CVE-2026-6342

Vulnerability

Mattermost Plugins versions 11.5 and earlier, 11.1.5 and earlier, 10.13.11 and earlier, and 11.3.4.0 and earlier do not properly validate group namespaces. This allows plugin users to create subscriptions to groups that were not whitelisted by creating groups that share the same prefix as a whitelisted group. The vulnerability is tracked as MMSA-2026-00601 [1].

Exploitation

An attacker must be an authenticated plugin user with the ability to create groups. By creating a group with a prefix that matches a whitelisted group, the attacker can then create a subscription to that group, bypassing the namespace whitelist check. No additional privileges or user interaction beyond standard plugin usage is required.

Impact

Successful exploitation allows the attacker to subscribe to groups that were not intended to be accessible, potentially leading to unauthorized access to group content, notifications, or other group-related features. The confidentiality and integrity of group communications may be compromised, depending on the group's purpose.

Mitigation

As of the publication date, no specific fix has been disclosed in the available references [1]. Users should monitor the Mattermost Security Updates page for patches and consider restricting plugin user permissions to mitigate the risk until an update is applied.