CVE-2023-51701 is a medium-severity (CVSS 5.3) CWE-444 vulnerability in @fastify/reply-from. A fix is available for @fastify/reply-from — see the affected versions and patch details below.
@fastify-reply-from JSON Content-Type parsing confusion
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2023-51701.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2023-51701 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 378,156 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
@fastify/reply-fromnpmDescription
Impact
The main repo of fastify use fast-content-type-parse to parse request Content-Type, which will trim after split.
The fastify-reply-from have not use this repo to unify the parse of Content-Type, which won't trim.
As a result, 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.
Patches
@fastify/reply-from v9.6.0 include the fix.
Workarounds
There are no known workarounds.
References
Hackerone Report: https://hackerone.com/reports/2295770.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | @fastify/reply-from | all versions | 9.6.0npm install @fastify/reply-from@9.6.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for @fastify/reply-from, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update @fastify/reply-from to 9.6.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2023-51701 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2023-51701 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2023-51701. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2023-51701 in your dependencies?
O3 Security finds CVE-2023-51701 across npm dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.