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Published by CyberDudeBivash • Date: Oct 31, 2025 (IST)

Threat Hunting Fail: The 5 Signatures Your SOC Must Deploy to Find Open-Source C2 Activity Now

Miss these 5 detections and open-source C2 (Sliver, Mythic, Covenant/Empire/PoshC2, Merlin, Koadic) will skate past your SIEM. Deploy the rules, then tune with our “reduce FPs” keys.CyberDudeBivash Ecosystem:Apps & Services · Threat Intel (Blogger) · CryptoBivash · News Portal · Subscribe: ThreatWire

TL;DR — The Five Detections

JA3/HTTP Beacon Fingerprints (TLS + URIs + header quirks)
LOLBIN Spawn Chains (Office → script host → living-off-land)
SMB Named Pipes & Service Abuse (lateral move orchestration)
In-Memory Stagers (AMSI/ETW tamper + reflective loaders)
Living-off-Cloud Callbacks (GitHub/GitLab/Notion/Cloud storage as C2)

Signature #1 — JA3/HTTP Beacon Fingerprints

Why it works: Many OSS C2 agents reuse TLS client-hello stacks and deterministic HTTP paths/headers. Combine JA3 + URI/Host + headers + intervals for high fidelity.

# Suricata (concept) — HTTP path quirks often seen in OSS C2 beacons
alert http $HOME_NET any -> $EXTERNAL_NET any (
  msg:"Possible OSS C2 beacon (odd keepalive path & header set)";
  http.uri; content:"/healthz"; nocase; distance:0; within:16;
  http.user_agent; pcre:"/Go-http-client|python-requests|curl\/7\./i";
  flow:to_server,established; classtype:trojan-activity; sid:9001001; rev:1;
)

# Zeek — flag rare JA3 + low-entropy beaconing
# (Implement in Zeek scripting: watch_conn_interval + ja3/ja3s rarity score)

Reduce FPs: Allowlist internal health checks; require interval regularity (±10%) across ≥6 hits; cross-check with rare UA + no Referer.

Signature #2 — LOLBIN Spawn Chains (Office → Script Host → Living-Off-Land)

Why it works: Sliver/Empire/PoshC2 chains commonly pivot from Office/PDF viewer into wscript/cscript/powershell, then into rundll32/regsvr32/mshta.

# Sigma — Office spawning script host and LOLBIN within 30s
title: Office->ScriptHost->LOLBIN Spawn Chain
id: 1c2-oss-lolbin-chain
logsource:
  product: windows
  category: process_creation
detection:
  sel1:
    ParentImage|endswith:
      - '\WINWORD.EXE'
      - '\EXCEL.EXE'
      - '\POWERPNT.EXE'
      - '\AcroRd32.exe'
  sel2:
    Image|endswith:
      - '\wscript.exe'
      - '\cscript.exe'
      - '\powershell.exe'
  sel3:
    CommandLine|contains|all:
      - 'Bypass'   # common, tune
      - 'EncodedCommand'
  timeframe: 30s
  condition: sel1 and sel2 and sel3
level: high

Reduce FPs: Exclude signed IT automation paths (SCCM/Intune/EDR scripts); add user context rule (no admin but spawning LOLBINs).

Signature #3 — SMB Named Pipes & Service Abuse

Why it works: Covenant/Empire/Sliver operators often push binaries and create services remotely, or use distinctive pipes during lateral movement.

# Sigma — Remote service creation from non-admin workstation
title: RemoteServiceCreate from Workstation
id: 1c2-oss-svc-remote
logsource:
  product: windows
  service: security
detection:
  sel:
    EventID: 4697
    ServiceFileName|contains:
      - '\AppData\Local\Temp\'
      - ':\Windows\Temp\'
  filter:
    SubjectUserName|endswith: '$'   # computer accounts
  condition: sel and not filter
level: high

# Zeek/SMB — alert on rare named pipes touched by client hosts (not servers)
# Example: flag new_pipe_name with low global frequency + WRITE_DATA within 5m

Reduce FPs: Allowlist deployment tools; require Temp path + recent SMB file write prior to service create.

