CVE-2026-18551 Mattermost guest account privilege escalation via member roles API
An authenticated attacker can escalate a guest account to team or channel admin by exploiting insufficient role validation in the member roles API.
- Exploited in the wild
- Not known
- Login needed
- An administrator
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-18551?
CVE-2026-18551 is a medium-severity vulnerability in Mattermost Mattermost Server, published on 16 Oct 2026. An attacker needs administrator access to exploit it, with no action from the victim.
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 validate the combination of guest and admin scheme roles which allows an authenticated attacker to escalate a guest account to team or channel admin via a crafted PUT request to the member roles API endpoint.. Mattermost Advisory ID: MMSA-2026-00752
Weakness: CWE-863
Which versions are affected by CVE-2026-18551?
It affects 1 product.
Products include Mattermost Mattermost Server.
How do I fix CVE-2026-18551?
No fix has been published for CVE-2026-18551 yet. Limit exposure of the affected software until one is available.
Is CVE-2026-18551 being exploited?
There is no known exploitation of CVE-2026-18551 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-rh7p-pp3q-6cf8.