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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.
Use CWE-20, Vercel vendor hub and Next.Js product page to widen CVE-2021-43803 into its surrounding weakness, vendor, and product context.
Compare it with CVE-2025-55182, CVE-2025-67779 and CVE-2025-55184 for nearby disclosures in the same product family. Additional editorial context is available in Cybersecurity Weekly Roundup: April 27, 2026 — Critical Zero-Days and Framework Failures.