App-only Graph access sweeping many mailboxes (BEC monitoring / mass exfil via app permission)
This rule detects potential mass email access or collection activities by an application (AppId). It monitors Microsoft 365 CloudAppEvents for actions like 'MailItemsAccessed', 'Send', or 'New-InboxRule' associated with an application. It triggers an alert when a single application interacts with 10 or more distinct mailboxes within a one-hour window, which is indicative of potential Business Email Compromise (BEC) or unauthorized mass data exfiltration via compromised OAuth applications.
Microsoft Sentinel (KQL)
