Newsbite analysis · Business, Security
BleepingComputer
Microsoft Outlook to block MSIX attachments starting November
Microsoft has announced a significant security update that will block .msix and .msixbundle file attachments in Outlook Web and the new Outlook Windows client. This policy will come into effect for Exchange Online users in early November. By preventing the sending, receiving, opening, or downloading of these file types, Microsoft aims to reduce the risk of attacks exploiting these modern Windows installation packages. The decision aligns with their broader initiative to enhance security following the abuse of certain Windows features in various attacks against users in recent years.
MonitorConfidence: HighShort term horizonStory: First report
EvidenceOfficial announcementIndependent evidenceCompany only
Published 7 Oct 2026, 16:50 UTC3 min read
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Brief
NewsBite reading:Microsoft to block MSIX attachments in Outlook for enhanced security
TL;DR
- Microsoft will start blocking .msix and .msixbundle attachments in November.
- The update is aimed at enhancing security in Outlook to prevent file-based attacks.
- Admins can whitelist these file types if necessary for their organization.
- This is part of Microsoft’s efforts to improve security against previously exploited features.
- Blocking of related file types has been ongoing since June 2022.
Interesting facts
- .msix files are modern installation packages designed for specific architectures.
- Microsoft previously blocked .library-ms and .search-ms file types due to exploitation.
- Most organizations are not expected to be affected by the change.