CVE-2026-6347 Mattermost Calls plugin exposes TURN server credentials in plaintext in support packets

What is CVE-2026-6347?

CVE-2026-6347 is a high-severity vulnerability in Mattermost Mattermost Server, published on 18 May 2026. An attacker needs administrator access to exploit it, with no action from the victim.

Official description

Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 fail to sanitize sensitive configuration fields in the Mattermost Calls plugin which allows an attacker with access to a support packet to obtain TURN server credentials via the plaintext values present in the exported plugin configuration.. Mattermost Advisory ID: MMSA-2026-00605

Weakness: CWE-200

Which versions are affected by CVE-2026-6347?

10 open source packages ship an affected version, including github.com/mattermost/mattermost-server and github.com/mattermost/mattermost-plugin-calls. It affects 1 product.

Products include Mattermost Mattermost Server.

How do I fix CVE-2026-6347?

Upgrade github.com/mattermost/mattermost-server to a fixed version for your release line: 10.11.14, 10.11.14+incompatible, 11.4.4+incompatible or 11.5.2+incompatible.

Is CVE-2026-6347 being exploited?

There is no known exploitation of CVE-2026-6347 in the wild and no public exploit code.

Other vulnerabilities in Mattermost Mattermost Server

• CVE-2025-12419 Account takeover on OAuth/OpenID-enabled servers Critical 9.9
• CVE-2025-12421 Account Takeover via Code Exchange Endpoint Critical 9.9
• CVE-2025-4981 Path Traversal Leading to RCE by Any Authenticated Mattermost User Critical 9.9
• CVE-2025-20051 Arbitrary file read via block duplication in Mattermost Boards Critical 9.9
• CVE-2025-25279 Arbitrary file read in Mattermost Boards via import & export board archive Critical 9.9

All Mattermost Mattermost Server vulnerabilities

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as GHSA-82j6-4fq7-fx62, GO-2026-5238.