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Mattermost ServerApplication · Mattermost

CVE-2026-5163

MEDIUM · 6.5 CVSS v3.1 Published 2026-05-18
Fix available
A fix is available. Upgrade to 11.5.2 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 fail to verify channel membership when processing AI-assisted message rewrites which allows an authenticated attacker to read the content of threads in private channels and direct messages they do not have access to via a crafted request to the post rewrite endpoint.. Mattermost Advisory ID: MMSA-2026-00645

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 versions 11.5.x through 11.5.1 fail to perform channel membership validation in the AI-assisted message rewrite endpoint. An authenticated attacker can craft requests to the post rewrite endpoint to read thread content from private channels and direct messages they are not a member of, bypassing access controls.

MitigationUpgrade Mattermost to a version beyond 11.5.1 when available. Alternatively, apply vendor-supplied patches and verify the post rewrite endpoint now correctly enforces channel membership authorization before returning message content.

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:>= 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
High
Integrity
None
Availability
None

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/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. Check Mattermost Server version
    Run 'mmctl version' or check the System Console under About > Mattermost, or inspect the /opt/mattermost/version.txt file if manually installed
    Affected if The installed version is 11.5.0, 11.5.1, or falls within the range >= 11.5.0 and < 11.5.2
  2. Verify AI plugin or assistant feature is enabled
    Navigate to System Console > Plugins > Plugin Management, or check /opt/mattermost/plugins directory for the mattermost-ai-plugin or similar AI integration
    Affected if An AI plugin or the native AI assistant feature is installed and enabled, as the vulnerability exists in the AI-assisted message rewrite endpoint
  3. Review audit logs for post rewrite API access
    Check Mattermost audit logs (System Console > Audit Logs) for POST requests to the '/api/v4/posts/.*/rewrite' or AI rewrite endpoint from users accessing channels they are not members of
    Affected if Unexpected API calls to the rewrite endpoint show access to private channels or DMs where the requesting user is not a member

You are affected if your Mattermost Server version is 11.5.0 or 11.5.1 AND you have any AI/assistant plugin or feature enabled that provides the message rewrite functionality.

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 11.5.2 or later
Fixed in 11.5.2
Interim mitigation

Upgrade Mattermost to a version beyond 11.5.1 when available. Alternatively, apply vendor-supplied patches and verify the post rewrite endpoint now correctly enforces channel membership authorization before returning message content.

Recommended fix High confidence

11.5.2

  1. 1. Backup your Mattermost database and configuration files before proceeding with the upgrade.
  2. 2. Review the Mattermost 11.5.2 release notes for any known issues or migration requirements.
  3. 3. Upgrade Mattermost Server to version 11.5.2 using your preferred installation method (e.g., apt/yum for Linux, Docker, or manual upgrade).
  4. 4. After upgrade, verify that the server starts successfully and all services are running.
  5. 5. Test that users can still authenticate and access their channels normally.
  6. 6. Confirm the fix by attempting (in a test environment) the previously vulnerable endpoint to verify authorization is now properly enforced.
Caveat Review the 11.5.2 release notes for any breaking changes or deprecations before upgrading production systems

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-2026-5163 in production — separate from our analysis above.

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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