huntrule

Possible Base64 Encoded User-Agent (via proxy)

This rule detects User Agent strings that end with an equal sign, that can be a sign of base64 encoding.

SigmamediumWebv1
sigma
title: Possible Base64 Encoded User-Agent (via proxy)
id: 48c48d7b-9612-563c-b726-77c725456899
status: stable
description: This rule detects User Agent strings that end with an equal sign, that can be a sign of base64 encoding.
references:
    - https://attack.mitre.org/techniques/T1071/001/
    - https://blogs.jpcert.or.jp/en/2022/07/yamabot.html
    - https://deviceatlas.com/blog/list-of-user-agent-strings#desktop
author: Huntrule Team
date: 2026-06-30
tags:
    - attack.command-and-control
    - attack.t1071.001
logsource:
    category: proxy
detection:
    selection:
        c-useragent|endswith: '='
    condition: selection
falsepositives:
    - Unknown
level: medium

Known false positives

  • Unknown

Detection content is published as a reviewed draft. Tune thresholds and exclusions against your own telemetry before enabling this rule for alerting.