Sigma rule: Suspicious User Agent

A detection rule from the SigmaHQ repository, shown as its author published it. Dataset version ref-bb9c81410872, updated 2022-10-31T00:00:00.000Z.

Rule

Title
Suspicious User Agent
Status
test
Level
high
Author
Florian Roth (Nextron Systems)
Date
2017-07-08
Modified
2022-10-31
File
rules/web/proxy_generic/proxy_ua_susp.yml
Commit
ccbb0369507f796db0b0ae6854c8a2be471b9d84

Detects suspicious malformed user agent strings in proxy logs

Log source

category
proxy

Detection

selection1:
    c-useragent|startswith:
        - 'user-agent'  # User-Agent: User-Agent:
        - 'Mozilla/3.0 '
        - 'Mozilla/2.0 '
        - 'Mozilla/1.0 '
        - 'Mozilla '  # missing slash
        - ' Mozilla/'  # leading space
        - 'Mozila/'  # single 'l'
        - 'Mozilla/4.0 (compatible; MSIE 6.0; MS Web Services Client Protocol'  # https://twitter.com/NtSetDefault/status/1303643299509567488
selection2:
    c-useragent|contains:
        - ' (compatible;MSIE '  # typical typo - missing space
        - '.0;Windows NT '  # typical typo - missing space
        - 'loader'  # https://twitter.com/securityonion/status/1522614635152744453?s=20&t=gHyPTSq5A27EqKwrCd9ohg
selection3:
    c-useragent:
        - '_'
        - 'CertUtil URL Agent'  # https://twitter.com/stvemillertime/status/985150675527974912
        - 'Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:60.0)'  # CobaltStrike Beacon https://unit42.paloaltonetworks.com/tracking-oceanlotus-new-downloader-kerrdown/
        - 'Mozilla/5.0 (Windows NT 6.3; WOW64; rv:28.0) Gecko/20100101 Firefox/28.0'  # used by APT28 malware https://threatvector.cylance.com/en_us/home/inside-the-apt28-dll-backdoor-blitz.html
        - 'HTTPS'  # https://twitter.com/stvemillertime/status/1204437531632250880
        - 'Erbium-UA-4ce7c27cb4be9d32e333bf032c88235a'  # https://www.cyfirma.com/outofband/erbium-stealer-malware-report
        - 'x'  # Use by Racoon Stealer but could be something else
        - 'xxx'  # Use by Racoon Stealer but could be something else
falsepositives:
    - c-useragent: 'Mozilla/3.0 * Acrobat *'  # Acrobat with linked content
    - cs-host|endswith: # Adobe product traffic, example: Mozilla/3.0 (compatible; Adobe Synchronizer 10.12.20000)
          - '.acrobat.com'
          - '.adobe.com'
          - '.adobe.io'
condition: 1 of selection* and not falsepositives

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_susp.yml

Sources and licences

  • SigmaHQ

    "Suspicious User Agent" by Florian Roth (Nextron Systems) (SigmaHQ). Rule: https://github.com/SigmaHQ/sigma/blob/ccbb0369507f796db0b0ae6854c8a2be471b9d84/rules/web/proxy_generic/proxy_ua_susp.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 User Agent",
entity page, dataset version ref-bb9c81410872, updated 2022-10-31T00:00:00.000Z,
https://ransomware-tracker.com/detections/sigma/7195a772-4b3f-43a4-a210-6a003d65caa1 (accessed 2026-11-03).
Underlying source: SigmaHQ rule 7195a772-4b3f-43a4-a210-6a003d65caa1, 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.