CVE-2026-9597 Deactivated guest accounts can authenticate via magic-link token in Mattermost REST API login endpoint
Deactivated guest users can bypass authentication controls to obtain active sessions via magic-link tokens in Mattermost.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-9597?
CVE-2026-9597 is a medium-severity authentication bypass vulnerability in Mattermost Mattermost Server, published on 13 Jul 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Official description
Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4 fail to verify whether a guest account is deactivated before creating a session in the magic-link token login path, which allows a deactivated guest user to obtain a fully functional session via a magic-link token issued prior to deactivation.. Mattermost Advisory ID: MMSA-2026-00681
Weakness: CWE-305
Which versions are affected by CVE-2026-9597?
It affects 1 product.
Products include Mattermost Mattermost Server.
How do I fix CVE-2026-9597?
No fix has been published for CVE-2026-9597 yet. Limit exposure of the affected software until one is available.
Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.
Is CVE-2026-9597 being exploited?
There is no known exploitation of CVE-2026-9597 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-42ff-p8fr-h5jr.