Microsoft Teams’ default cross-tenant collaboration settings has been observed creating an overlooked security exposure that attackers could weaponize at scale.
Ontinue’s research revealed that users who accepted guest invitations to external Microsoft 365 tenants were no longer protected by their own organization’s Microsoft Defender for Office 365 controls. Instead, all protections, such as Safe Links, Zero-hour Auto Purge, and Safe Attachments, were enforced by the hosting tenant.
If that external tenant lacked Defender or had disabled security features, users effectively operated inside an unprotected environment. Ontinue researchers noted that this design reflected Microsoft’s established architecture for Business-to-Business (B2B) guest access, rather than a defect in Teams. However, the practical impact is significant.
The risk increased following Microsoft’s rollout of the MC1182004 “Chat with anyone with an email address” feature, which was enabled by default across all Microsoft 365 tenants. This capability allows any Teams user to initiate a chat with any email address, generating a legitimate Microsoft invitation. Because attackers could create low-cost or trial tenants without Defender capabilities, they could easily build “protection-free zones” and invite targets into them.
Many organizations remained unaware that disabling outbound guest invitations did not block inbound invitations from malicious tenants. As a result, threat actors could deliver phishing links and malware without triggering security alerts in the victim’s home environment.
Industry Experts Call for Stronger Controls and Governance
Security leaders said the findings reinforced the need for tighter controls around cross-tenant communication.
“To protect against the risks of users joining risky external tenants, organizations must ensure that credentials are well governed, access is appropriately limited, and activity tied to sensitive systems is consistently monitored,” Shane Barney, CISO at Keeper Security told Expert Insights.
Recommended mitigations from Ontinue include:
- Enforcing cross-tenant access policies through Microsoft Entra ID
- Limiting external communication in the Teams Admin Center
- Allow listing trusted domains
- Training users to treat unexpected Teams invitations with caution
“Don’t assume your Defender for Office 365 investment follows your users everywhere,” Ontinue warned. “It doesn’t.”
