huntrule

Proxy User-Agent starts with Base64-like prefixes associated with encoded client strings

Identifies proxy requests with User-Agent values starting with known Base64-encoded prefixes.

FreeReviewedSigma · Medium · v2
Category
proxy
Author
Nasreddine Bencherchali (Nextron Systems) (SigmaHQ), DRL 1.1
Published
2023-05-04
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 HTTP proxy logs where the User-Agent header starts with specific Base64-like prefixes that indicate an encoded client identity. Attackers and malware may use encoded User-Agent values to evade simple filtering, blend into traffic patterns, or obscure their tooling. Detection relies on proxy telemetry capturing the User-Agent field and matching the specified starting substrings.

Reporting behind it

Related detections9 linkedT1071.001 — drag to rearrange
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Possible PS1Bot C2 Beacon With Drive Serial URI Pattern (via proxy)
Suspicious WarmCookie C2 Beacon With Fixed Firefox User-Agent
Malicious Renamed Python Interpreter Executing nvidia.py RAT
Malicious Self-Signed TLS Certificate With Exploit Subject via Zeek X509
Suspicious Network Connection From wabmig.exe (Turian Injection)
Suspicious wmicodegen.dll Sideload by inetinfo.exe (UAT-8099 Cobalt Strike)
Proxy User-Agent starts with Base64-like prefixes associated with encoded client strings
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.