LoFP LoFP / 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.

Techniques

Sample rules

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.

Detection logic

`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())) 
| 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 
| `security_content_ctime(firstTime)` 
| `security_content_ctime(lastTime)` 
| search NOT auth_msg="MFA successfully completed" 
| sort 0 - firstTime 
| `azure_ad_authentication_failed_during_mfa_challenge_filter`