O365 Excessive File Access for Exfiltration Staging

This analytic detects when an excessive number of files are accessed from Office 365 by the same user over a short period of time. This behavior may indicate an attacker staging data for exfiltration or an insider threat removing organizational data. The rule specifically looks for `fileaccessed` operations in the Office 365 Universal Audit Log, excluding SharePoint app access and common image file extensions. It then calculates the count of unique source file names accessed per user and flags users whose access count significantly exceeds the average (3 standard deviations above the mean) and is greater than 50 files. This can help identify malicious actors abusing "open in app" functionality or Graph API-based access to evade typical file download events.