CVE-2017-18920
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 3.6.2. The WebSocket feature does not follow the Same Origin Policy.
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 Server before version 3.6.2 has a WebSocket implementation that fails to enforce the Same Origin Policy, allowing cross-origin WebSocket connections. This vulnerability could enable cross-site WebSocket hijacking (CSWSH) attacks where a malicious website can establish authenticated WebSocket connections to the Mattermost server on behalf of an authenticated user.
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< 3.6.2CVSS 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 installed Mattermost Server versionCheck the version by logging into the Mattermost System Console and navigating to About > Version, or check the version.json file in the Mattermost installation directory.Affected if The installed version is lower than 3.6.2 (e.g., 3.6.0, 3.5.x, etc.)
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Verify WebSocket endpoint is accessibleAttempt to establish a WebSocket connection to the Mattermost server at /api/v3/websocket or /ws/api/v3/websocket using a WebSocket client or curl with the Upgrade header.Affected if WebSocket connections can be established to the Mattermost server (the vulnerability only applies if WebSocket is enabled and in use)
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Check for origin header validation in WebSocket handshakeInspect the Mattermost server configuration files (config.json) and WebSocket handler source code for any origin validation logic during the WebSocket upgrade process. Alternatively, test by sending a WebSocket handshake request with an Origin header pointing to a different domain.Affected if The WebSocket handshake accepts connections from arbitrary origins without validating the Origin header, allowing cross-origin connections
You are affected if your Mattermost Server version is below 3.6.2 AND WebSocket connections are enabled/operational on your server.
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 · scoped3.6.2
Implement proper origin header validation during the WebSocket handshake process to ensure only legitimate origins can establish connections. Update to version 3.6.2 or later which contains the fix.
3.6.2
- Backup the Mattermost Server database and configuration files
- Stop the Mattermost Server service
- Upgrade Mattermost Server to version 3.6.2 or later
- Verify the WebSocket functionality is working correctly after upgrade
- Restart the Mattermost Server service
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation4.0 h
- Implementation8.0 h
- Testing6.0 h
- Review / QA3.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $5,888.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2017-18920 — 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-2017-18920 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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