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
CVE-2026-18551
CVSS base score
6.5 (v3.1)
Severity
MEDIUM
Attack vector
Network
Weakness
CWE-863
Last modified
October 29, 2026
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:N
Affected products
mattermost mattermost server
References
- mattermost.com Vendor Advisory