Sigma rule: HTTP Request With Empty User Agent

A detection rule from the SigmaHQ repository, shown as its author published it. Dataset version ref-f33237777db0, updated 2021-11-27T00:00:00.000Z.

Rule

Title
HTTP Request With Empty User Agent
Status
test
Level
medium
Author
Florian Roth (Nextron Systems)
Date
2017-07-08
Modified
2021-11-27
File
rules/web/proxy_generic/proxy_ua_empty.yml
Commit
ccbb0369507f796db0b0ae6854c8a2be471b9d84

Detects a potentially suspicious empty user agent strings in proxy log.
Could potentially indicate an uncommon request method.

Log source

category
proxy

Detection

selection:
  # Empty string - as used by Powershell's (New-Object Net.WebClient).DownloadString
    c-useragent: ''
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

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_empty.yml

Sources and licences

  • SigmaHQ

    "HTTP Request With Empty User Agent" by Florian Roth (Nextron Systems) (SigmaHQ). Rule: https://github.com/SigmaHQ/sigma/blob/ccbb0369507f796db0b0ae6854c8a2be471b9d84/rules/web/proxy_generic/proxy_ua_empty.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: HTTP Request With Empty User Agent",
entity page, dataset version ref-f33237777db0, updated 2021-11-27T00:00:00.000Z,
https://ransomware-tracker.com/detections/sigma/21e44d78-95e7-421b-a464-ffd8395659c4 (accessed 2026-11-03).
Underlying source: SigmaHQ rule 21e44d78-95e7-421b-a464-ffd8395659c4, 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.