Sigma rule: Potential Base64 Encoded User-Agent

A detection rule from the SigmaHQ repository, shown as its author published it. Dataset version ref-20fdff879caa, updated 2023-05-04T00:00:00.000Z.

Rule

Title
Potential Base64 Encoded User-Agent
Status
test
Level
medium
Author
Florian Roth (Nextron Systems), Brian Ingram (update)
Date
2022-07-08
Modified
2023-05-04
File
rules/web/proxy_generic/proxy_ua_susp_base64.yml
Commit
ccbb0369507f796db0b0ae6854c8a2be471b9d84

Detects User Agent strings that end with an equal sign, which can be a sign of base64 encoding.

Log source

category
proxy

Detection

selection:
    c-useragent|endswith: '='
condition: selection

False positives (as the rule lists them)

  • Unknown

Tags

  • attack.command-and-control
  • attack.t1071.001

MITRE ATT&CK ids this rule names

Related rules

References

Read on 2026-11-03T00:58:21.399Z under licence sigma-drl-1.1@d5f4c36a from https://github.com/SigmaHQ/sigma/blob/ccbb0369507f796db0b0ae6854c8a2be471b9d84/rules/web/proxy_generic/proxy_ua_susp_base64.yml

Sources and licences

  • SigmaHQ

    "Potential Base64 Encoded User-Agent" by Florian Roth (Nextron Systems), Brian Ingram (update) (SigmaHQ). Rule: https://github.com/SigmaHQ/sigma/blob/ccbb0369507f796db0b0ae6854c8a2be471b9d84/rules/web/proxy_generic/proxy_ua_susp_base64.yml.
    Licensed under the Detection Rule License (DRL) 1.1: https://github.com/SigmaHQ/Detection-Rule-License

    Detection Rule License (DRL) 1.1

Cite this page

Dragons Eye Ransomware Tracker, "Sigma rule: Potential Base64 Encoded User-Agent",
entity page, dataset version ref-20fdff879caa, updated 2023-05-04T00:00:00.000Z,
https://ransomware-tracker.com/detections/sigma/894a8613-cf12-48b3-8e57-9085f54aa0c3 (accessed 2026-11-03).
Underlying source: SigmaHQ rule 894a8613-cf12-48b3-8e57-9085f54aa0c3, commit ccbb0369507f796db0b0ae6854c8a2be471b9d84.

Pages change. The dataset version identifies the data, but no archive of old versions is kept, so this citation cannot be replayed later.