CVE-2025-12756: 
vulnerability analysis and mitigation

CVE-2025-12756 is an incorrect authorization vulnerability in Mattermost Server that allows authenticated users with the editor role to delete comments created by other users in the Boards feature. It affects Mattermost versions 11.0.x ≤ 11.0.2, 10.12.x ≤ 10.12.1, 10.11.x ≤ 10.11.4, and 10.5.x ≤ 10.5.12. The vulnerability was published on December 1, 2025, with patches released shortly after. It carries a CVSS v3.1 base score of 4.3 (Medium) (Mattermost Security, Red Hat CVE).

The root cause is classified as CWE-863 (Incorrect Authorization): Mattermost Boards fails to validate whether the requesting user has ownership or sufficient privileges over a comment before processing a delete request. An authenticated user with only the editor role can send a delete request targeting comments authored by other users, and the server will process it without enforcing ownership checks. The attack vector is network-based, requires low privileges (a valid editor-role account), and no user interaction is needed (Mattermost Security, Red Hat CVE).

Successful exploitation allows an authenticated editor-role user to permanently delete comments authored by other users within Mattermost Boards, resulting in unauthorized content removal and potential loss of critical project context or audit trail. There is no confidentiality or availability impact; the impact is limited to integrity (Low). While lateral movement is not applicable, the ability to silently erase other users' contributions could disrupt team communication and undermine trust in collaborative workflows (Mattermost Security).

There is no public proof-of-concept exploit and no evidence of in-the-wild exploitation at this time. The EPSS score is approximately 0.022% (0.000220), indicating a very low probability of exploitation in the near term. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog. Exploitation requires a valid authenticated account with at least the editor role, limiting the attack surface to internal or invited users (Mattermost Security, Red Hat CVE).

1. Obtain editor-role access: Authenticate to a vulnerable Mattermost instance using an account that has been granted the editor role in a Boards workspace.
2. Identify target comments: Browse the Boards feature to locate comments authored by other users that the attacker wishes to delete.
3. Send unauthorized delete request: Issue a delete API request (e.g., via the Mattermost Boards REST API) targeting the comment ID of another user's comment, without the server enforcing ownership validation.
4. Confirm deletion: Verify that the comment authored by another user has been removed, confirming the authorization bypass was successful (Mattermost Security).

• Logs: Mattermost server logs showing delete comment API calls where the requesting user's ID does not match the comment author's ID in the Boards audit trail.
• Application Activity: Unexpected disappearance of comments in Mattermost Boards, particularly comments authored by users other than the one performing the deletion.
• Audit Trail: Gaps or missing entries in board comment history that correlate with editor-role user activity but no corresponding author-initiated deletions.

Mattermost has released patched versions addressing this vulnerability: upgrade to 10.5.13, 10.11.5, 10.12.2, or 11.0.3 as appropriate for your release branch. As a temporary workaround if immediate patching is not possible, restrict editor-role permissions or demote potentially malicious users to viewer roles. Additionally, monitor Mattermost Boards activity logs for unexpected comment deletions by editor-role users (Mattermost Security).