Auth bypass in Mattermost

CVE-2026-3473

Mattermost versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to validate file ownership and access control, which allows an authenticated user to access and download files belonging to other users or teams via crafted Boards API requests using valid file IDs.. Mattermost Advisory ID: MMSA-2026-00620

Vulnerability class: IDOR (Insecure Direct Object Reference)

Published · last modified .

CVSS v3 metric

CVSS v3 base score 5.9 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:L/A:N.

EPSS exploit prediction

EPSS: 0.002 (14.4th percentile), scored .

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 14th percentile — 14.4% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS trend (30 days)EPSS over the last 30 days for CVE-2026-3473: held from 0.001 to 0.002.

EPSS over last 30 days · oldest: 0.001 · newest: 0.002 · change: +0.001

Affected products

Weakness classification (CWE)

References

Frequently asked questions

What is CVE-2026-3473?
CVE-2026-3473 is a medium-severity vulnerability in Mattermost, classified under Authorization Bypass Through User-Controlled Key. CVSS score: 5.9/10. Published 2026-05-22.
How severe is CVE-2026-3473?
Medium severity. CVSS v3 base score is 5.9 out of 10.