Microsoft Teams guest chat exposes cross-tenant blind spot
🔒 Security researchers warn that a cross-tenant collaboration design in Microsoft Teams can cause a user's Defender for Office 365 protections to be dropped when they accept a guest invitation and join another tenant. The default-enabled feature MC1182004 (chat with any email) lowers the bar for attackers to spin up hostile tenants and deliver links or files that bypass URL scanning, Safe Links, file sandboxing and zero-hour auto purge. Administrators are advised to treat guest access as a trust boundary: restrict B2B invites to vetted domains, enforce Entra ID cross-tenant policies, and disable the 'chat with Anyone' capability where appropriate.
