Is CVE-2026-18551 being exploited?

No public exploit is currently tracked.

How severe is CVE-2026-18551?

CVE-2026-18551 has a CVSS v3 severity of MEDIUM (6.5).

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CVE-2026-18551

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