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Next.js is a React framework. Starting with version 12.0.0 and prior to version 12.0.9, vulnerable code could allow a bad actor to trigger a denial of service attack for anyone using i18n functionality. In order to be affected by this CVE, one must use next start or a custom server and the built-in i18n support. Deployments on Vercel, along with similar environments where invalid requests are filtered before reaching Next.js, are not affected. A patch has been released, `next@12.0.9`, that mitigates this issue. As a workaround, one may ensure `/${locale}/_next/` is blocked from reaching the Next.js instance until it becomes feasible to upgrade.
Use Vercel vendor hub and Next.Js product page to widen CVE-2022-21721 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.