Azure AD Multiple Denied MFA Requests For User

name: Azure AD Multiple Denied MFA Requests For User
id: d0895c20-de71-4fd2-b56c-3fcdb888eba1
version: 13
creation_date: '2023-11-16'
modification_date: '2026-05-13'
author: Mauricio Velazco, Splunk
status: production
type: TTP
description: The following analytic detects an unusually high number of denied Multi-Factor Authentication (MFA) requests for a single user within a 10-minute window, specifically when more than nine MFA prompts are declined. It leverages Azure Active Directory (Azure AD) sign-in logs, focusing on "Sign-in activity" events with error code 500121 and additional details indicating "MFA denied; user declined the authentication." This behavior is significant as it may indicate a targeted attack or account compromise attempt, with the user actively declining unauthorized access. If confirmed malicious, it could lead to data exfiltration, lateral movement, or further malicious activities.
data_source:
    - Azure Active Directory Sign-in activity
search: |-
    `azure_monitor_aad` category=SignInLogs operationName="Sign-in activity"
      | rename properties.* as *
      | search status.errorCode=500121 status.additionalDetails="MFA denied; user declined the authentication"
      | bucket span=10m _time
      | rename userAgent as user_agent
      | fillnull
      | stats count min(_time) as firstTime max(_time) as lastTime values(dest) as dest values(user_agent) as user_agent values(src) as src
        BY user status.additionalDetails vendor_account
           vendor_product signature _time
      | where count > 9
      | `security_content_ctime(firstTime)`
      | `security_content_ctime(lastTime)`
      | `azure_ad_multiple_denied_mfa_requests_for_user_filter`
how_to_implement: You must install the latest version of Splunk Add-on for Microsoft Cloud Services from Splunkbase (https://splunkbase.splunk.com/app/3110/#/details). You must be ingesting Azure Active Directory events into your Splunk environment through an EventHub. This analytic was written to be used with the azure:monitor:aad sourcetype leveraging the Signin log category.
known_false_positives: Multiple denifed MFA requests in a short period of span may also be a sign of authentication errors. Investigate and filter as needed.
references:
    - https://www.mandiant.com/resources/blog/russian-targeting-gov-business
    - https://arstechnica.com/information-technology/2022/03/lapsus-and-solar-winds-hackers-both-use-the-same-old-trick-to-bypass-mfa/
    - https://therecord.media/russian-hackers-bypass-2fa-by-annoying-victims-with-repeated-push-notifications/
    - https://attack.mitre.org/techniques/T1621/
    - https://attack.mitre.org/techniques/T1078/004/
    - https://www.cisa.gov/sites/default/files/publications/fact-sheet-implement-number-matching-in-mfa-applications-508c.pdf
finding:
    title: User $user$ denied more than 9 MFA requests in a timespan of 10 minutes.
    entity:
        field: user
        type: user
        score: 50
analytic_story:
    - Azure Active Directory Account Takeover
asset_type: Azure Active Directory
mitre_attack_id:
    - T1621
product:
    - Splunk Enterprise
    - Splunk Enterprise Security
    - Splunk Cloud
category: cloud
security_domain: identity