CVE-2018-21251
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 · uneditedAn issue was discovered in Mattermost Server before 5.2 and 5.1.1. Authorization could be bypassed if the channel name were not the same in the params and the body.
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 confidenceAuthorization bypass vulnerability in Mattermost Server where channel access validation can be circumvented by providing mismatched channel names in request parameters versus request body, allowing unauthorized users to access channels they shouldn't have permission to view.
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< 5.1.1= 5.2.0CVSS 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
- High
- Integrity
- High
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/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 checksWork through these to decide whether this CVE applies to you.
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Determine Mattermost Server versionLog into the Mattermost system console or check the /api/v4/system endpoint for the version information, or run 'grep -i version' in the Mattermost installation directoryAffected if Version is less than 5.1.1 or equals exactly 5.2.0
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Verify channel access feature is in useConfirm the Mattermost deployment accepts API requests to channel-related endpoints (such as /api/v4/channels) - this is a core feature enabled by default in any running instanceAffected if The server processes channel API requests from users
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Check for mismatched channel name requestsReview web server and API logs for requests where the channel name in the URL parameter differs from the channel name in the request body, focusing on POST and PUT requests to channel endpointsAffected if Such mismatched requests exist in the logs and were processed successfully (indicating the bypass may have occurred)
You are affected if your Mattermost Server version is less than 5.1.1 or exactly 5.2.0, and the server processes channel API requests (which is default behavior).
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 · scoped5.1.1
Upgrade Mattermost Server to version 5.2, 5.1.1, or later to patch the authorization bypass vulnerability.
Mattermost Server 5.1.1 or later (for 5.1.x branch); or 5.2.1 or later (for 5.2.x branch)
- 1. Back up your current Mattermost Server database and configuration files.
- 2. Stop the Mattermost Server service.
- 3. Upgrade to Mattermost Server version 5.1.1 (if on 5.1.x branch) or version 5.2.1 or later (if on 5.2.x branch).
- 4. Verify the upgrade by checking the server version in System Console > About.
- 5. Restart the Mattermost Server service.
- 6. Test that users can only access channels they are authorized to access.
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation4.0 h
- Implementation8.0 h
- Testing6.0 h
- Review / QA4.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $6,176.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2018-21251 — 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-2018-21251 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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