The following analytic identifies multiple failed multi-factor authentication requests for a single user within an Azure AD tenant. Error Code 500121 represents a failed attempt to authenticate using a second factor. Specifically, the analytic triggers when more than 10 MFA user prompts fail within 10 minutes. Azure AD tenants can be very different depending on the organization, Security teams should test this detection and customize these arbitrary thresholds. The detected behavior may represent an adversary who has obtained legitimate credentials for a user and continuously repeats login attempts in order to bombard users with MFA push notifications, SMS messages, and phone calls potentially resulting in the user finally accepting the authentication request. Threat actors like the Lapsus team and APT29 have leveraged this technique to bypass multi-factor authentication controls as reported by Mandiant and others.
- Type: Anomaly
Product: Splunk Enterprise, Splunk Enterprise Security, Splunk Cloud
- Last Updated: 2022-08-25
- Author: Mauricio Velazco, Splunk
- ID: 264ea131-ab1f-41b8-90e0-33ad1a1888ea
Kill Chain Phase
- CIS 3
- CIS 5
- CIS 16
1 2 3 4 5 6 `azuread` body.category=SignInLogs body.properties.status.errorCode=500121 | rename body.properties.* as * | bucket span=10m _time | stats dc(_raw) AS mfa_prompts values(userPrincipalName) by userPrincipalName, status.additionalDetails, appDisplayName, userAgent, _time | where mfa_prompts > 10 | `azure_ad_multiple_failed_mfa_requests_for_user_filter`
The SPL above uses the following Macros:
azure_ad_multiple_failed_mfa_requests_for_user_filter is a empty macro by default. It allows the user to filter out any results (false positives) without editing the SPL.
List of fields required to use this analytic.
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. Specifically, this analytic leverages the SignInLogs log category.
Known False Positives
Multiple Failed MFA requests may also be a sign of authentication or application issues. Filter as needed.
Associated Analytic Story
|63.0||70||90||Multiple Failed MFA requests for user $body.properties.userPrincipalName$|
The Risk Score is calculated by the following formula: Risk Score = (Impact * Confidence/100). Initial Confidence and Impact is set by the analytic author.
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