Medium · published September 14, 2026
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
| Product | Versions | Fixed in |
|---|---|---|
| mattermost/mattermost_server | ≥ 11.7.0, < 11.7.8 | 11.7.8 |
| mattermost/mattermost_server | ≥ 11.8.0, < 11.8.5 | 11.8.5 |
| mattermost/mattermost_server | ≥ 11.9.0, < 11.9.1 | 11.9.1 |