The O365 Multiple AppIDs and UserAgents Authentication Spike rule identifies potential malicious activity within an Office 365 (O365) environment by monitoring user authentication attempts. It specifically looks for scenarios where a single user account shows an unusual spike in authentication attempts, defined as more than eight attempts within a short timeframe while using three or more unique application IDs and over five different user agents. This type of behavior often suggests that an adversary is probing for vulnerabilities, particularly with multi-factor authentication, which may lead to account compromise and unauthorized access. The rule utilizes O365 audit logs focused on UserLoggedIn and UserLoginFailed events. It applies statistical thresholds to detect anomalies and alert security teams for further investigation. If confirmed as malicious, it can indicate an ongoing attack that leads to privilege escalation or data exfiltration, hence emphasizing the importance of early detection.