This detection rule identifies potential "MFA fatigue" attacks targeting Office 365 users by monitoring for abnormal numbers of Multi-Factor Authentication (MFA) prompts experienced by users in a given time period. The rule specifically looks for more than nine successful prompts (as indicated by ResultStatus = Success) within a ten-minute timeframe from the Azure Active Directory logs. It captures events with an ErrorNumber of 500121, which pertains to specific failed login attempts tied to MFA. The significance of this rule lies in the fact that attackers may initiate multiple MFA requests to overwhelm users, making it easier for them to gain unauthorized access. When a user faces numerous prompts, they might mistakenly approve one, leading to possible data breaches and unauthorized data access within the Office 365 environment. Therefore, any detection triggered by this rule warrants immediate investigation and ideally, mitigations such as alerting the affected users and reviewing access logs for suspicious behavior.