CVE-2026-9571 Deactivated user accounts can continue to obtain valid OAuth access tokens via refresh token grant in Mattermost
Deactivated users or attackers with refresh tokens can obtain valid access tokens, bypassing account deactivation controls in Mattermost.
- Exploited in the wild
- Not known
- Login needed
- An administrator
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-9571?
CVE-2026-9571 is a medium-severity authentication bypass vulnerability in Mattermost Mattermost Server, published on 13 Jul 2026. An attacker needs administrator access to exploit it, with no action from the victim.
Official description
Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, 10.11.x <= 10.11.19 fail to invalidate OAuth refresh tokens upon user account deactivation, which allows a deactivated user or an attacker in possession of a valid refresh token to obtain new functional access tokens via the OAuth refresh token grant endpoint.. Mattermost Advisory ID: MMSA-2026-00680
Weakness: CWE-305
Which versions are affected by CVE-2026-9571?
It affects 1 product.
Products include Mattermost Mattermost Server.
How do I fix CVE-2026-9571?
Upgrade to a fixed version.
Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.
Is CVE-2026-9571 being exploited?
There is no known exploitation of CVE-2026-9571 in the wild and no public exploit code.
Other vulnerabilities in Mattermost Mattermost Server
- CVE-2025-12419 Account takeover on OAuth/OpenID-enabled servers
- CVE-2025-12421 Account Takeover via Code Exchange Endpoint
- CVE-2025-4981 Path Traversal Leading to RCE by Any Authenticated Mattermost User
- CVE-2025-20051 Arbitrary file read via block duplication in Mattermost Boards
- CVE-2025-25279 Arbitrary file read in Mattermost Boards via import & export board archive
Sources
Also known as GHSA-4v7j-4mvr-5975, MINI-3xgw-xw35-68pj, MINI-62q2-848r-5hmw.