CVE-2026-20796 Time-of-check time-of-use vulnerability in common teams API

What is CVE-2026-20796?

CVE-2026-20796 is a low-severity vulnerability in Mattermost Mattermost Server, published on 13 Feb 2026. Any signed-in user can exploit it over the network, with no action from the victim.

Official description

Mattermost versions 10.11.x <= 10.11.9 fail to properly validate channel membership at the time of data retrieval which allows a deactivated user to learn team names they should not have access to via a race condition in the /common_teams API endpoint.. Mattermost Advisory ID: MMSA-2025-00549

Weakness: CWE-367

Which versions are affected by CVE-2026-20796?

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

Products include Mattermost Mattermost Server.

How do I fix CVE-2026-20796?

Upgrade github.com/mattermost/mattermost-server to a fixed version for your release line: 10.11.10 or 10.11.10+incompatible.

Is CVE-2026-20796 being exploited?

There is no known exploitation of CVE-2026-20796 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-2xf7-hmf6-p64j, GO-2026-4495.