The reported vulnerability concerns a cross-tenant security gap in Microsoft Teams guest access functionality that allows attackers to circumvent Microsoft Defender for Office 365 protections. When a user accepts a guest invitation to join another tenant, their security protections are governed by the hosting tenant's policies rather than their home organization's. This architectural design creates a blind spot where an attacker can establish a malicious Microsoft 365 tenant—potentially using low-cost licenses like Teams Essentials or Business Basic that lack Defender protections—and invite victims as guests. The invitation email is sent from Microsoft's own infrastructure, bypassing SPF, DKIM, and DMARC email authentication checks, making it unlikely to be flagged by email security solutions. Once the victim accepts the invitation, subsequent communications occur within the attacker's tenant, where Defender's Safe Links and Safe Attachments protections do not apply. This enables attackers to deliver phishing links or malware-laden attachments without detection by the victim's home security controls. The victim's organization remains unaware since the attack happens outside its security boundary. The vulnerability is exacerbated by Microsoft's rollout of a new Teams feature allowing chat invitations via email to anyone, including non-Teams users, increasing the attack surface. The fundamental issue is that guest access shifts security responsibility to the hosting tenant, which can be exploited to create 'protection-free zones.' To mitigate this risk, organizations should restrict guest invitations to trusted domains, enforce cross-tenant access policies, limit external Teams communication, and educate users to recognize unsolicited Teams invitations from unknown sources.

For European organizations, this vulnerability poses a significant risk to confidentiality and integrity due to the potential for phishing and malware delivery through trusted Microsoft Teams channels. The bypass of Microsoft Defender protections means that malicious payloads or links can reach users without triggering alerts or scans, increasing the likelihood of successful compromise. The attack can lead to credential theft, lateral movement, data exfiltration, or ransomware deployment. Since Teams is widely used across Europe for collaboration, especially in sectors like finance, healthcare, and government, the impact could be substantial. The stealthy nature of the attack—originating from legitimate Microsoft infrastructure—makes detection difficult, potentially delaying incident response. Additionally, the cross-tenant nature complicates attribution and remediation. Organizations may face regulatory and compliance challenges if breaches occur due to insufficient controls over guest access. The threat also undermines trust in inter-organizational collaboration, potentially hindering productivity and external partnerships.

European organizations should implement strict B2B collaboration policies by allowing guest invitations only from verified and trusted domains to reduce exposure to malicious tenants. Employ cross-tenant access controls available in Azure AD to enforce conditional access policies and restrict guest user permissions. Disable or tightly control the new Teams feature that allows chat invitations via email, especially if external collaboration is not essential. Regularly audit guest access and remove unnecessary or inactive guest accounts. Enhance user awareness training to help employees recognize unsolicited Teams invitations and suspicious external collaboration requests. Integrate Microsoft Cloud App Security or similar tools to monitor and alert on anomalous guest access activities. Coordinate with Microsoft support to stay informed about patches or updates addressing this vulnerability. Consider deploying additional endpoint detection and response (EDR) solutions that monitor Teams client behavior for suspicious activity. Finally, establish incident response playbooks specifically addressing cross-tenant collaboration risks.