Vulnerability in Mattermost

[ Description ]Official description

Mattermost versions 11.10.x <= 11.10.0, 11.7.x <= 11.7.9, 11.9.x <= 11.9.1, 11.8.x <= 11.8.5 fail to verify the integrity of the SAML RelayState POST parameter which allows an authenticated SAML user to join any team, including invite-only teams, via forging the team_id or invite_id fields in the RelayState.. Mattermost Advisory ID: MMSA-2026-00727

[ Fixes ]Fixes and patches

No fix or interim patch is linked from our sources yet. Check the vendor references below and limit exposure of affected systems until a fix is available.

[ Advisories ]Government CERT advisories

• BSI CERT-Bund15 Oct 2026WID-SEC-2026-4009

  Mattermost: Multiple vulnerabilities Translated from German

  CVE-2026-18551CVE-2026-16251CVE-2026-19621+11 more CVEs

[ References ]References

• mattermost.com/security-updates