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Mattermost ServerApplication · Mattermost

CVE-2025-9076

MEDIUM · 6.5 CVSS v3.1 Published 2025-09-15
Fix available
A fix is available. Upgrade to 10.10.2 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 10.10.x <= 10.10.1 fail to properly sanitize user data during shared channel membership synchronization, which allows malicious or compromised remote clusters to access sensitive user information via unsanitized user objects. This vulnerability affects Mattermost Server instances with shared channels enabled.

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 · moderate confidence

Mattermost versions 10.10.x through 10.10.1 contain a sanitization failure in the shared channel membership synchronization process. When remote clusters synchronize channel membership data, user objects are not properly sanitized before being processed, allowing malicious or compromised remote clusters to inject or expose sensitive user information through unsanitized user objects.

MitigationUpgrade Mattermost Server to version 10.10.2 or later. If immediate upgrade is not possible, consider disabling shared channels or implementing network-level controls to restrict communication with untrusted remote clusters.

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:>= 10.10.0, < 10.10.2

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. Identify installed Mattermost Server version
    Run 'mattermost version' or check the /opt/mattermost/version file, or inspect the Docker image tag if running containerized
    Affected if Version is 10.10.0 or 10.10.1 (versions 10.10.x through 10.10.1 are affected)
  2. Confirm shared channels feature is enabled
    Check the Mattermost configuration file (config.json) for the 'EnableSharedChannels' setting under ServiceSettings, or query the database: SELECT * FROM Systems WHERE Name = 'EnableSharedChannels';
    Affected if EnableSharedChannels is set to true (the vulnerability only applies when shared channels are active)
  3. Verify remote cluster connections exist
    Check the remote_clusters table in the database: SELECT * FROM RemoteClusters; or navigate to System Console > Shared Channels > Remote Clusters in the web UI
    Affected if One or more remote clusters are configured and connected (these are the synchronization endpoints that could be exploited)
  4. Inspect channel synchronization status
    Query the SharedChannel table: SELECT * FROM SharedChannels WHERE RemoteId IS NOT NULL; to see which channels are configured for synchronization with remote clusters
    Affected if Channels are actively being synchronized with remote clusters (indicates the vulnerable synchronization code path is in use)

You are affected if your Mattermost Server version is 10.10.0 or 10.10.1 AND shared channels are enabled with at least one remote cluster 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 10.10.2 or later
Fixed in 10.10.2
Interim mitigation

Upgrade Mattermost Server to version 10.10.2 or later. If immediate upgrade is not possible, consider disabling shared channels or implementing network-level controls to restrict communication with untrusted remote clusters.

Recommended fix High confidence

Mattermost Server 10.10.2

  1. 1. Create a complete backup of your Mattermost Server database and configuration files.
  2. 2. Review the Mattermost 10.10.2 release notes for any known issues or migration requirements.
  3. 3. Upgrade Mattermost Server to version 10.10.2 or later.
  4. 4. After upgrade, verify the server starts successfully and all services are running.
  5. 5. Confirm shared channels functionality is operational by testing synchronization between clusters.
  6. 6. Review system logs for any errors related to shared channel membership synchronization.
Caveat Standard minor version upgrade; review release notes for any configuration changes

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
  • Implementation8.0 h
  • Testing6.0 h
  • Review / QA3.0 h
21.0 hours of engineering $3,680
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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-2025-9076 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