NextJS Critical security vulnerability CVE-2025-29927

​A critical security vulnerability, identified as CVE-2025-29927, has been discovered in Next.js, a widely used React framework. This flaw allows attackers to bypass middleware authorization checks, potentially granting unauthorized access to sensitive areas of applications.​

Understanding the Vulnerability

Next.js utilizes middleware functions to handle tasks such as authentication, authorization, and request processing before routing. To prevent infinite loops where middleware might call itself repeatedly, Next.js employs the x-middleware-subrequest header. By manipulating this header, an attacker can trick the application into skipping critical middleware checks, effectively bypassing authorization mechanisms.

Affected Versions

The vulnerability impacts the following versions of Next.js:​

• 15.x versions prior to 15.2.3​

• 14.x versions prior to 14.2.25​

• 13.x versions prior to 13.5.9

• 12.x versions prior to 12.3.5​

Developers using these versions, especially those relying on middleware for authorization without additional security layers, are at significant risk.

Potential Impact

Exploiting this vulnerability could lead to unauthorized access to restricted sections of an application, exposure of sensitive data, and possible cache poisoning or denial-of-service attacks. The simplicity of the exploit—requiring only the addition of a specific HTTP header—makes it particularly concerning.

Mitigation Steps

To protect your Next.js applications:

1. Update Immediately: Upgrade to the latest patched versions:​

   • 15.2.3 or later​

   • 14.2.25 or later​

   • 13.5.9 or later​

   • 12.3.5 or later​

2. Implement Additional Authorization Checks: Relying solely on middleware for authorization is insufficient. Incorporate server-side authentication mechanisms to ensure robust security.

3. Sanitize Incoming Headers: Configure your server to remove or ignore the x-middleware-subrequest header from incoming requests to prevent unauthorized manipulation.

The discovery of CVE-2025-29927 underscores the importance of maintaining up-to-date dependencies and implementing multiple layers of security in web applications. Developers should act promptly to update their Next.js versions and review their authorization strategies to safeguard against potential exploits.