What is CVE-2026-21386?

CVE-2026-21386 is a medium-severity vulnerability in Mattermost, classified under Observable Discrepancy. CVSS score: 4.3/10. Published 2026-03-16.

How severe is CVE-2026-21386?

Medium severity. CVSS v3 base score is 4.3 out of 10.

Vulnerability in Mattermost

CVE-2026-21386

Mattermost versions 11.3.x <= 11.3.0, 11.2.x <= 11.2.2, 10.11.x <= 10.11.10 fail to use consistent error responses when handling the /mute command which allows an authenticated team member to enumerate private channels they are not authorized to know about via differing error messages for nonexistent versus private channels. Mattermost Advisory ID: MMSA-2026-00588

EPSS: 0.002 (8.3th percentile) — read the EPSS interpretation.

CVSS v3 metric

CVSS v3 base score 4.3 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N.

Affected products

Mattermost — versions 11.3.0, 11.2.0, 10.11.0
Mattermost Mattermost_server

Weakness classification (CWE)

CWE-203 — Observable Discrepancy

References

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

Frequently asked questions

What is CVE-2026-21386?: CVE-2026-21386 is a medium-severity vulnerability in Mattermost, classified under Observable Discrepancy. CVSS score: 4.3/10. Published 2026-03-16.
How severe is CVE-2026-21386?: Medium severity. CVSS v3 base score is 4.3 out of 10.