What is CVE-2026-5139?

CVE-2026-5139 is a medium-severity vulnerability in Mattermost, classified under Missing Authorization. CVSS score: 5.4/10. Published 2026-06-22.

How severe is CVE-2026-5139?

Medium severity. CVSS v3 base score is 5.4 out of 10.

Auth bypass in Mattermost

CVE-2026-5139

Mattermost versions 11.7.x <= 11.7.0, 11.6.x <= 11.6.2, 11.5.x <= 11.5.5, 10.11.x <= 10.11.17 fail to enforce administrator authorization on the {{setDefaultInstance}} call within the {{/gitlab connect}} command handler, which allows any a…

Vulnerability class: Broken Access Control

CVSS v3 metric

CVSS v3 base score 5.4 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:L.

Affected products

Mattermost — versions 11.7.0, 11.6.0, 11.5.0

Weakness classification (CWE)

CWE-862 — Missing Authorization

References

responsibledisclosure@mattermost.com (vendor-advisory)

Frequently asked questions

What is CVE-2026-5139?: CVE-2026-5139 is a medium-severity vulnerability in Mattermost, classified under Missing Authorization. CVSS score: 5.4/10. Published 2026-06-22.
How severe is CVE-2026-5139?: Medium severity. CVSS v3 base score is 5.4 out of 10.