Signature #4 — In-Memory Stagers (AMSI/ETW Tamper + Reflective Load)

Why it works: Empire/PoshC2/Sliver tradecraft frequently tampers with AMSI/ETW or uses reflection to load in memory.

# Sigma — Known AMSI/ETW tamper switches (tune for your EDR)
title: AMSI/ETW Tamper Attempt
id: 1c2-oss-amsi-tamper
logsource:
  product: windows
  category: process_creation
detection:
  sel1:
    Image|endswith: '\powershell.exe'
    CommandLine|contains:
      - 'amsiutils'
      - 'AmsiScanBuffer'
      - 'EtwEventWrite'
  sel2:
    CommandLine|contains:
      - 'Add-Type'
      - 'Reflection.Assembly'
  condition: sel1 or (sel1 and sel2)
level: high

Reduce FPs: Exclude signed blue-team scripts; require network egress within 2 minutes to non-allowlisted hosts.

Signature #5 — Living-off-Cloud Callbacks (GitHub/GitLab/Notion/Cloud Storage)

Why it works: OSS C2 frequently hides in plain sight using paste/issue gists, object storage, wikis, or note apps as tasking/dead-drop.

# Suricata — suspicious API pull cadence to public repos with executable MIME drift
alert http $HOME_NET any -> $EXTERNAL_NET any (
  msg:"Possible Living-off-Cloud C2 (public code host periodic pulls)";
  http.host; pcre:"/(api\.github\.com|raw\.githubusercontent\.com|gitlab\.com|pastebin\.com|notion\.so)/";
  http.method; content:"GET";
  file.mime; pcre:"/(application\/octet-stream|text\/plain)/";
  threshold:type both, track by_src, count 6, seconds 420;
  classtype:trojan-activity; sid:9001011; rev:1;
)

# Proxy SIEM — rule of 3s:
# same host → same path prefix → 3xx/200 mix → ~60s jitter ⇒ flag

Reduce FPs: Carve out dev teams’ known repos; require new agent process on the host reaching these endpoints.

Deployment & Tuning (Fast Path)

Stage in “detect-only” for 48h; capture FPs and add allowlists (fleet mgmt, backup, EDR updaters).
Correlate two domains: endpoint + network (e.g., LOLBIN chain + JA3 rarity).
Escalate to “block/quarantine” only when both domains fire within a 5-minute window.
Dashboards: “New Service Create in Temp”, “Beacon Interval Regularity”, “Cloud API Pull Cadence”.
Maintenance: Re-score monthly; expire stale allowlists; rotate thresholds.

Incident Response: First 60 Minutes

Scope quickly: search for same parent hash/pipe/JA3 across 7 days.
Contain: isolate hosts; block egress to matched domains/ASNs; disable newly created services.
Collect: prefetch Windows event logs 4688/4697/7045, Prefetch, Shimcache, Zeek conn/http, Suricata eve.json.
Remediate: remove autoruns, rotate creds, push EDR full scan; review GPO/Intune baselines for script hosts.

FAQ

Why not just IOC feeds?

OSS C2 rotates infra rapidly. Behavioral signatures (spawn chains, intervals, named pipes) outlast single IP/domain IOCs.

Will these block legit IT tasks?

They can. Start in detect-only, add allowlists for IT automation, then add correlation (process + network) before blocking.

Which frameworks benefit most?

Sliver/Mythic/Covenant/Empire/PoshC2/Merlin/Koadic patterns are most impacted by these five families of detections.

CyberDudeBivash — Services, Apps & Ecosystem

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Detection Engineering (Sigma → SIEM, Suricata/Zeek → NDR, EDR custom rules)
IR Retainers (containment runbooks, purple-team simulations, hardening)

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