Microsoft has fixed a critical security vulnerability inMicrosoft Teamsthat allowed an unauthorized attacker to obtain information about a network.
The vulnerability is registered asCVE-2026-21535and was caused by an error in access control, also referred to as Improper access control.
According to Microsoft, an attack did not require authentication, user interaction, or elevated privileges. The vulnerability therefore had a high impact on data confidentiality, resulting in a critical classification and a CVSS score of 8.2. The error was located in the Teams cloud service itself and has been completely resolved on the server side. Users and IT administrators do not need to install updates or take additional measures.
Microsoft says it is not aware of any active exploitation of the leak. The vulnerability was reported by an external researcher and was disclosed after mitigation via the Microsoft Security Update Guide. The company deliberately released few technical details about the nature of the information that could be viewed or the exact attack path in order to limit the risk of reuse or derivative attacks.
Access control weak spot in cloud software
For some time now, there has been a trend whereby access control errors are a recurring problem on large SaaS platforms. Unlike classic software leaks, these often do not involve bugs in client software, but rather complex backend logic or authorization models that unintentionally disclose more data than intended. Such vulnerabilities are difficult to detect and often only come to light through external security researchers.
The fact that Microsoft’s solution does not require user action confirms that the problem was not in the Teams clients, but in the underlying cloud infrastructure. For organizations, this means that the immediate risk has now been eliminated, but the incident does underscore the importance of trust in cloud providers and their internal security processes. At the same time, the publication of CVE-2026-21535 shows that Microsoft is continuing its transparency policy around cloud services by also publicly documenting vulnerabilities that have already been resolved.