Auth bypass in Mattermost

CVE-2026-18223

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 invited recipient's email address when redeeming team and guest invitation tokens, which allows an authenticated attacker to join a team or gain unauthorized access to private channels via a forwarded or leaked invitation link.. Mattermost Advisory ID: MMSA-2026-00747

Vulnerability class: Broken Access Control

Published · last modified .

CVSS v3 metric

CVSS v3 base score 5.0 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:H/UI:N/S:U/C:H/I:L/A:N.

EPSS exploit prediction

No EPSS score from FIRST.org yet. New CVEs usually get one within a few days; no score means no signal yet, not low risk. How EPSS works.

Affected products

Weakness classification (CWE)

References

Frequently asked questions

What is CVE-2026-18223?
CVE-2026-18223 is a medium-severity vulnerability in Mattermost, classified under Incorrect Authorization. CVSS score: 5.0/10. Published 2026-10-16.
How severe is CVE-2026-18223?
Medium severity. CVSS v3 base score is 5.0 out of 10.