Microsoft will block two Windows installer formats in Outlook from November
Microsoft will add .msix and .msixbundle to Outlook’s blocked attachment list in early November, closing an email route for a packaging format its own threat researchers watched four criminal groups abuse in 2023. Renaming the extension still gets a package through, and the change lands as a default policy edit in every Exchange Online tenant, including those of European governments now trying to leave.
From early November, Outlook on the web and the new Outlook for Windows will stop users opening or downloading two kinds of attachment. Both are Microsoft’s own Windows installer formats. Microsoft says most organisations will not notice.
Notice MC1488841, published on 5 October, adds both to the default Outlook web mailbox policy and to every custom policy in a tenant. Rollout runs from early November to mid-month. Administrators needing the formats must add them to an allowed list first, per Microsoft’s support documentation.
MSIX is...
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