What is CVE-2026-9812?

CVE-2026-9812 is a medium-severity vulnerability in Mattermost, classified under Authorization Bypass Through User-Controlled Key. CVSS score: 6.5/10. Published 2026-09-14.

How severe is CVE-2026-9812?

Medium severity. CVSS v3 base score is 6.5 out of 10.

Auth bypass in Mattermost

CVE-2026-9812

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 that a property field belongs to the specified run before updating its value which allows an authenticated user with run property-management access to crash the Playbooks plugin via a REST request referencing a property field that belongs to a different run. Mattermost Advisory ID: MMSA-2026-00684

Vulnerability class: IDOR (Insecure Direct Object Reference)

CVSS v3 metric

CVSS v3 base score 6.5 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H.

Affected products

Mattermost

Weakness classification (CWE)

CWE-639 — Authorization Bypass Through User-Controlled Key

References

responsibledisclosure@mattermost.com (vendor-advisory)

Frequently asked questions

What is CVE-2026-9812?: CVE-2026-9812 is a medium-severity vulnerability in Mattermost, classified under Authorization Bypass Through User-Controlled Key. CVSS score: 6.5/10. Published 2026-09-14.
How severe is CVE-2026-9812?: Medium severity. CVSS v3 base score is 6.5 out of 10.