CVE-2021-43803

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. Note that prior version 0.9.9 package next hosted a different utility (0.4.1 being the latest version of that codebase), and this advisory does not apply to those versions.

The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths. Typical impact: varies by context: data corruption, logic bypass, or denial of service.

CVE-2021-43803 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (12.0.5, 11.1.3); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2021-43803? CVE-2021-43803 is a high-severity improper input validation vulnerability in next (npm), affecting versions >= 12.0.0, < 12.0.5. It is fixed in 12.0.5, 11.1.3. The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths.
2. How severe is CVE-2021-43803? CVE-2021-43803 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of next are affected by CVE-2021-43803? next (npm) versions >= 12.0.0, < 12.0.5 is affected.
4. Is there a fix for CVE-2021-43803? Yes. CVE-2021-43803 is fixed in 12.0.5, 11.1.3. Upgrade to this version or later.
5. Is CVE-2021-43803 exploitable, and should I be worried? Whether CVE-2021-43803 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2021-43803 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2021-43803?
   • Upgrade next to 12.0.5 or later
   • Upgrade next to 11.1.3 or later