Open Redirect in Mattermost
CVE-2026-12985
Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7 Mattermost failed to validate Dynamic Client Registration redirect URIs by URL component (matching glob patterns against the raw URI string instead) which allows a remote unauthenticated attacker to register an OAuth client with an attacker-controlled callback host that bypasses the configured redirect URI allowlist via a crafted redirect URI that places an allowlisted host/path suffix inside the query string.. Mattermost Advisory ID: MMSA-2026-00700
Vulnerability class: Open Redirect
CVSS v3 metric
CVSS v3 base score 6.8 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:N.
Affected products
Weakness classification (CWE)
References
- responsibledisclosure@mattermost.com (vendor-advisory)
Frequently asked questions
- What is CVE-2026-12985?
- CVE-2026-12985 is a medium-severity vulnerability in Mattermost, classified under URL Redirection to Untrusted Site (Open Redirect). CVSS score: 6.8/10. Published 2026-09-14.
- How severe is CVE-2026-12985?
- Medium severity. CVSS v3 base score is 6.8 out of 10.