CVE-2019-20866
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 5.12.0. Use of a Proxy HTTP header, rather than the source address in an IP packet header, for obtaining IP address information was mishandled.
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 confidenceMattermost Server before 5.12.0 was trusting Proxy HTTP headers (such as X-Forwarded-For or X-Real-IP) from incoming requests instead of using the actual source IP address from the TCP/IP packet header. This allows remote attackers to spoof their IP address by manipulating these headers, potentially bypassing IP-based rate limiting, access controls, or ban enforcement.
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< 5.12.0CVSS 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
- Low
- Integrity
- None
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/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 checksWork through these to decide whether this CVE applies to you.
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Check Mattermost Server versionRun 'mattermost server version' or 'mmctl version' from the command line, or check the version displayed in the Mattermost UI under System Console > About. Compare the installed version against the affected range: versions before 5.12.0Affected if Installed version is below 5.12.0
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Verify if server is deployed behind a reverse proxy or load balancerInspect your infrastructure setup. Check if a reverse proxy (such as nginx, Apache, HAProxy, AWS ALB, Cloudflare) or load balancer sits in front of the Mattermost Server handling incoming trafficAffected if A reverse proxy or load balancer forwards traffic to Mattermost and may pass X-Forwarded-For or X-Real-IP headers
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Check ProxyHeaders configuration in config.jsonLocate the Mattermost config.json file (typically in /opt/mattermost/config/ or /etc/mattermost/). Look for the 'ProxyHeaders' setting under the 'ServiceSettings' section. If present and enabled, the server trusts incoming proxy headersAffected if ProxyHeaders setting is enabled or not explicitly disabled in the configuration
You are affected if Mattermost Server version is below 5.12.0 and the server trusts proxy headers from requests (commonly when deployed behind a reverse proxy without stripping or restricting those headers).
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.
From vendor data5.12.0
Upgrade to Mattermost Server 5.12.0 or later. Additionally, configure any reverse proxies or load balancers to either strip untrusted proxy headers or restrict trust to known proxy IPs only.
- Consultation2.0 h
- Implementation1.0 h
- Testing2.0 h
- Review / QA1.0 h
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Free · runs locallyCheck whether your project pulls in CVE-2019-20866 — 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-2019-20866 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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