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

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Malicious BlueNoroff C2 Communication via cur1-agent User-Agent (via proxy)
Possible GammaSteel C2 Using Spoofed iPhone User Agent
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Suspicious Interlock Backdoor C2 Beacon to init1234 Endpoint (via proxy)
Possible FakeBat Loader Payload Download and C2 URI Patterns
Suspicious HATVIBE C2 Beacon via HTTP PUT to PHP Endpoints (via proxy)
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.