CVE-2025-29927

Remediation/Mitigation Strategy for CVE-2025-29927: Next.js Middleware Authorization Bypass

This document outlines a remediation/mitigation strategy for CVE-2025-29927, a vulnerability affecting Next.js applications where authorization checks in middleware can be bypassed.

1. Vulnerability Description:

  • Vulnerability: Authorization Bypass in Next.js Middleware
  • Affected Product: Next.js (React Framework)
  • Affected Versions: Versions prior to 14.2.25 and 15.2.3
  • CVE ID: CVE-2025-29927
  • Description: An attacker can bypass authorization checks within a Next.js application if those checks are implemented in the middleware. This bypass is potentially achieved by manipulating the x-middleware-subrequest header.
  • Root Cause (Inferred): The x-middleware-subrequest header, likely intended for internal Next.js routing purposes, can be manipulated by external requests to circumvent the intended authorization logic within the middleware.

2. Severity Assessment:

  • CVSS Score: 9.1 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N) - Based on the information provided.
  • Severity: Critical
  • Rationale: The ability to bypass authorization checks constitutes a critical security vulnerability. It allows unauthorized users to access protected resources, potentially leading to data breaches, privilege escalation, and other severe consequences.

3. Known Exploitation:

While the provided information does not explicitly detail the exact exploit, it implies the manipulation of the x-middleware-subrequest header. An attacker can likely send requests with a modified or forged x-middleware-subrequest header to bypass the authentication logic implemented in the middleware. Specific exploitation details would depend on the exact implementation of the affected Next.js application’s middleware. Therefore, understanding how your specific middleware implementation uses this header is crucial.

4. Remediation Strategy:

  • Primary Remediation: Upgrade Next.js:

    • Action: Upgrade your Next.js application to version 14.2.25 or 15.2.3 or later. These versions contain the fix for the vulnerability.
    • Procedure:
      1. Update your package.json file to specify the new Next.js version. For example: "next": "14.2.25" or "next": "15.2.3".
      2. Run npm install or yarn install to install the updated Next.js package.
      3. Test your application thoroughly after the upgrade to ensure that all features are working as expected and that the vulnerability is resolved. Pay special attention to testing all protected endpoints and authorization flows.
      4. Redeploy your application with the updated version.

  • Secondary Remediation (Mitigation - if Upgrade is Infeasible):

    • Action: Prevent external user requests from containing the x-middleware-subrequest header from reaching your Next.js application.
    • Methods:

      • Web Server Configuration (Recommended): Configure your web server (e.g., Nginx, Apache, Vercel) to strip or reject any incoming requests that contain the x-middleware-subrequest header. This is the preferred mitigation method as it prevents the malicious requests from even reaching the Next.js application.

        • Example (Nginx): location / { if ($http_x_middleware_subrequest) { return 403; # or 400, depending on desired behavior } # … your other configuration }

        • Example (Vercel): (using vercel.json):

          You’ll need to use Middleware within Vercel to achieve this. This is more complex and less efficient than configuring at the edge. // middleware.js (Example) import { NextResponse } from ’next/server' import type { NextRequest } from ’next/server'

          export function middleware(request: NextRequest) { if (request.headers.has(‘x-middleware-subrequest’)) { return new NextResponse(null, { status: 403 }); // Or redirect, or other response } return NextResponse.next(); }

          export const config = { matcher: [ /* * Match all request paths except for the ones starting with: * - api (API routes) * - _next/static (static files) * - _next/image (image optimization files) * - favicon.ico (favicon file) / ‘/((?!api|_next/static|_next/image|favicon.ico).)’, ], }

      • Middleware-Level Mitigation (Less Recommended): Implement middleware within your Next.js application to reject requests containing the x-middleware-subrequest header if they originate from external sources. This is less ideal than web server configuration because the request still reaches the Next.js application and consumes resources.

        • Caveats: This approach is more complex to implement correctly and relies on accurately identifying external requests. It also introduces more overhead to your application. It is much better to handle this at the webserver layer.

5. Validation:

  • After Upgrading: Thoroughly test your application after upgrading to the patched version. Focus on all authorization flows and attempt to bypass authentication using techniques that may have exploited the vulnerability previously. Review your middleware logic to ensure it’s robust.
  • After Implementing Mitigation: Test your mitigation by sending requests with the x-middleware-subrequest header from an external source. Verify that the requests are blocked by your web server or middleware (as configured).
  • Penetration Testing: Consider conducting a penetration test of your application to ensure that the vulnerability has been completely resolved.

6. Monitoring:

  • Web Server Logs: Monitor your web server logs for any attempts to send requests with the x-middleware-subrequest header. This can help you identify potential attackers.
  • Application Logs: Review your application logs for any unusual behavior or errors related to authorization checks.
  • Security Audits: Conduct regular security audits of your Next.js application to identify and address any potential vulnerabilities.

7. Communication:

  • Communicate the vulnerability and remediation steps to all relevant stakeholders, including developers, security teams, and system administrators.
  • Inform users if there is any potential impact to their data or accounts.

Important Considerations:

  • This remediation strategy is based on the limited information provided in the security advisory. The specific steps may need to be adjusted based on the specific implementation of your Next.js application.
  • Always test any changes in a non-production environment before deploying them to production.
  • Keep your Next.js version up to date with the latest security patches.
  • Implement a comprehensive security strategy that includes regular vulnerability scanning, penetration testing, and security audits.
  • Consult the Next.js documentation and community resources for more information on securing your Next.js applications.

Assigner

Date

  • Published Date: 2025-03-21 15:15:43
  • Updated Date: 2025-03-21 15:15:43

More Details

CVE-2025-29927