CVE-2026-28736: Focalboard IDOR in file content endpoint allows cross-user file access (unsupported product, no fix)

Severity: medium (CVSS 4.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-04-03 · updated 2026-10-23
Weakness: CWE-639

Description

** 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.

Affected products

Mattermost Focalboard: 8.0.0

References

https://github.com/mattermost-community/focalboard

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