Potential MFA Bypass Using Legacy Client Authentication

Status: test
Severity: high
Log source: product azure, service signinlogs
Author: Harjot Singh, '@cyb3rjy0t'
Source: github.com/SigmaHQ/sigma

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.

Known false positives

Known Legacy Accounts

MITRE ATT&CK coverage

Tactic	Techniques
Initial Access	`T1078.004` Valid Accounts: Cloud Accounts
Persistence	`T1078.004` Valid Accounts: Cloud Accounts
Privilege Escalation	`T1078.004` Valid Accounts: Cloud Accounts
Stealth	`T1078.004` Valid Accounts: Cloud Accounts
Credential Access	`T1110` Brute Force

Telemetry coverage

Platform	Record / event type
Entra ID	`SigninLogs` sign-in event: Successful sign-in

Rule body

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

Stages and Predicates

Stage 0: `condition`

selection

Stage 1: `selection`

selection:
    Status: 'Success'
    userAgent|contains:
        - 'BAV2ROPC'
        - 'CBAinPROD'
        - 'CBAinTAR'

Indicators

These rows show field, operator, and value matches.

Field	Kind	Values	Search
`Status`	eq	`Success`	`field:"Status" kind:eq value:"Success"`
`userAgent`	match	`BAV2ROPC` `CBAinPROD` `CBAinTAR`	`field:"userAgent" kind:match`