This analytic targets the detection of an unusually high number of denied Multi-Factor Authentication (MFA) requests for a single user within a 10-minute window, specifically identifying instances where more than nine MFA prompts were declined by the user. Utilizing Azure Active Directory (Azure AD) sign-in logs, particularly focusing on "Sign-in activity" events, it filters for scenarios where the MFA request was denied due to the user declining the authentication, as indicated by error code 500121 and additional details stating "MFA denied; user declined the authentication." The data is then aggregated into 10-minute intervals, counting distinct raw events and capturing the earliest and latest times of occurrence for each user. This behavior is significant for a Security Operations Center (SOC) as it could be an early indicator of a targeted attack or an account compromise attempt, with an attacker having obtained the user's credentials and the user actively declining the MFA prompts, preventing unauthorized access. A true positive detection would imply that an attacker is on the verge of gaining full access to the user's account, posing a threat that could lead to data exfiltration, lateral movement, or further malicious activities within the organization, necessitating immediate investigation and response to safeguard the organization's assets.
- Type: TTP
Product: Splunk Enterprise, Splunk Enterprise Security, Splunk Cloud
- Last Updated: 2023-10-31
- Author: Mauricio Velazco, Splunk
- ID: d0895c20-de71-4fd2-b56c-3fcdb888eba1
Kill Chain Phase
- CIS 10
1 2 3 4 5 6 7 8 9 `azure_monitor_aad` category=SignInLogs operationName="Sign-in activity" | rename properties.* as * | search status.errorCode=500121 status.additionalDetails="MFA denied; user declined the authentication" | bucket span=10m _time | stats dc(_raw) AS mfa_prompts earliest(_time) as firstTime latest(_time) as lastTime by user, status.additionalDetails, appDisplayName, userAgent, _time | where mfa_prompts > 9 | `security_content_ctime(firstTime)` | `security_content_ctime(lastTime)` | `azure_ad_multiple_denied_mfa_requests_for_user_filter`
The SPL above uses the following Macros:
azure_ad_multiple_denied_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 through an EventHub. This analytic was written to be used with the azure:monitor:aad sourcetype leveraging the Signin log category.
Known False Positives
Multiple denifed MFA requests in a short period of span may also be a sign of authentication errors. Investigate and filter as needed.
Associated Analytic Story
|54.0||60||90||User $user$ denied more than 9 MFA requests in a timespan of 10 minutes.|
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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