CVE-2021-43803 is a high-severity (CVSS 7.5) Improper Input Validation vulnerability in next. O3 Security confirms whether CVE-2021-43803 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
Unexpected server crash in Next.js.
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.
nextnpmDescription
Next.js is a React framework. In versions of Next.js prior to 12.0.5 or 11.1.3, invalid or malformed URLs could lead to a server crash. In order to be affected by this issue, the deployment must use Next.js versions above 11.1.0 and below 12.0.5, Node.js above 15.0.0, and next start or a custom server. Deployments on Vercel are not affected, along with similar environments where invalid requests are filtered before reaching Next.js. Versions 12.0.5 and 11.1.3 contain patches for this issue.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | next | ≥ 12.0.0&&< 12.0.5 | 12.0.5 |
| 📦npm | next | ≥ 0.9.9&&< 11.1.3 | 11.1.3 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for next. 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 next to 12.0.5 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-43803 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-2021-43803 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-2021-43803. 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-2021-43803 in your dependencies?
O3 detects CVE-2021-43803 across npm dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.