CVE-2026-3473: Improper file ownership validation in the Boards API allows unauthorised file access

Severity: high (CVSS 7.1 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.1% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-05-22 · updated 2026-07-23
Weakness: CWE-639

Description

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

Affected products

Mattermost: from 10.11.0 before 10.11.15; from 11.4.0 before 11.4.5; from 11.5.0 before 11.5.4; from 11.6.0 before 11.6.1 — fixed in 10.11.15, 11.4.5, 11.5.4, 11.6.1

How to fix

Upgrade to a fixed version: Mattermost 10.11.15, 11.4.5, 11.5.4, 11.6.1. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

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