CVE-2026-4274
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 · uneditedMattermost versions 11.2.x <= 11.2.2, 10.11.x <= 10.11.10, 11.4.x <= 11.4.0, 11.3.x <= 11.3.1 fail to restrict team-level access when processing membership sync from a remote cluster, which allows a malicious remote cluster to grant a user access to an entire private team instead of only the shared channel via sending crafted membership sync messages that trigger team membership assignment. Mattermost Advisory ID: MMSA-2026-00574
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 confidenceMattermost fails to enforce proper team-level access controls during membership synchronization from remote clusters. A malicious remote cluster can send crafted membership sync messages that incorrectly assign users to entire private teams, bypassing the intended restriction where users should only gain access to shared channels.
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>= 10.11.0, < 10.11.11>= 11.2.0, < 11.2.3>= 11.3.0, < 11.3.2>= 11.4.0, < 11.4.1CVSS 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
- Low
- Integrity
- Low
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/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 checksWork through these to decide whether this CVE applies to you.
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Check Mattermost Server versionAccess the Mattermost System Console or run the command: `mattermost version` or check the /api/v4/system/info endpointAffected if The installed version falls within any of these ranges: 10.11.0-10.11.10, 11.2.0-11.2.2, 11.3.0-11.3.1, or 11.4.0-11.4.0
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Verify remote cluster feature statusNavigate to System Console > Integrations > Remote Clusters, or query the /api/v4/remote_clusters endpointAffected if Remote clusters are enabled and at least one remote cluster connection is active
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Confirm membership sync is configuredCheck the remote cluster settings for each active cluster, looking for the 'Sync Member' or 'Membership Sync' option being enabledAffected if Membership synchronization is turned on for any remote cluster connection
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Audit recent team membership changesReview Mattermost audit logs or check the Team Members page in System Console for any unexpected private team memberships, particularly from users associated with remote cluster syncAffected if Users have been added to private teams without manual invitation, especially if those users are linked to remote cluster synchronization activities
You are affected if your Mattermost Server version is within the vulnerable ranges AND remote clusters with membership sync are enabled, and unauthorized private team memberships exist.
Generated from the published advisory. Verify against your own configuration.
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 · scoped10.11.1111.2.311.3.2
Upgrade to patched Mattermost versions when released, or disable/restrict remote cluster membership sync until patches are available. Monitor for unauthorized team membership grants.
Minimum fixed versions: 10.11.11, 11.2.3, 11.3.2, or 11.4.1 (choose based on your current major.minor branch)
- Upgrade Mattermost Server to version 10.11.11 or later for the 10.11.x branch
- Upgrade Mattermost Server to version 11.2.3 or later for the 11.2.x branch
- Upgrade Mattermost Server to version 11.3.2 or later for the 11.3.x branch
- Upgrade Mattermost Server to version 11.4.1 or later for the 11.4.x branch
- After upgrading, verify that the team membership sync from remote clusters now correctly restricts access to shared channels only and does not grant full private team membership
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation3.0 h
- Implementation12.0 h
- Testing6.0 h
- Review / QA3.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-4274 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2026-4274 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- 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
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