title: Suspicious Base64 User Agent
id: 894a8613-cf12-48b3-8e57-9085f54aa0c3
status: experimental
description: Detects suspicious User Agent strings that end with an equal sign, which can be a sign of base64 encoded values used as User Agent string
references:
    - https://blogs.jpcert.or.jp/en/2022/07/yamabot.html
author: Florian Roth (Nextron Systems)
date: 2022/07/08
modified: 2022/11/27
tags:
    - attack.command_and_control
    - attack.t1071.001
logsource:
    category: proxy
detection:
    selection:
        c-useragent|endswith:
            - '='
            - 'TW96aWxsY'  # base64 encoded Mozilla/ as used by YamaBot
    condition: selection
fields:
    - ClientIP
    - c-uri
    - c-useragent
falsepositives:
    - Unknown
level: high