Users’ .zip files attached to S/MIME encrypted messages may not be displayed in Outlook on the web

Title: Users’ .zip files attached to S/MIME encrypted messages may not be displayed in Outlook on the web

User impact: Users’ .zip files attached to S/MIME encrypted messages may not have displayed in Outlook on the web.

More info: Users may have been able to bypass impact by clicking the “Reply to All,” “Reply,” or “Forward” button on the mail item that had the issue to launch the compose message screen. After that, users could save the message as a draft and check the draft item to see the attachment. Users could have used the Outlook desktop client without error.

Final status: Our investigation has determined that a recent update to the Outlook on the web code base responsible for displaying encrypted attachments introduced an error that prevented some attachments from displaying correctly, leading to impact. We reverted the offending update to repair this issue, and we’ve received confirmation from previously affected users that impact is remediated.

Scope of impact: Users serviced by the affected infrastructure who received attached .zip files to Secure/Multipurpose internet Mail Extensions (S/MIME) encrypted messages may have been unable to view them when using Outlook on the web.

Start time: Monday, March 4, 2024, at 8:00 AM UTC

End time: Monday, March 18, 2024, at 10:07 PM UTC

Root cause: A recent update to the Outlook on the web code base responsible for displaying encrypted attachments introduced an error that prevented some attachments from displaying correctly, leading to impact.

Next steps:
– We’re reviewing our Outlook on the web code update procedures pertaining to encrypted attachment behaviors to prevent similar impact in future updates.

This is the final update for the event.