Microsoft Teams Guest Chat Cross-Tenant Security Bypass
A significant security weakness has been identified in Microsoft Teams' guest chat feature, allowing attackers to bypass Defender for Office 365 protections when users accept invitations to external tenants. Security researchers from Ontinue revealed that when a user joins another organization's Teams environment as a guest, the security policies of the hosting tenant apply, not those of the user's home organization. This architectural flaw means that if the external tenant has minimal or no security controls, all advanced protections such as URL scanning, Safe Links, file sandboxing, and Zero-hour Auto Purge are effectively disabled for the guest user.
Attackers can exploit this by creating their own Microsoft 365 tenants with security features turned off and inviting targets to collaborate, thereby exposing them to phishing, malware, and other threats without the usual safeguards. The issue is not a software bug but a fundamental limitation of how cross-tenant collaboration is managed in Microsoft Teams. Security experts warn that organizations may have a false sense of security, believing their protections follow users across tenants, when in reality, attackers can easily create "protection-free zones" to deliver malicious content undetected.
Timeline
Nov 27, 2025
Microsoft had not responded publicly to the reported Teams issue
Coverage of the research noted that Microsoft had not yet responded to requests for comment about the cross-tenant Teams guest chat exposure. This indicated there was no public vendor statement or fix at the time of reporting.
Nov 27, 2025
Security guidance issued to restrict external Teams guest access
Following the disclosure, researchers recommended treating external guest access as a trust boundary, restricting guest invitations to vetted domains, disabling the 'chat with anyone' feature, and enforcing Microsoft Entra cross-tenant access policies. These mitigations were presented as the primary way for organizations to reduce exposure.
Nov 27, 2025
Ontinue discloses Teams guest chat cross-tenant security blind spot
Ontinue published research showing that when users join an external Microsoft 365 tenant as a guest in Teams, their home tenant’s Defender for Office 365 protections no longer apply. The company described the issue as an architectural behavior rather than a software bug, warning it could let attackers deliver phishing links, malware, and social engineering from poorly secured attacker-controlled tenants.
Nov 26, 2025
Microsoft enables Teams chat with any email address by default
Microsoft introduced update MC1182004, a default-enabled Teams feature that allows users to chat with anyone using an email address. Researchers said this expanded the reach of cross-tenant guest invitations and made the exposure easier to exploit.
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