Vulnerability · Published May 22, 2026 · Assigned by Mattermost
CVE-2026-3473Improper file ownership validation in the Boards API allows unauthorised file access
Medium 5.9
In plain English
A medium-severity authorization bypass via user-controlled key in Mattermost from Mattermost. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.1% (higher than 4% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official 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 packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Go | github.com/mattermost/mattermost-server | 11.6.1 | GHSA-7pf2-9c95-w332 |
| Go | github.com/mattermost/mattermost-server | 11.5.4 | GHSA-7pf2-9c95-w332 |
| Go | github.com/mattermost/mattermost-server | 11.4.5 | GHSA-7pf2-9c95-w332 |
| Go | github.com/mattermost/mattermost-server | 10.11.15 | GHSA-7pf2-9c95-w332 |
| Go | github.com/mattermost/mattermost-server | 10.11.15+incompatible, 11.4.5+incompatible, 11.5.4+incompatible, 11.6.1+incompatible | GO-2026-5817 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| Mattermost | Mattermost | 11.6.0 – through 11.6.011.5.0 – through 11.5.311.4.0 – through 11.4.410.11.0 – through 10.11.14 |
Weakness: CWE-639 · Vector: CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:L/A:N
References
- MMSA-2026-00620 vendor-advisory