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

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Suspicious WarmCookie C2 Beacon With Fixed Firefox User-Agent
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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.