Detection: Azure AD Authentication Failed During MFA Challenge

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.

Search

 1`azure_monitor_aad` category=SignInLogs properties.status.errorCode=500121 
 2| rename properties.* as *, authenticationDetails{}.* as * 
 3| eval time=strptime(authenticationStepDateTime, "%Y-%m-%dT%H:%M:%S") 
 4| eval auth_detail=mvzip(strftime(time, "%Y-%m-%dT%H:%M:%S"),authenticationStepResultDetail," - "), auth_msg=mvappend('status.additionalDetails', authenticationStepResultDetail) 
 5| eval auth_method=mvmap(authenticationMethod, if(isnull(mvfind('mfaDetail.authMethod', authenticationMethod)), authenticationMethod, null())) 
 6| stats min(_time) as firstTime, max(_time) as lastTime, values(user) as user, values(src_ip) as src_ip, values(mfaDetail.authMethod) as mfa_method, values(auth_method) as auth_method, values(auth_detail) as auth_detail, values(auth_msg) as auth_msg, values(appDisplayName) as appDisplayName, values(user_agent) as user_agent by tenantId originalRequestId 
 7| `security_content_ctime(firstTime)` 
 8| `security_content_ctime(lastTime)` 
 9| search NOT auth_msg="MFA successfully completed" 
10| sort 0 - firstTime 
11| `azure_ad_authentication_failed_during_mfa_challenge_filter`

Data Source

Name	Platform	Sourcetype	Source
Azure Active Directory	Azure	'azure:monitor:aad'	'Azure AD'

Macros Used

Annotations

Default Configuration

This detection is configured by default in Splunk Enterprise Security to run with the following settings:

Implementation

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. Further reductions in notable 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.

Associated Analytic Story

• Azure Active Directory Account Takeover

Risk Based Analytics (RBA)

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

Detection Testing

Replay any dataset to Splunk Enterprise by using our replay.py tool or the UI. Alternatively you can replay a dataset into a Splunk Attack Range

Source: GitHub | Version: 6