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MediumSplunk security contentMauricio Velazco +3

Azure AD Authentication Failed During MFA Challenge

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…

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

Detection requirements

Platform
IaaSIdentity ProviderLinuxmacOSOffice SuitePRESaaSWindows

The rule states no platform. This is derived from the 3 ATT&CK techniques it maps to.

Log source category
azure_active_directory

Known benign triggers

  • 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 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.

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