In this post
Microsoft is switching file attachments in external Teams chats from off to on by default, with rollout from late October to late November 2026. Admins who want it off should set the policy explicitly now.
What is changing
Microsoft Message Center post MC1479514 changes the default of file sharing in external (federated) Teams chats from Disabled to Enabled. Until now, admins had to switch it on before users could attach files in chats with people outside the organization. Users could only paste links to files that already existed.
Once the default flips, users in external 1:1 chats, group chats and meeting chats will see the paperclip Attach file option and can drag and drop files into the compose box. Teams automatically grants chat participants the permissions they need, so senders no longer adjust sharing by hand. Senders can still review, change or remove access before sending, and revoke it afterward.
The change applies to Teams on Windows, Mac and the web. Files uploaded from a device are stored in the sender’s OneDrive, files already in the cloud stay where they are, and recipients receive a link.
Who is affected and when
Rollout starts in late October 2026 and should finish by late November 2026. It affects tenants that use Teams external access (federation) and have never configured the FileSharingInChatswithExternalUsers policy setting, which has been Disabled by default since Microsoft introduced it in July 2025.
The wording has shifted during the rollout. The original Message Center text said organizations wanting sharing off must reapply the disabled setting after rollout, and some commentators read it as overriding earlier disabled settings. Microsoft’s updated post says it only affects tenants that have not adjusted the policy, and Topedia reports that explicitly disabled tenants are not touched. Treat the explicit setting as the safe path, and confirm the current value in your own tenant.
What admins need to do
Action is required only if you want external file attachments to stay off. Run these in Teams PowerShell before the rollout reaches your tenant.
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- Check the current value of the file attachment setting:
Get-CsTeamsFilesPolicy -Identity Global | select FileSharingInChatswithExternalUsers | fl-
- Set it to Disabled explicitly so the new default cannot apply:
Set-CsTeamsFilesPolicy -Identity Global -FileSharingInChatsWithExternalUsers Disabled-
- Check the second setting, automatic permission assignment, which is enabled by default:
Get-CsTeamsMessagingPolicy -Identity Global | select AutoShareFilesInExternalChats | fl-
- If you want to stop Teams granting permissions automatically, set it to Disabled:
Set-CsTeamsMessagingPolicy -Identity Global -AutoShareFilesInExternalChats DisabledThe examples target the Global policy. If you assign custom policies to users, repeat the checks for each one. Also tell your helpdesk about the change, whichever way you decide.
Security and governance considerations
The change does not override OneDrive or SharePoint external sharing settings. If your organization limits sharing to “Only people in your organization”, external recipients still cannot open attached files. Tenants with external sharing set to existing guests or broader are where the new default has real effect, so review those settings before the rollout lands.
The larger shift is behavioral. Users no longer need to stop and think about permissions, which makes accidental oversharing easier. Consider whether your data loss prevention rules, sensitivity labels and external access allow lists cover chats with federated partners. Microsoft also suggests considering Entra B2B integration for SharePoint and OneDrive, which helps external recipients authenticate and open shared files.