Researchers have recently identified two techniques that pose serious security threats to Microsoft SharePoint users by facilitating covert file theft without triggering standard audit logs. These methods, unveiled by Varonis Threat Labs, exploit inherent features of SharePoint to bypass detection mechanisms and manipulate log entries during file downloads.
The first technique leverages SharePoint’s “Open in App” functionality, allowing users to access documents directly through applications like Microsoft Word instead of using the web browser. By exploiting this feature, attackers can execute silent data exfiltration without generating the typical “FileDownloaded” audit log entries. Instead, the activity is logged as a less conspicuous “Access” event, potentially evading detection by security teams.
In a separate approach, attackers can spoof the User-Agent string of file access requests to emulate Microsoft SkyDriveSync, a service utilized for file synchronization between SharePoint and local computers. This tactic alters the audit log entries to mimic legitimate data syncing events (“FileSyncDownloadedFull”), reducing the likelihood of scrutiny by security personnel. Both techniques enable hackers to steal sensitive information while maintaining a low profile, posing significant risks to SharePoint data security.
While these vulnerabilities were disclosed to Microsoft in November 2023, immediate fixes are not available as the issues were rated as moderate severity. SharePoint administrators are urged to remain vigilant and implement mitigation measures to safeguard against potential exploitation. Monitoring for anomalous access activity, scrutinizing sync events for irregularities, and maintaining awareness of unusual data exfiltration patterns are crucial steps in mitigating these emerging threats.
