CVE-2025-12756: Mattermost Server Auth Bypass Vulnerability

CVE-2025-12756 Overview

CVE-2025-12756 is an authorization vulnerability in Mattermost Server affecting the Boards feature. The flaw allows authenticated users with the editor role to delete comments created by other users. Mattermost fails to validate user permissions when processing comment deletion requests in Boards. The issue is tracked under CWE-863: Incorrect Authorization and affects multiple release branches of the Mattermost Server.

Critical Impact
Authenticated editors can delete other users' comments in Boards, enabling destruction of collaborative content and potential loss of audit trail integrity.

Affected Products

Mattermost Server versions 11.0.x up to and including 11.0.2
Mattermost Server versions 10.12.x up to and including 10.12.1, and 10.11.x up to and including 10.11.4
Mattermost Server versions 10.5.x up to and including 10.5.12

Discovery Timeline

2025-12-01 - CVE-2025-12756 published to the National Vulnerability Database
2026-10-19 - Last updated in NVD database

Technical Details for CVE-2025-12756

Vulnerability Analysis

The vulnerability resides in the Boards module of Mattermost Server. The comment deletion endpoint does not verify that the requesting user is the original author of the comment or holds elevated administrative rights. An authenticated user assigned the editor role can submit a delete request targeting any comment within a board they can access. The server executes the deletion without enforcing ownership checks, resulting in horizontal privilege escalation within the Boards collaboration space.

Root Cause

The root cause is missing authorization logic in the comment deletion handler. Mattermost assigns the editor role sufficient permissions to create and manage board content, but the deletion path omits the ownership verification step. This maps to CWE-863, where a security decision is made without validating that the actor is entitled to perform the operation on the specific resource.

Attack Vector

Exploitation requires an authenticated session with the editor role on a shared board. The attacker issues a standard delete request against a comment identifier owned by another user. No special tooling, elevated privileges, or user interaction is required. The impact is limited to integrity of comment data in Boards. There is no direct impact on confidentiality or availability of the broader Mattermost deployment.

No verified public proof-of-concept code is available. Refer to the Mattermost Security Updates page for vendor technical details.

Detection Methods for CVE-2025-12756

Indicators of Compromise

Unexpected deletion of Boards comments reported by content owners or discovered during audits
API or audit log entries showing DELETE operations on comment resources where the acting user differs from the comment author
Spikes in comment deletion events originating from a single editor-role account

Detection Strategies

Query Mattermost audit logs for Boards comment deletion events and correlate the acting user_id with the comment author field
Alert when a non-admin editor account performs comment deletions across multiple boards or multiple authors within a short window
Baseline normal deletion patterns per user and flag deviations for review

Monitoring Recommendations

Forward Mattermost application and audit logs to a centralized SIEM for retention and correlation
Enable verbose logging on Boards API endpoints to capture request metadata including acting user and target resource
Review editor-role membership regularly and reduce privileges where editor access is unnecessary

How to Mitigate CVE-2025-12756

Immediate Actions Required

Upgrade Mattermost Server to a fixed release on your current branch as published on the Mattermost Security Updates page
Audit Boards comment deletion activity since the vulnerable versions were deployed to identify unauthorized removals
Review and tighten role assignments in Boards, limiting editor role grants to users who require content modification

Patch Information

Mattermost has released fixed versions above 11.0.2, 10.12.1, 10.11.4, and 10.5.12 for the respective maintenance branches. Administrators should consult the Mattermost Security Updates advisory for the specific patched build matching their deployment branch and apply the upgrade following the standard Mattermost upgrade procedure.

Workarounds

Restrict editor role assignments on sensitive boards until patches are applied
Export or back up Boards content regularly so that unauthorized deletions can be reconstructed
Monitor and alert on anomalous comment deletion activity through audit log review until the upgrade is completed