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CVE-2026-64642 – Authentication Bypass – Next.js 16.0.0–16.2.10

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Summary

CVE-2026-64642 is a high-severity authentication bypass vulnerability in Next.js, the widely deployed React web framework maintained by Vercel. Affecting versions 16.0.0 through 16.2.10, the flaw allows unauthenticated remote attackers to craft requests that completely bypass middleware- and proxy-based authentication in applications using the App Router with Turbopack and a single configured i18n locale. The issue was disclosed and patched as part of Vercel’s July 21, 2026 security release, with a CVSS 4.0 score of 8.3 (High).

Technical details

  • Root cause: The middleware request-routing logic in Next.js App Router applications built with Turbopack improperly handles locale configuration when config.i18n.locales contains only a single entry, causing certain request paths to bypass validation and skip the middleware execution chain entirely.
  • Trigger conditions: Exploitation requires all three of the following to be true simultaneously: (1) the application uses the Next.js App Router, (2) it is built with Turbopack (Webpack-based builds are unaffected), and (3) the application has exactly one locale defined in config.i18n.locales.
  • Attack vector: Network — crafted HTTP requests sent remotely, requiring no authentication or user interaction.
  • Impact: Any authentication or security enforcement implemented purely in middleware or a reverse proxy is silently bypassed. This can expose protected routes, admin panels, API endpoints, and sensitive application data to completely unauthenticated attackers.

Affected software

  • next (npm) — versions >= 16.0.0 and < 16.2.11

Severity

  • CVSS 4.0 Score: 8.3 (High)
  • CVSS 4.0 Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:L/VA:N/SC:N/SI:N/SA:N

Mitigation and recommended actions

  • Immediate — apply the vendor patch:
    • Upgrade to Next.js 16.2.11 (v16 Active LTS) or Next.js 15.5.21 (v15 Maintenance LTS) and redeploy.
    npm install [email protected]   # for v16 applications
npm install [email protected]   # for v15 applications
  • If immediate patching is not feasible — workaround:
    • Per the vendor advisory: enforce authorization checks within the server-side data-fetching methods of pages (e.g., in server components or route handlers) rather than relying solely on middleware or proxy-layer authentication. This ensures access control is enforced even if the middleware layer is bypassed.

IONIX Status

The IONIX research team is tracking ongoing exploitation attempts and recommends immediate patching. Potentially affected assets are outlined in this post.

References

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