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Authentication Bypass vulnerability in Zeit Next.Js CVE-2025-29927

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Summary

CVE-2025-29927 is a critical authorization bypass vulnerability in Next.js. Applications may be affected when authorization checks are implemented in Next.js middleware, allowing a remote unauthenticated attacker to bypass those checks.

Technical details

  • Root cause: improper authorization handling in Next.js middleware.

  • Trigger conditions: the application performs authorization checks in middleware.

  • Attack vector: network-based exploitation using HTTP requests that include the x-middleware-subrequest header.

  • Impact: bypass of middleware-based authorization checks, with high confidentiality and integrity impact.

Affected software

  • Next.js versions >= 11.1.4 and < 12.3.5

  • Next.js versions >= 13.0.0 and < 13.5.9

  • Next.js versions >= 14.0.0 and < 14.2.25

  • Next.js versions >= 15.0.0 and < 15.2.3

Severity

CVSS v3.1 base score: 9.1 Critical
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

Mitigation and recommended actions

  • Immediate: upgrade Next.js to 12.3.5, 13.5.9, 14.2.25, or 15.2.3, depending on the deployed major version.

  • If patching is not immediately feasible: prevent external user requests containing the x-middleware-subrequest header from reaching the Next.js application.

References

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