CVE-2026-20796: Mattermost doesn't properly validate channel membership at the time of data retrieval

February 13, 2026

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

References

github.com/advisories/GHSA-2xf7-hmf6-p64j
github.com/mattermost/mattermost
mattermost.com/security-updates
nvd.nist.gov/vuln/detail/CVE-2026-20796

Code Behaviors & Features

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