Microsoft Outlook to block MSIX attachments starting November
BLUF
Microsoft is blocking MSIX package attachments in Outlook to prevent malware delivery via this vector.
NEWS
Starting November 2026, Outlook Web and the new Windows client will automatically block .msix and .msixbundle files from being sent or received. This move expands Microsoft's existing list of blocked executable types due to rising abuse in phishing campaigns.
Why I Care
This change disrupts legitimate software distribution workflows relying on email for MSIX packages but significantly reduces the attack surface for malware disguised as application installers. IT administrators and end-users must adjust file transfer methods immediately to avoid communication failures.
Next Steps
Administrators should notify users of the upcoming block by October 31, 2026, and establish alternative secure file sharing protocols for legitimate MSIX deployments before November begins.
Source: BleepingComputer ·