CVE-2026-3637: Mattermost Server Auth Bypass Vulnerability

CVE-2026-3637 Overview

CVE-2026-3637 is a missing authorization vulnerability [CWE-862] affecting Mattermost Server. The flaw exists because the application does not verify the create_post channel permission during post edit operations. An authenticated user whose posting privileges have been revoked can still modify their existing posts by sending direct API requests to the post update and patch endpoints. Mattermost tracks this issue as advisory MMSA-2026-00627. The vulnerability impacts content integrity within channels where administrators expected posting restrictions to apply.

Critical Impact

Authenticated users with revoked posting privileges can bypass channel permission checks to modify their existing posts via direct API calls to update and patch endpoints.

Affected Products

• Mattermost Server versions 11.5.x through 11.5.1
• Mattermost Server versions 11.4.x through 11.4.3
• Mattermost Server versions 10.11.x through 10.11.13

Discovery Timeline

• 2026-05-18 - CVE-2026-3637 published to NVD
• 2026-05-19 - Last updated in NVD database

Technical Details for CVE-2026-3637

Vulnerability Analysis

The vulnerability is a broken access control issue in Mattermost's post management API. When a user edits an existing post, the server processes the request through the post update and patch endpoints. These endpoints fail to validate whether the requesting user currently holds the create_post permission on the target channel. As a result, administrators who revoke a user's posting rights cannot prevent that user from editing prior posts. Attackers exploit this gap by issuing API requests directly rather than through the standard client workflow, which would normally enforce the permission check at the UI layer.

Root Cause

The root cause is a missing authorization check [CWE-862] in the post update and patch handlers. The code path validates ownership of the post but does not re-evaluate the channel-level create_post permission at edit time. This violates the principle that authorization must be enforced on every state-changing operation, not only at initial post creation.

Attack Vector

An authenticated attacker sends an HTTP request directly to the post edit API endpoints. The attacker must have valid session credentials and a previously authored post in the target channel. No user interaction is required, and the attack proceeds over the network with low complexity. Successful exploitation allows the attacker to modify post content even after an administrator has revoked their posting privileges, undermining moderation controls.

No verified public exploit code is available. See the Mattermost Security Updates advisory for additional technical context.

Detection Methods for CVE-2026-3637

Indicators of Compromise

• API requests to PUT /api/v4/posts/{post_id} or PUT /api/v4/posts/{post_id}/patch originating from accounts that lack create_post permission in the referenced channel.
• Post edit events in audit logs for users whose channel role does not include posting rights.
• Unexpected modifications to historical post content following permission revocation actions.

Detection Strategies

• Correlate Mattermost audit logs for post edit events against the current channel permission state of the acting user.
• Alert on post update API calls where the user's effective permissions on the target channel do not include create_post.
• Compare post edit_at timestamps against the time of any role or permission change for the same user.

Monitoring Recommendations

• Forward Mattermost server logs and audit events to a centralized logging platform for retention and analysis.
• Track API endpoint usage patterns to baseline normal edit activity and surface anomalies.
• Review administrative actions that revoke posting privileges and verify that subsequent edits from affected users are blocked.

How to Mitigate CVE-2026-3637

Immediate Actions Required

• Upgrade Mattermost Server to a version that contains the fix for MMSA-2026-00627 as documented in the vendor advisory.
• Audit current channel members with revoked posting permissions and review their recent post edit history.
• Restrict network access to the Mattermost API to trusted sources where feasible.

Patch Information

Mattermost has addressed this vulnerability in updated releases tracked under advisory MMSA-2026-00627. Administrators running affected branches 11.5.x, 11.4.x, or 10.11.x should consult the Mattermost Security Updates page for the corresponding fixed version and apply the upgrade following standard deployment procedures.

Workarounds

• Remove affected users from channels rather than relying solely on revoking create_post permission, until the patch is applied.
• Delete or archive historical posts authored by users whose posting privileges have been revoked when content integrity is critical.
• Monitor and review post edit audit logs to identify abuse before remediation is complete.