CVE-2024-39839: 
vulnerability analysis and mitigation

Mattermost versions 9.9.x <= 9.9.0, 9.5.x <= 9.5.6, 9.7.x <= 9.7.5, and 9.8.x <= 9.8.1 contain a security vulnerability related to shared channels functionality. The vulnerability was disclosed on August 1, 2024, and affects the user authentication mechanism in Mattermost server installations (NVD).

The vulnerability stems from a failure to properly restrict users from setting their own remote usernames when shared channels are enabled. This allows users on a remote system to set their remote username property to an arbitrary string, which would then be synchronized to the local server if the user hasn't been previously synced. The vulnerability has been assigned a CVSS v3.1 base score of 4.3 (Medium) with the vector string CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N (NVD).

When exploited, this vulnerability allows attackers to manipulate their remote username identifiers, potentially leading to identity confusion or impersonation within the shared channel environment. The impact is primarily limited to integrity concerns, with no direct effect on confidentiality or availability of the system (NVD).

Organizations should upgrade to the following fixed versions: 9.5.7 or later for the 9.5.x series, 9.7.6 or later for the 9.7.x series, 9.8.2 or later for the 9.8.x series, or versions after 9.9.0 for the 9.9.x series (NVD).