Outlook: Import emails from PST file to mailbox in new Outlook for Windows

🚨 The Signal: New Outlook for Windows now allows importing PST files directly into mailboxes. This increases the risk of data exfiltration, unmanaged data ingress, and introduces potential for malware injection via legacy file formats.

The Impact

All users are affected, increasing risks of data exfiltration, unmanaged data, and malware.

• End Users: Increased risk of inadvertently importing malicious content.
• Security Teams: New vector for data exfiltration and unmanaged data ingress.
• Compliance Teams: Challenges in maintaining data governance and eDiscovery completeness.
• Administrators: Potential for increased storage consumption and data sprawl.

The Action

1. Review and enforce existing data loss prevention (DLP) policies to monitor PST file content.
2. Educate users on the risks associated with importing untrusted PST files.
3. Implement or strengthen email attachment scanning for known malicious file types within PSTs.
4. Consider Group Policy or Intune settings to restrict PST import functionality if not business-critical.

Domain: Exchange · Impact: high · Workload: Exchange Online