On March 21, 2025, a critical authorization bypass vulnerability in Next.js, identified as CVE-2025-29927, was disclosed with a CVSS score of 9.1. This framework’s middleware handling flaw enables attackers to bypass authentication and authorization, exposing sensitive routes to unauthorized access. Exploiting this vulnerability does not require authentication, providing attackers with direct access to protected routes. In this blog, we will discuss the vulnerability in detail. 

What is Next.js Vulnerability (CVE-2025-29927)?

Next.js, an open-source web framework developed by Vercel, powers React-based applications with features like static and server-side rendering. A recently disclosed critical vulnerability (CVE) allows attackers to bypass middleware-based authorization checks. The vulnerability specifically targets the middleware functionality in Next.js, which is widely used for tasks such as authorization, path rewriting, server-side redirects, and adding response headers like Content Security Policy (CSP).

Exploitation of this vulnerability allows attackers to bypass middleware controls by including a specifically crafted x-middleware-subrequest header in their HTTP requests. Affected versions include Next.js from 11.1.4 to 13.5.6, 14.x prior to 14.2.25, and 15.x before 15.2.3. Applications relying on middleware for access control face severe risks, as attackers can gain unauthorized access to protected resources without requiring authentication.

The Vulnerability Mechanism

The CVE-2025-29927 vulnerability arises from a design flaw in how Next.js handles the x-middleware-subrequest header, which was intended for internal use to prevent infinite middleware execution loops. In applications utilizing middleware, the runMiddleware function processes incoming requests and checks for this header. If the header is present with a specific value, middleware execution is bypassed, and the request is directly forwarded to its intended destination through NextResponse.next().

The vulnerability stems from the ability of external users to exploit the header check. By including the x-middleware-subrequest header with the correct value in a request, attackers can entirely bypass middleware-based protection mechanisms. Below is an explanation of how this vulnerability operates at the code level: JavaScript.

The following HTTP GET request demonstrates how the CVE-2025-29927 vulnerability can be exploited to bypass authentication and gain unauthorized access to the admin dashboard.

How does Next.js Vulnerability Work?

Next.js middleware relies on an internal header, x-middleware-subrequest, to prevent recursive requests and avert infinite loops. However, a security vulnerability allows attackers to tamper with this header, enabling them to bypass middleware execution entirely.

The exploit varies across different Next.js versions:

• Older versions (prior to 12.2): x-middleware-subrequest: pages/_middleware
• Modern versions: x-middleware-subrequest: middleware:middleware:middleware:middleware:middleware (or src/middleware:src/middleware:src/middleware:src/middleware:src/middleware, if the src directory is utilized)

When this header contains the appropriate value, the middleware is bypassed, allowing the request to proceed directly to its destination without undergoing the security checks or modifications typically applied by the middleware layer.

Potential Consequences of the Vulnerability

The vulnerability can be exploited via various methods:

• Authorization Bypass: Attackers may gain access to protected routes without undergoing proper authentication or authorization checks.

• Content Security Policy (CSP) Bypass: If middleware is responsible for adding CSP headers, attackers can circumvent these safeguards, potentially facilitating cross-site scripting (XSS) attacks.

• Denial of Service through Cache Poisoning: In specific configurations, attackers could bypass middleware responsible for setting cache control headers, enabling them to contaminate caches with unauthorized content.

This vulnerability is particularly alarming due to its ease of exploitation and the extensive adoption of Next.js for developing modern web applications. Exploitation requires no specialized tools or advanced techniques—simply including a single HTTP header with the appropriate value is enough to bypass security measures.

What Should Organizations Do Next?

To address this vulnerability effectively, organizations must act swiftly to mitigate the risks and secure their applications. Here are the recommended steps to ensure protection: 

Update to Patched Versions Immediately

• For Next.js 15.x: Update to version 15.2.3 or later
• For Next.js 14.x: Update to version 14.2.25 or later
• For Next.js 13.x: Update to version 13.5.9 or later
• For Next.js 12.x: Update to version 12.3.5 or later

If an Immediate Update is Not Feasible

• Set up network-level filtering to block the x-middleware-subrequest header.
• Enhance monitoring efforts to detect and respond to unusual access patterns.
• Prioritize updates based on the application’s external exposure and the sensitivity of the data it handles.

Confirm and Authenticate

• Ensure the patch has been applied successfully.
• Verify that authorization controls continue to operate as intended.

How Can Kratikal Help Mitigate Security Gaps?

Kratikal offers specialized services to address the security vulnerabilities found in Next.js applications, including those caused by CVE-2025-29927. With our Vulnerability Assessment and Penetration Testing (VAPT) services, Kratikal rigorously examines application middleware to identify exploitable weaknesses. This detailed analysis empowers organizations to remediate vulnerabilities before attackers can exploit them. Additionally, Kratikal’s Patch Management Services can ensure seamless and timely updates to affected Next.js versions. By prioritizing and implementing critical patches, businesses can prevent unauthorized access to sensitive resources. Together, these proactive measures help organizations strengthen their defenses and reduce the risk of future exploits.

FAQs