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Mattermost ServerApplication · Mattermost

CVE-2026-6340

MEDIUM · 6.5 CVSS v3.1 Published 2026-05-18
Fix available
A fix is available. Upgrade to 10.11.14 / 11.4.4 or later.
See remediation →
71/100
Remediation priority · Elevated
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 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 fail to validate 7zip archive structure before processing which allows an authenticated attacker to cause server memory exhaustion and denial of service via uploading a specially crafted 7zip file with excessive folder declarations.. Mattermost Advisory ID: MMSA-2026-00573

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

Mattermost fails to validate 7zip archive structure before processing, allowing authenticated attackers to upload specially crafted 7zip files containing excessive folder declarations that cause server memory exhaustion and denial of service.

MitigationUpgrade Mattermost to versions 11.5.2+, 10.11.14+, or 11.4.4+ which include proper 7zip archive structure validation. Until patched, restrict or disable 7zip file uploads in the system.

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, < 10.11.14>= 11.4.0, < 11.4.4>= 11.5.0, < 11.5.2

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
None
Integrity
None
Availability
High

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

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. Check Mattermost Server version
    Access the system console or run 'grep Version /opt/mattermost/config/config.json' or check the About section in the system console. Compare the reported version against the affected ranges: 10.11.0 through 10.11.13, 11.4.0 through 11.4.3, and 11.5.0 through 11.5.1.
    Affected if The installed version falls within any of these ranges: >=10.11.0 <10.11.14, >=11.4.0 <11.4.4, or >=11.5.0 <11.5.2.
  2. Verify if file uploads are enabled
    Navigate to System Console > Environment > File Storage, or inspect the config.json file for 'EnableFileAttachments' setting set to true.
    Affected if File uploads are enabled (EnableFileAttachments is true).
  3. Confirm 7zip file type is allowed in uploads
    Navigate to System Console > Environment > File Storage > Allowable Files, or inspect the config for 'AllowedFileExtensions' to see if '7z' or '7zip' is listed.
    Affected if The 7zip extension (7z or 7zip) is included in allowed file extensions.
  4. Check if authenticated users can upload files
    Navigate to System Console > Users and Roles > Permissions, or check if the system allows file uploads from team members/ authenticated users without additional restrictions.
    Affected if Authenticated users have the ability to upload files to the server.

You are affected if your Mattermost Server version is in the vulnerable range AND 7zip file uploads are enabled for authenticated users.

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 10.11.14 / 11.4.4 / 11.5.2 or later
Fixed in 10.11.1411.4.411.5.2
Interim mitigation

Upgrade Mattermost to versions 11.5.2+, 10.11.14+, or 11.4.4+ which include proper 7zip archive structure validation. Until patched, restrict or disable 7zip file uploads in the system.

Recommended fix High confidence

Upgrade to 10.11.14 (if on 10.11.x), 11.4.4 (if on 11.4.x), or 11.5.2 (if on 11.5.x)

  1. 1. Back up your Mattermost Server database and configuration files before upgrading.
  2. 2. Identify your current Mattermost Server version by checking the System Console or running `mattermost version`.
  3. 3. If running 10.11.x (10.11.0 through 10.11.13), upgrade to version 10.11.14 or later.
  4. 4. If running 11.4.x (11.4.0 through 11.4.3), upgrade to version 11.4.4 or later.
  5. 5. If running 11.5.x (11.5.0 through 11.5.1), upgrade to version 11.5.2 or later.
  6. 6. Follow Mattermost's standard upgrade procedure for your deployment method (e.g., Ansible, Docker, Kubernetes, or manual).
  7. 7. After upgrade, verify the server is running and accessible.
  8. 8. Test that file upload functionality works correctly with 7zip archives to confirm the fix.
Caveat Check the relevant Mattermost version release notes for any breaking changes between your current version and the target fixed version

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

Fix this in Mattermost Server Scoped from the published advisory
  • Consultation4.0 h
  • Implementation12.0 h
  • Testing6.0 h
  • Review / QA3.0 h
25.0 hours of engineering $4,400
Get the upgrade done

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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-2026-6340 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