Microsoft 365 Alert – Service Degradation – Exchange Online – Users’ .zip files attached to S/MIME encrypted messages may not be displayed in Outlook on the web – RESOLVED

19/03/2024 08:55:00 AM

NHSMail Reference: INC42014856

Microsoft Reference: EX742346

Issue Status: RESOLVED

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

More info: Users may be able to bypass impact by clicking the “Reply to All,” “Reply,” or “Forward” button on the mail item that has the issue to launch the compose message screen. After that, save the message as a draft and check the draft item to see the attachment. As an alternative method, users can use the Outlook desktop client without error

Final Update: 20/03/2024 09:03:00 AM – Microsoft’s 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. They reverted the offending update to repair this issue, and they have 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 be unable to view them when using Outlook on the web.

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: Microsoft are reviewing their Outlook on the web code update procedures pertaining to encrypted attachment behaviors to prevent similar impact in future updates.