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Mattermost ServerApplication · Mattermost

CVE-2026-3473

HIGH · 7.1 CVSS v3.1 Published 2026-05-22
Fix available
A fix is available. Upgrade to 10.11.15 / 11.4.5 or later.
See remediation →
77/100
Remediation priority · High
Remotely reachable Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

This is an Insecure Direct Object Reference (IDOR) vulnerability in Mattermost's Boards API where authenticated users can access and download files belonging to other users or teams by using valid file IDs in crafted API requests. The application fails to validate file ownership and access control permissions before serving file content.

MitigationApply the vendor-supplied patch to upgrade Mattermost to version 11.6.1, 11.5.4, 11.4.5, or 10.11.15 (or later) to resolve the missing authorization checks in the Boards API file access functions.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Mattermost ServerApplication
Affected:>= 10.11.0, < 10.11.15>= 11.4.0, < 11.4.5>= 11.5.0, < 11.5.4>= 11.6.0, < 11.6.1

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
Low
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
Low
Availability
None

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:N

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify installed Mattermost Server version
    Run 'sudo mattermost version' or check the Mattermost system console under About > Mattermost or inspect the /opt/mattermost/version file
    Affected if The installed version falls within these ranges: >= 10.11.0 and < 10.11.15; >= 11.4.0 and < 11.4.5; >= 11.5.0 and < 11.5.4; >= 11.6.0 and < 11.6.1
  2. Confirm Boards plugin is enabled
    Navigate to System Console > Plugins > Plugin Management or query the plugins API endpoint /api/v4/plugins
    Affected if The Mattermost Boards plugin is installed and enabled (the vulnerability exists only in the Boards API)
  3. Check if file access logging is enabled
    Review Mattermost logs under /var/log/mattermost or via the System Console > Logs, looking for API endpoints under /api/v4/boards/files
    Affected if File access logging is disabled or no historical API logs exist to audit for unauthorized file downloads

You are affected if your Mattermost Server version is within the affected ranges AND the Boards plugin is enabled, as this combination allows the missing file ownership checks to be exploitable.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 10.11.15 / 11.4.5 / 11.5.4 or later
Fixed in 10.11.1511.4.511.5.4
Interim mitigation

Apply the vendor-supplied patch to upgrade Mattermost to version 11.6.1, 11.5.4, 11.4.5, or 10.11.15 (or later) to resolve the missing authorization checks in the Boards API file access functions.

Recommended fix High confidence

Upgrade to 10.11.15 (for 10.11.x), 11.4.5 (for 11.4.x), 11.5.4 (for 11.5.x), or 11.6.1 (for 11.6.x) depending on your current branch

  1. 1. Identify the current Mattermost Server version by checking the system console or running the CLI command: mattermost version
  2. 2. Determine the appropriate upgrade target based on current version: For 10.11.x upgrade to 10.11.15; For 11.4.x upgrade to 11.4.5; For 11.5.x upgrade to 11.5.4; For 11.6.x upgrade to 11.6.1
  3. 3. Back up the Mattermost database and file storage before upgrading
  4. 4. Download the target version from https://mattermost.com/download/ or the official Mattermost repository
  5. 5. Follow the standard Mattermost upgrade procedure: stop the server, replace the binaries, run any database migrations, then restart the server
  6. 6. Verify the upgrade was successful by checking the version in the system console
  7. 7. Test that file access controls work correctly by attempting to access files from another user's workspace (should be denied)
Caveat Standard Mattermost minor version upgrades typically have minimal breaking changes; review the release notes for your target version for any migration notes

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Mattermost Server Scoped from the published advisory
  • Consultation3.0 h
  • Implementation6.0 h
  • Testing4.0 h
  • Review / QA2.0 h
15.0 hours of engineering $2,640
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-3473 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data