Auth bypass in Mattermost Focalboard

CVE-2026-28736

** UNSUPPORTED WHEN ASSIGNED ** Focalboard version 8.0 fails to validate file ownership when serving uploaded files. This allows an authenticated attacker who knows a victim's fileID to read the content of the file. NOTE: Focalboard as a standalone product is not maintained and no fix will be issued.

Vulnerability class: IDOR (Insecure Direct Object Reference)

Published · last modified .

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.

EPSS exploit prediction

EPSS: 0.003 (17.3th percentile), scored .

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 17th percentile — 17.3% 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-2026-28736: held from 0.002 to 0.003.

EPSS over last 30 days · oldest: 0.002 · newest: 0.003 · change: +0.000

Affected products

Weakness classification (CWE)

References

Frequently asked questions

What is CVE-2026-28736?
CVE-2026-28736 is a medium-severity vulnerability in Mattermost Focalboard, classified under Authorization Bypass Through User-Controlled Key. CVSS score: 4.3/10. Published 2026-04-03.
How severe is CVE-2026-28736?
Medium severity. CVSS v3 base score is 4.3 out of 10.