What is CVE-2026-5163?

CVE-2026-5163 is a medium-severity vulnerability in Mattermost, classified under Missing Authorization. CVSS score: 6.5/10. Published 2026-05-18.

How severe is CVE-2026-5163?

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

Auth bypass in Mattermost

CVE-2026-5163

Mattermost versions 11.5.x <= 11.5.1 fail to verify channel membership when processing AI-assisted message rewrites which allows an authenticated attacker to read the content of threads in private channels and direct messages they do not h…

Vulnerability class: Broken Access Control

EPSS: 0.000 (13.6th percentile) — read the EPSS interpretation.

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:H/I:N/A:N.

Affected products

Mattermost — versions 11.5.0, 11.6.0, 11.5.2
Mattermost Mattermost_server

Weakness classification (CWE)

CWE-862 — Missing Authorization

References

responsibledisclosure@mattermost.com (vendor-advisory, Vendor Advisory)

Frequently asked questions

What is CVE-2026-5163?: CVE-2026-5163 is a medium-severity vulnerability in Mattermost, classified under Missing Authorization. CVSS score: 6.5/10. Published 2026-05-18.
How severe is CVE-2026-5163?: Medium severity. CVSS v3 base score is 6.5 out of 10.