Yara Memory Hunting

YARA Memory Hunting

Maps to MITRE ATT&CK T1059 (Command and Scripting Interpreter) and T1027 (Obfuscated Files or Information) when classifying samples. For rule sources see Elastic protections-artifacts, YARA-Forge, and vendor reports.

When to Use

• You have IoCs (strings, byte patterns, config layouts) and want to confirm / scope
• Triage surfaced a suspect process and you want to classify the family
• Sweeping an image for commodity C2 frameworks (Cobalt Strike, Sliver, Brute Ratel, Metasploit, etc.)

Scoping Decisions First

Whole-image scans are slow and noisy. Use them when you don't yet have a suspect, then narrow to PIDs.

Workflow

1. Pick the rule set

For commodity C2 and known malware, start with community-maintained packs:

• Elastic Protections Artifacts (protections-artifacts/yara/rules/)
• YARA-Forge aggregate
• Vendor-published rules attached to report writeups (Mandiant, Volexity, CrowdStrike blogs)

For custom IoCs derived from triage, write the rule inline — don't manage a file for a one-shot.

2. Run the scan

volatility_yara_scan(image, rules_file="/path/to/rules.yar", pid=None)

or

volatility_yara_scan(image, rules_inline="""
rule MyCustom { strings: $a = "CONFIG:" condition: $a }
""", pid=1234)

Exactly one of rules_file / rules_inline. The pid filter dramatically speeds up scans when you have a suspect.

3. Interpret hits

Each hit gives you: rule name, PID, process, virtual address, matching string(s). Triage each:

• High specificity rule (tight strings, multi-condition) — trust the verdict, move to analysis
• Generic rule (single common string) — corroborate before acting
• Hit on a `csrss`, `services`, `svchost` — elevated priority; system processes don't normally contain arbitrary strings
• Hit on a user-mode browser / IDE / chat app — often benign (strings in page cache, clipboard, logs)

4. Pivot

For each confirmed hit:

• volatility_malfind --pid N to see if the hit sits inside an injected region
• volatility_dll_list --pid N to see what module the offset maps into (if any)
• volatility_dump_process --pid N --mode vad and carve around the hit offset for offline triage
• Derive new IoCs (neighboring strings, config blobs, mutex names) and re-scan

Rule Patterns That Work Well Against Memory

Config-block pattern (Cobalt Strike / Sliver style)

rule CS_Beacon_Config {
  strings:
    $magic = { 2E 2E 2E 2E ?? ?? 2E 2E 2E 2E }   // XOR-key surrounded config
    $sleep = "Beacon_mask" ascii wide
    $post  = "post-ex" ascii wide
  condition:
    2 of them
}

Decoded-in-memory string pattern

Decrypted strings appear in memory even if encrypted on disk. Target what only shows up after unpacking:

rule FamilyX_Runtime_Strings {
  strings:
    $cmd1 = "!@#run_payload@#!" ascii wide
    $cmd2 = "!@#beacon_check@#!" ascii wide
    $err  = "FamX: failed to allocate" ascii wide
  condition:
    any of them
}

API-resolution pattern

Malware that dynamically resolves APIs leaves the names in memory:

rule Injector_API_Resolution {
  strings:
    $a1 = "NtAllocateVirtualMemory"
    $a2 = "NtWriteVirtualMemory"
    $a3 = "NtCreateThreadEx"
    $a4 = "RtlAdjustPrivilege"
  condition:
    3 of them
}

Benign software rarely stores these as plaintext strings (they're in import tables, not data).

Mutex / named-pipe pattern

rule Beacon_NamedPipe {
  strings:
    $p1 = "\\\\.\\pipe\\msagent_" ascii wide
    $p2 = "\\\\.\\pipe\\postex_" ascii wide
    $p3 = "\\\\.\\pipe\\status_" ascii wide
  condition:
    any of them
}

False-Positive Control

• Never rely on single-string rules unless the string is a hash / GUID / long unique phrase
• Pair "would appear in malware" strings with "would not appear in benign software" strings
• Scope by process type: system processes (lsass, services, svchost) have a tiny legitimate string set; hits there are almost always real
• Every hit in a browser, email client, or IDE needs corroboration — those processes scrape the internet into memory and will match almost anything

Timeouts and Budgets

Whole-image YARA is slow. The MCP defaults to 600s; raise timeout for larger images or heavy rule packs. When iterating rules, scope to pid=N first to get fast feedback, then widen.

Common Pitfalls

• Rules with only wide strings miss ASCII-compiled binaries; use ascii wide liberally
• Very long strings may not match if the memory range they land in is paged out — prefer shorter signatures with stronger condition logic
• Vol3 yarascan wants a compiled or source `.yar` file, not a compiled .yarc — pass source