huntrule

Suspicious APT User-Agent Strings in Proxy Logs

Alerts when proxy requests contain known APT-style user agent strings indicating likely malicious client behavior.

FreeReviewedSigma · High · v2
Category
proxy
Author
Florian Roth (Nextron Systems), Markus Neis (SigmaHQ), DRL 1.1
Published
2019-11-12
Updated
2026-07-31
AnalysisDetection

ATT&CK techniques

C2

  1. Recon

  2. Resource Dev

  3. Initial Access

  4. Execution

  5. Persistence

  6. Priv Esc

  7. Defense Evasion

  8. Cred Access

  9. Discovery

  10. Lateral Movement

  11. Collection

  12. C2

  13. Exfiltration

  14. Impact

What it detects

This rule flags proxy-log requests whose User-Agent strings match a set of suspicious, APT-associated values. Attackers often disguise automated malware traffic by spoofing legacy or unusual browser identifiers to blend in with normal web activity. It relies on proxy telemetry containing the client User-Agent field and matches it against the predefined suspicious strings.

Reporting behind it

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Possible PlugX and NPS Proxy Command and Control Domain Lookup
Possible Backdoor Mistic Command and Control Domain Lookup
Suspicious Weaver Ant In-Memory Web Shell Marker in Web Request (via webserver)
Malicious BlueNoroff C2 Communication via cur1-agent User-Agent (via proxy)
Suspicious APT User-Agent Strings in Proxy Logs
Pivot detection · T1071.001 · 9 related

Changelog

v2
  1. v2
    Candidate ingested via manual entry.2026-07-31
  2. v1
    No changelog recorded for this version.2026-07-30

Detection content is published as a reviewed draft. Tune thresholds and exclusions against your own telemetry before enabling this rule for alerting.