Potential MFA Bypass Using Legacy Client Authentication — Sigma Rule

Rule metadata and linked tags

Author: Harjot Singh, '@cyb3rjy0t'. Source status: test; severity: high. Source dates: 2023-03-20 / not supplied.

{
  "product": "azure",
  "service": "signinlogs"
}

Pinned original Sigma rule · Detection Rule License 1.1

Source SHA-256: ddb6668f7a6b2c5549c163d98efc020aa981b2ad058d2def4cba9d338438d09f

Detection logic

Original source YAML. Source-tag agreement is not proof of complete semantic coverage. This rule has not been compiled for a SIEM backend or validated against live telemetry here.

title: Potential MFA Bypass Using Legacy Client Authentication
id: 53bb4f7f-48a8-4475-ac30-5a82ddfdf6fc
status: test
description: Detects successful authentication from potential clients using legacy authentication via user agent strings. This could be a sign of MFA bypass using a password spray attack.
references:
    - https://web.archive.org/web/20230217071802/https://blooteem.com/march-2022
    - https://www.microsoft.com/en-us/security/blog/2021/10/26/protect-your-business-from-password-sprays-with-microsoft-dart-recommendations/
author: Harjot Singh, '@cyb3rjy0t'
date: 2023-03-20
tags:
    - attack.privilege-escalation
    - attack.persistence
    - attack.initial-access
    - attack.credential-access
    - attack.stealth
    - attack.t1078.004
    - attack.t1110
logsource:
    product: azure
    service: signinlogs
detection:
    selection:
        Status: 'Success'
        userAgent|contains:
            - 'BAV2ROPC'
            - 'CBAinPROD'
            - 'CBAinTAR'
    condition: selection
falsepositives:
    - Known Legacy Accounts
level: high

Original YAML and metadata in JSON

False positives

• Known Legacy Accounts

Source references

• https://web.archive.org/web/20230217071802/https://blooteem.com/march-2022
• https://www.microsoft.com/en-us/security/blog/2021/10/26/protect-your-business-from-password-sprays-with-microsoft-dart-recommendations/