Azure AD Authentication Failed During MFA Challenge

name: Azure AD Authentication Failed During MFA Challenge
id: e62c9c2e-bf51-4719-906c-3074618fcc1c
version: 13
creation_date: '2022-07-14'
modification_date: '2026-05-13'
author: Mauricio Velazco, Gowthamaraj Rajendran, Splunk, 0xC0FFEEEE
status: production
type: TTP
description: The following analytic identifies failed authentication attempts against an Azure AD tenant during the Multi-Factor Authentication (MFA) challenge, specifically flagged by error code 500121. It leverages Azure AD SignInLogs to detect these events. This activity is significant as it may indicate an adversary attempting to authenticate using compromised credentials on an account with MFA enabled. If confirmed malicious, this could suggest an ongoing effort to bypass MFA protections, potentially leading to unauthorized access and further compromise of the affected account.
data_source:
    - Azure Active Directory
search: "`azure_monitor_aad` category=SignInLogs properties.status.errorCode=500121 | rename properties.* as *, authenticationDetails{}.* as * | eval time=strptime(authenticationStepDateTime,\"%Y-%m-%dT%H:%M:%S\") | eval auth_detail=mvzip(strftime(time, \"%Y-%m-%dT%H:%M:%S\"),authenticationStepResultDetail,\" - \"), auth_msg=mvappend('status.additionalDetails', authenticationStepResultDetail) | eval auth_method=mvmap(authenticationMethod, if(isnull(mvfind('mfaDetail.authMethod',authenticationMethod)), authenticationMethod, null())) | search NOT auth_msg=\"MFA successfully completed\" | rename userAgent as user_agent | fillnull | stats count min(_time) as firstTime max(_time) as lastTime by dest user src vendor_account vendor_product auth_method auth_msg user_agent signature | `security_content_ctime(firstTime)` | `security_content_ctime(lastTime)` | `azure_ad_authentication_failed_during_mfa_challenge_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 SignInLogs log category.
known_false_positives: "False positives have been minimized by removing attempts that result in 'MFA successfully completed messages', which were found to be generated when a user opts to use a different MFA method than the default.\nFurther reductions in finding events can be achieved through filtering 'MFA denied; duplicate authentication attempt' messages within the auth_msg field, as they could arguably be considered as false positives."
references:
    - https://attack.mitre.org/techniques/T1621/
    - https://attack.mitre.org/techniques/T1078/004/
    - https://docs.microsoft.com/en-us/azure/active-directory/authentication/concept-mfa-howitworks
    - https://learn.microsoft.com/en-us/entra/identity/monitoring-health/concept-sign-in-log-activity-details
finding:
    title: User $user$ failed to pass MFA challenge
    entity:
        field: user
        type: user
        score: 50
threat_objects:
    - field: src
      type: ip_address
analytic_story:
    - Azure Active Directory Account Takeover
asset_type: Azure Active Directory
mitre_attack_id:
    - T1078.004
    - T1586.003
    - T1621
product:
    - Splunk Enterprise
    - Splunk Enterprise Security
    - Splunk Cloud
category: cloud
security_domain: identity