Sigma rule: Suspicious Base64 Encoded User-Agent

A detection rule from the SigmaHQ repository, shown as its author published it. Dataset version ref-af81c87b8420, updated 2023-05-04T00:00:00.000Z.

Rule

Title
Suspicious Base64 Encoded User-Agent
Status
test
Level
medium
Author
Nasreddine Bencherchali (Nextron Systems)
Date
2023-05-04
File
rules/web/proxy_generic/proxy_ua_base64_encoded.yml
Commit
ccbb0369507f796db0b0ae6854c8a2be471b9d84

Detects suspicious encoded User-Agent strings, as seen used by some malware.

Log source

category
proxy

Detection

selection:
    c-useragent|startswith:
        - 'Q2hyb21l' # Chrome Encoded with offset to not include padding
        - 'QXBwbGVXZWJLaX' # AppleWebKit Encoded with offset to not include padding
        - 'RGFsdmlr' # Dalvik Encoded with offset to not include padding
        - 'TW96aWxsY'  # Mozilla Encoded with offset to not include padding (as used by YamaBot)
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_base64_encoded.yml

Sources and licences

  • SigmaHQ

    "Suspicious Base64 Encoded User-Agent" by Nasreddine Bencherchali (Nextron Systems) (SigmaHQ). Rule: https://github.com/SigmaHQ/sigma/blob/ccbb0369507f796db0b0ae6854c8a2be471b9d84/rules/web/proxy_generic/proxy_ua_base64_encoded.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: Suspicious Base64 Encoded User-Agent",
entity page, dataset version ref-af81c87b8420, updated 2023-05-04T00:00:00.000Z,
https://ransomware-tracker.com/detections/sigma/d443095b-a221-4957-a2c4-cd1756c9b747 (accessed 2026-11-03).
Underlying source: SigmaHQ rule d443095b-a221-4957-a2c4-cd1756c9b747, 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.