Suspicious Malformed User-Agent Strings in Proxy Logs

ATT&CK techniques

C2

Recon
Resource Dev
Initial Access
Execution
Persistence
Priv Esc
Defense Evasion
Cred Access
Discovery
Lateral Movement
Collection
C2
- T1071.001Web Protocols
Exfiltration
Impact

What it detects

This rule flags proxy requests with malformed or suspicious User-Agent header values, including misspelled or incorrectly formatted browser identifiers and known suspicious substrings. Attackers may use abnormal User-Agent strings to blend in, evade simple filtering, or tag automated tooling for command-and-control behavior. It relies on proxy telemetry containing the client User-Agent field (c-useragent) to match specific malformed patterns while excluding specified Adobe/Acrobat-related false positives.

Reporting behind it

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Suspicious Malformed User-Agent Strings in Proxy Logs

Pivot detection · T1071.001 · 9 related

Changelog

v2

v2
Candidate ingested via manual entry.2026-07-31
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.

suspicious-and-malformed-user-agent-strings-in-proxy-logs-7195a772

title: Suspicious Malformed User-Agent Strings in Proxy Logs
id: 5e31a491-6a12-4f9f-98b4-6d410ba321aa
status: test
description: This rule flags proxy requests with malformed or suspicious User-Agent header values, including misspelled or incorrectly formatted browser identifiers and known suspicious substrings. Attackers may use abnormal User-Agent strings to blend in, evade simple filtering, or tag automated tooling for command-and-control behavior. It relies on proxy telemetry containing the client User-Agent field (c-useragent) to match specific malformed patterns while excluding specified Adobe/Acrobat-related false positives.
references:
  - https://github.com/fastly/waf_testbed/blob/8bfc406551f3045e418cbaad7596cff8da331dfc/templates/default/scanners-user-agents.data.erb
  - https://github.com/SigmaHQ/sigma/blob/master/rules/web/proxy_generic/proxy_ua_susp.yml
author: Florian Roth (Nextron Systems), Huntrule Team
date: 2017-07-08
modified: 2022-10-31
tags:
  - attack.command-and-control
  - attack.t1071.001
logsource:
  category: proxy
detection:
  selection1:
    c-useragent|startswith:
      - user-agent
      - "Mozilla/3.0 "
      - "Mozilla/2.0 "
      - "Mozilla/1.0 "
      - "Mozilla "
      - " Mozilla/"
      - Mozila/
      - Mozilla/4.0 (compatible; MSIE 6.0; MS Web Services Client Protocol
  selection2:
    c-useragent|contains:
      - " (compatible;MSIE "
      - ".0;Windows NT "
      - loader
  selection3:
    c-useragent:
      - _
      - CertUtil URL Agent
      - Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:60.0)
      - Mozilla/5.0 (Windows NT 6.3; WOW64; rv:28.0) Gecko/20100101 Firefox/28.0
      - HTTPS
      - Erbium-UA-4ce7c27cb4be9d32e333bf032c88235a
      - x
      - xxx
  falsepositives:
    - c-useragent: Mozilla/3.0 * Acrobat *
    - cs-host|endswith:
        - .acrobat.com
        - .adobe.com
        - .adobe.io
  condition: 1 of selection* and not falsepositives
falsepositives:
  - Unknown
level: high
license: DRL-1.1
related:
  - id: 7195a772-4b3f-43a4-a210-6a003d65caa1
    type: derived

Convert

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