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Mattermost ServerApplication · Mattermost

CVE-2026-5308

HIGH · 7.5 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 →
84/100
Remediation priority · High
Remotely reachable No privileges 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 enforce request body size limits on plugin HTTP endpoints which allows an attacker to cause a denial of service via crafted oversized HTTP requests.. Mattermost Advisory ID: MMSA-2026-00646

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 enforce HTTP request body size limits on plugin API endpoints, allowing attackers to send crafted oversized requests that exhaust server resources and cause denial of service.

MitigationUpgrade Mattermost to patched versions (11.6.1, 11.5.4, 11.4.5, 10.11.15 or later) or implement request body size limits at the reverse proxy/load balancer level for plugin endpoints.

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
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/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
    Run `mattermost version` or inspect the version string in the system console under About > Mattermost, or check the Docker/container image tag if using containers
    Affected if The installed version falls within these ranges: 10.11.0 through 10.11.14, 11.4.0 through 11.4.4, 11.5.0 through 11.5.3, or 11.6.0
  2. Identify plugin HTTP endpoints in use
    Review the Mattermost configuration file (config.json) for enabled plugins with HTTP APIs, or list installed plugins via the system console under Plugin Management
    Affected if Any custom or third-party plugins with exposed HTTP endpoints are installed and active
  3. Confirm plugin API access is enabled
    Check the config.json for `PluginSettings` > `Enable` is set to true, or verify plugins can make outbound HTTP requests via `PluginSettings` > `AllowedUntrustedInternalConnections`
    Affected if Plugins with HTTP endpoint capabilities are enabled without additional request size constraints

You are affected if your Mattermost Server version is 10.11.0-10.11.14, 11.4.0-11.4.4, 11.5.0-11.5.3, or 11.6.0 and you have plugins with HTTP endpoints enabled without explicit request body size limits configured.

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

Upgrade Mattermost to patched versions (11.6.1, 11.5.4, 11.4.5, 10.11.15 or later) or implement request body size limits at the reverse proxy/load balancer level for plugin endpoints.

Recommended fix High confidence

Upgrade to the latest version in your current branch: 10.11.15+, 11.4.5+, 11.5.4+, or 11.6.1+ (recommended: latest stable release)

  1. 1. Identify the currently installed Mattermost Server version by checking the system settings or running 'mattermost version'
  2. 2. Determine which version branch you are currently on (10.11.x, 11.4.x, 11.5.x, or 11.6.x)
  3. 3. For 10.11.x branch: Upgrade to version 10.11.15 or later
  4. 4. For 11.4.x branch: Upgrade to version 11.4.5 or later
  5. 5. For 11.5.x branch: Upgrade to version 11.5.4 or later
  6. 6. For 11.6.x branch: Upgrade to version 11.6.1 or later
  7. 7. After upgrade, verify that plugin HTTP endpoints now enforce request body size limits
  8. 8. Test that legitimate plugin functionality continues to work correctly after the patch
Caveat Standard Mattermost upgrade considerations apply - review release notes for any configuration or feature changes between versions

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

Fix this in Mattermost Server Scoped from the published advisory
  • Consultation2.0 h
  • Implementation4.0 h
  • Testing3.0 h
  • Review / QA2.0 h
11.0 hours of engineering $1,930
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $3,088.

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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-2026-5308 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