What is CVE-2026-8821?

CVE-2026-8821 is a high-severity vulnerability in Mattermost, classified under Missing Authorization. CVSS score: 7.1/10. Published 2026-09-14.

How severe is CVE-2026-8821?

High severity. CVSS v3 base score is 7.1 out of 10.

Auth bypass in Mattermost

CVE-2026-8821

Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7, 10.11.x <= 10.11.22 fail to validate channel member-management permission during playbook run creation, allowing an authenticated channel member to add an arbitrary user to a restricted channel via the run owner field.. Mattermost Advisory ID: MMSA-2026-00677

Vulnerability class: Broken Access Control

CVSS v3 metric

CVSS v3 base score 7.1 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:N.

Affected products

Mattermost

Weakness classification (CWE)

CWE-862 — Missing Authorization

References

responsibledisclosure@mattermost.com (vendor-advisory)

Frequently asked questions

What is CVE-2026-8821?: CVE-2026-8821 is a high-severity vulnerability in Mattermost, classified under Missing Authorization. CVSS score: 7.1/10. Published 2026-09-14.
How severe is CVE-2026-8821?: High severity. CVSS v3 base score is 7.1 out of 10.