A missing authorization vulnerability in Mattermost allows authenticated attackers to bypass team access restrictions, posing a significant risk to internal data confidentiality and integrity.

The application fails to perform a capability check when users join a team using an invite token. By manipulating the OAuth state, any authenticated user can join unauthorized teams, effectively bypassing established membership controls.

Successful exploitation allows unauthorized users to gain access to private team channels and sensitive communications, leading to potential data breaches and intellectual property loss. Given the CVSS score of 8.1, this vulnerability is classified as High severity because it allows lateral movement within the application environment and unauthorized access to restricted information.

Immediate Action: Upgrade your Mattermost installation to versions 10.12.0, 10.11.2, 10.10.3, 10.5.11, or higher to apply the necessary authorization checks.

Proactive Monitoring: Review audit logs for unusual team join events or spikes in OAuth-related activity from standard user accounts.

Compensating Controls: Ensure that strict team-level access policies are enforced at the identity provider level where possible, though direct patching remains the only reliable mitigation.

Public Exploit Available: Unknown

This vulnerability represents a significant lapse in access control that permits unauthorized access to private team data. Organizations using the affected versions of Mattermost must prioritize the deployment of the provided security updates to prevent potential unauthorized data exposure. Immediate patching is strongly advised to maintain the security posture of your collaboration platform.