CVE-2026-18551

Medium

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

SeverityMedium

CVSS6.5

Exploit probability—

Product

Mattermost Server

PublishedOct 16, 2026

First seenOct 16, 2026

Weakness typeCWE-863Incorrect authorization

How to fix

Update

mattermost mattermost_server11.7.10, 11.8.6, 11.9.2 or later

Vendor advisories

mattermost.com/security-updates

Affected products

Product	Affected versions	Fixed in
mattermost mattermost_server	11.7.0 ≤ v < 11.7.10, 11.8.0 ≤ v < 11.8.6, 11.9.0 ≤ v < 11.9.2, 11.10.0	11.7.10, 11.8.6, 11.9.2

Check your version

Choose the product and enter the version you run to see whether it falls in the affected range.

In the Intelligence 360 plan

Attack Surface Management

See the assets tied to your domains, the technologies they run and known vulnerabilities in one list.