CVE-2023-51701
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 · uneditedfastify-reply-from is a Fastify plugin to forward the current HTTP request to another server. A reverse proxy server built with `@fastify/reply-from` could misinterpret the incoming body by passing an header `ContentType: application/json ; charset=utf-8`. This can lead to bypass of security checks. This vulnerability has been patched in '@fastify/reply-from` version 9.6.0.
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 confidenceThe @fastify/reply-from plugin for Fastify incorrectly parses the Content-Type header when set to 'application/json ; charset=utf-8' (with a space before the semicolon). This malformed header causes the plugin to misinterpret the incoming HTTP body, allowing attackers to bypass security checks that rely on proper Content-Type validation.
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< 9.6.0CVSS 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
- None
- Integrity
- High
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/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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Verify @fastify/reply-from is installedCheck your package.json dependencies or run 'npm list @fastify/reply-from' to see if the plugin is present in your projectAffected if @fastify/reply-from appears in your dependencies
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Check the installed versionRun 'npm list @fastify/reply-from' or inspect node_modules/@fastify/reply-from/package.json to find the exact version numberAffected if The version is lower than 9.6.0
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Identify Content-Type header processingSearch your codebase for places where incoming request Content-Type headers are validated or used for security decisions (e.g., content-type checks, allowlist logic, parsing decisions)Affected if Your code performs security checks based on the Content-Type header value
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Test for vulnerable header parsingSend a request with 'Content-Type: application/json ; charset=utf-8' (note the space before the semicolon) and observe whether your application accepts it as JSON or rejects itAffected if The malformed header bypasses your Content-Type validation or is processed as valid JSON when it should not be
You are affected if @fastify/reply-from version is below 9.6.0 AND your application performs security checks on Content-Type headers that could be bypassed by the space-before-semicolon pattern.
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 · scoped9.6.0
Upgrade @fastify/reply-from to version 9.6.0 or later to patch the Content-Type parsing vulnerability. Review all incoming request handling code to ensure proper header validation is in place.
9.6.0
- Upgrade @fastify/reply-from to version 9.6.0 or later using your package manager (e.g., npm install @fastify/reply-from@^9.6.0 or yarn add @fastify/reply-from@^9.6.0)
- Test the upgrade in a staging environment to ensure the reverse proxy functionality works correctly
- Deploy the updated version to production
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation2.0 h
- Implementation1.0 h
- Testing3.0 h
- Review / QA2.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2023-51701 — 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-2023-51701 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
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