huntrule

Proxy Log User-Agent Ending with '=' Suggesting Base64 Encoding

Alerts on proxy requests with User-Agent values ending in '=' that may indicate Base64-encoded content.

FreeReviewedSigma · Medium · v2
Category
proxy
Author
Florian Roth (Nextron Systems), Brian Ingram (update) (SigmaHQ), DRL 1.1
Published
2022-07-08
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 where the User-Agent field ends with an equals sign, a common base64 padding character. Attackers may use base64-encoded identifiers or payload markers within User-Agent to obfuscate command-and-control related data. It relies on proxy telemetry capturing the full User-Agent string and evaluates only the trailing character pattern.

Reporting behind it

Related detections9 linkedT1071.001 — drag to rearrange
Malicious HarborWatch RAT Command and Control Beacon by User Agent (via proxy)
Suspicious UAT-10608 Credential Harvesting C2 Beacon via HTTP
Malicious BadIIS C2 Communication via lwxatisme User-Agent
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 Log User-Agent Ending with '=' Suggesting Base64 Encoding
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.