CVE-2021-21321 is a critical-severity (CVSS 10) Improper Input Validation vulnerability in fastify-reply-from. O3 Security confirms whether CVE-2021-21321 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
Prefix escape
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, a proxy for how much of the ecosystem is exposed.
fastify-reply-fromnpmDescription
fastify-reply-from is an npm package which is a fastify plugin to forward the current http request to another server. In fastify-reply-from before version 4.0.2, by crafting a specific URL, it is possible to escape the prefix of the proxied backend service. If the base url of the proxied server is "/pub/", a user expect that accessing "/priv" on the target service would not be possible. In affected versions, it is possible. This is fixed in version 4.0.2.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | fastify-reply-from | all versions | 4.0.2 |
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. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update fastify-reply-from to 4.0.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21321 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 pinpoints whether CVE-2021-21321 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2021-21321. 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-2021-21321 in your dependencies?
O3 detects CVE-2021-21321 across npm dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.