Mattermost Plugins versions older than the ones listed fail to properly validate namespace prefixes when creating group subscriptions. This allows a plugin user to craft a group name that shares the same prefix as an approved group, thereby obtaining a subscription to that group without having the required authorization. The flaw is an authorization bypass in which an attacker can subscribe to arbitrary groups, potentially accessing messages or data that should be restricted.

The check applies to Mattermost Plugins for the Mattermost collaboration platform. Vulnerable versions include those up to 11.5, 11.1.5, 10.13.11, and 11.3.4.0. Administrators should confirm the installed plugin version and apply the available updates as soon as possible.

The CVSS score of 4.3 categorizes this issue as moderate risk. While a plugin user with sufficient permissions could exploit the flaw by creating a synthetic group name, no exploitation code is publicly documented and the EPSS score is not available, indicating a low but uncertain probability of exploitation. The vulnerability is also not listed in the CISA KEV catalog.