huntrule

Suspicious Malformed User-Agent Strings in Proxy Logs

Flags proxy requests whose User-Agent headers are malformed or match suspicious automation/tooling patterns, excluding known Adobe/Acrobat traffic.

FreeReviewedSigma · High · v2
Category
proxy
Author
Florian Roth (Nextron Systems) (SigmaHQ), DRL 1.1
Published
2017-07-08
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 requests with malformed or suspicious User-Agent header values, including misspelled or incorrectly formatted browser identifiers and known suspicious substrings. Attackers may use abnormal User-Agent strings to blend in, evade simple filtering, or tag automated tooling for command-and-control behavior. It relies on proxy telemetry containing the client User-Agent field (c-useragent) to match specific malformed patterns while excluding specified Adobe/Acrobat-related false positives.

Reporting behind it

Related detections9 linkedT1071.001 — drag to rearrange
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Malicious VoidLink Kernel Rootkit C2 Endpoints via HTTP (via proxy)
Malicious EtherRAT C2 Exfiltration Endpoints via HTTP (via proxy)
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)
Possible GammaSteel C2 Using Spoofed iPhone User Agent
Suspicious PolarEdge Backdoor C2 URL with Module and putdata Command (via proxy)
Suspicious Malformed 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.