/
# JSON output for downstream processing
yara -r rules/ /samples/ -m -p 4    # -m: metadata, -p: parallel threads

# Fast pre-filter with capa first
capa /samples/sample.exe   # extracts capabilities; rule-driven, JSON-output

Common pitfalls

Mass-hunt workflow

1. Triage a sample (see reverser/malware-triage skill)
2. Extract unique byte patterns, strings, capabilities
3. Write a YARA rule with meta for context + bounded condition
4. Test against:
   • The known sample (must match)
   • A clean corpus (must NOT match — false positive rate)
   • A different malware family corpus (must NOT match)
5. Submit to VT Retrohunt + MalwareBazaar
6. Track results, iterate

Tooling

• yara / yara-python (writing + running)
• yarGen (auto-generate rules from a sample set)
• valhalla.nextron-systems.com (commercial YARA feed by Florian Roth)
• Neo23x0/signature-base (open-source canonical rule set)
• capa (Mandiant — capability-level rules)

References

• VirusTotal YARA guide
• "YARA, the pattern matching swiss knife for malware researchers"
• Florian Roth's blog (the canonical YARA author of our era)
• "Practical YARA Rules" — Andre Tavares