CVE-2025-29927: Middleware Authorization Bypass in Next.js and How InvisiRisk BAF Prevents it

Introduction

In the ever-evolving landscape of web development, security remains a paramount concern. A recent discovery of a critical vulnerability in Next.js, identified as CVE-2025-29927, underscores the necessity for robust security measures within build processes. This blog post delves into the specifics of this vulnerability and illustrates how InvisiRisk's Build Application Firewall (BAF) proactively mitigates such threats, ensuring the integrity of your applications.

Understanding CVE-2025-29927: The Next.js Middleware Vulnerability

Next.js, a widely adopted React framework for building full-stack web applications, was found to have a critical security flaw affecting versions 11.1.4 through 15.2.2 on March 21st, 2025. This vulnerability allows attackers to bypass authentication mechanisms by manipulating the x-middleware-subrequest HTTP header. By appending a specific sequence to this header, an attacker can effectively disable all middleware security checks, granting unauthorized access to protected resources and routes like “/dashboard/admin”. The vulnerability was given the CVSS score of 9.1 (Critical) by NVD.

Technical Breakdown of the Vulnerability

Middleware in Next.js serves as an intermediary that processes requests before they reach the application's core logic, handling tasks such as authentication and authorization. The identified vulnerability exploits a mechanism intended to prevent infinite recursion in middleware execution.

By crafting a request with the x-middleware-subrequest header containing a repeated pattern (e.g., middleware:middleware:middleware:middleware:middleware), an attacker can trick the application into skipping critical middleware functions. This manipulation results in unauthorized access to restricted areas of the application.

Figure: Risk of Vulnerable Packages being Deployed.

InvisiRisk BAF: Real-time Defense Against Critical Vulnerabilities

InvisiRisk's Build Application Firewall (BAF) is designed to fortify the software build process by enforcing stringent security policies that detect and block known vulnerabilities. Upon integrating the Default Security Policy, BAF automatically identifies and prevents the inclusion of packages with critical vulnerabilities, such as the affected versions of Next.js.

BAF in Action: Blocking Vulnerable Next.js Packages

Detection of Vulnerable Versions: BAF maintains an up-to-date repository of packages with known critical vulnerabilities. When a build process attempts to incorporate a compromised version of Next.js (e.g., versions 11.1.4 through 15.2.2), BAF detects the presence of these versions. Furthermore, if a package in an existing scan was not initially a known critical vulnerability but is later discovered as one, users watching the product will be notified as well.

Figure: In-app notification of Package with critical vulnerability detected in our Product.

“Blocks Critical Vulnerable Packages” Policy: Upon detection, BAF intervenes by halting the build process, preventing the integration of the vulnerable package. This immediate action serves as a first line of defense, ensuring that compromised code does not enter the production environment.

Figure: Invisirisk BAF blocking Vulnerable Next.js Package.

Notification and Reporting: BAF provides alerts and reports to development teams, highlighting the specific vulnerability and recommending corrective actions, such as upgrading to a secure version of the package.

Figure: Mail notification of vulnerable package detected.

Outcome: Enhanced Security and Compliance

By proactively blocking the integration of vulnerable Next.js versions, InvisiRisk BAF mitigates the risk of authentication bypass exploits, safeguarding sensitive data and maintaining the integrity of the application. This approach not only enhances security but also streamlines compliance with industry standards and best practices.

Figure: InvisiRisk BAF Blocking Vulnerable Packages.

Conclusion

The discovery of CVE-2025-29927 in Next.js serves as a critical reminder of the importance of real-time security measures in the software development lifecycle. InvisiRisk's Build Application Firewall offers a robust, adaptable solution that detects and blocks known vulnerabilities during the build process, providing peace of mind and reducing the need for reactive incident responses. By integrating BAF into your development pipeline, you ensure that your applications remain resilient against emerging threats.

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