Mattermost Legal Hold Plugin Authentication Bypass Vulnerability

The Mattermost Legal Hold plugin, in versions 1.1.4 and earlier, contains an authentication bypass vulnerability (CVE-2026-3524) that can be exploited by authenticated attackers. The vulnerability lies in the ServeHTTP function, where a failed authorization check does not properly halt request processing. This flaw allows attackers to craft malicious API requests to the plugin’s endpoints, enabling them to access, create, download, and delete legal hold data without proper authorization. The vulnerability is identified by Mattermost Advisory ID MMSA-2026-00621 and poses a significant risk to organizations using the affected plugin versions, potentially leading to data breaches and compliance violations.

Attack Chain

1. Attacker authenticates to the Mattermost server with valid user credentials.
2. Attacker crafts a malicious API request targeting the Legal Hold plugin’s endpoints.
3. The request is sent to the Mattermost server.
4. The ServeHTTP function in the Legal Hold plugin processes the request.
5. Authorization check fails due to insufficient privileges or incorrect parameters.
6. Instead of halting request processing, the plugin continues to execute the request.
7. The attacker gains unauthorized access to legal hold data or performs unauthorized actions (create, download, delete).
8. The attacker successfully exfiltrates or manipulates sensitive legal hold information.

Impact

Successful exploitation of this vulnerability (CVE-2026-3524) allows authenticated attackers to bypass authorization controls within the Mattermost Legal Hold plugin. This can result in unauthorized access, creation, modification, or deletion of sensitive legal hold data. The vulnerability affects versions 1.1.4 and earlier of the plugin. Organizations using the affected versions are at risk of data breaches, compliance violations, and reputational damage. A CVSS v3.1 score of 8.8 indicates a high level of severity due to the potential for significant data compromise.

Recommendation

• Upgrade the Mattermost Legal Hold plugin to a version later than 1.1.4 to remediate CVE-2026-3524.
• Deploy the Sigma rules provided in this brief to detect exploitation attempts targeting the vulnerable Legal Hold plugin endpoints (see rules section).
• Monitor Mattermost server logs for unusual API requests to the Legal Hold plugin, specifically those resulting in unexpected data access or modification, as a potential sign of exploitation (webserver log source).