A critical security vulnerability has recently been discovered in Next.js, the open-source web framework developed by Vercel that enhances React applications with server-side and static rendering capabilities.
The identified vulnerability (CVE) enables malicious actors to circumvent authorization checks implemented through middleware, potentially compromising application security.
If your application uses Next.js for your web applications you should prioritise addressing this critical vulnerability, to fix this exploitable and dangerous hole in your security posture.
What is Authorization Bypass?
Authentication Bypass is a security vulnerability that occurs when an attacker is able to access a system, resource, or functionality without the proper credentials or prior authorization.
This flaw allows unauthorized users to bypass the authentication mechanisms that are typically in place to prevent access to sensitive areas of an application.
How Authorization Bypass Works in Next.js
In Next.js, authorization mechanisms are typically implemented using middleware to ensure that only authenticated users can access protected routes or resources. However, in the case of CVE-2025-29927, attackers can get middleware execution bypass authorization checks due to a vulnerability in how the x-middleware-subrequest header is processed. This header is used internally by Next.js middleware to differentiate subrequests, but it is not properly validated, allowing attackers to manipulate it and bypass authorization controls.
By crafting a request that includes the x-middleware-subrequest header, attackers can trick the system into treating the request as if it has passed the authorization checks, even though the user is unauthorized. This can lead to unauthorized access to restricted areas of the application, such as admin dashboards, login,, private APIs, or sensitive user data.
Example of exploitation: An attacker might send a request like:
GET /admin HTTP/1.1
Host: example.com
x-middleware-subrequest: trueIf your Next.js application relies on middleware to do web server to enforce authentication, this request might trick the application into allowing access to restricted pages or resources without proper authorization.
What are the Nextjs versions that are affected by CVE-2025-29927
11.1.4
12.x.x
13.x.x (up to 13.5.8)
14.x.x (up to 14.2.24)
15.x.x (up to 15.2.2)
Why is CVE-2025-29927 important?
This vulnerability is particularly severe because it:
Widespread Impact: Next.js is widely adopted by both startups and enterprises, meaning this vulnerability affects a large number of applications.
Ease of Exploitation: Exploiting this vulnerability requires minimal effort. An attacker can bypass security simply by adding a specific header to an HTTP request.
Serious Consequences: If exploited, this vulnerability can lead to severe consequences. Attackers could:
Gain access to admin dashboards without logging in.
Retrieve sensitive user data.
Perform unauthorized actions on behalf of legitimate users, potentially compromising the integrity of the application.
Difficult to Detect: The attack doesn’t involve traditional techniques like brute force or SQL injection, making it hard to detect using conventional security tools.
How to Mitigate the Threat
The only way to mitigate this vulnerability is to update your Next.js applications. Next.js team has patched this vulnerability in the following latest versions:
13.5.9
14.2.25
15.2.3
Upgrade your Next.js version by running these commands:
npm update nextor
yarn upgrade nextThis ensures your application is protected from this exploit.
Strengthen Middleware Authorization
If your application relies on middleware code for authentication or authorization, make sure that:
Middleware does not depend on client-supplied headers like x-middleware-subrequest.
Implement strict access control checks at both route-level and middleware-level.
Validate user authentication tokens properly before granting access.
Implement Additional Security Measures
Use a Web Application Firewall (WAF): A WAF can detect and block unauthorized request patterns.
Restrict HTTP Headers: Prevent external requests from sending x-middleware-subrequest.
Regular Security Audits: Review application logs for suspicious activity.
Is Next.js Secure?
If you're building web applications with Next.js, you’ll be happy to know that it’s generally considered a secure framework.
It comes with built-in security features like automatic static optimization, API route protection, and strict Content Security Policy (CSP) support. These features help safeguard your application, but like any framework, Next.js isn’t entirely immune to security vulnerabilities.
The security of your Next.js application ultimately depends on implementation quality, update frequency, and adherence to security best practices. Strengthening your Next.js security posture requires attention to several key areas. Automatic Static Optimization pre-generates pages during build time, effectively reducing the attack surface available to potential intruders.
When implementing Server-Side Rendering with authentication, you ensure proper session management across your application. Implementing middleware for thorough request validation significantly reinforces your authentication and authorization controls.
Additionally, Next.js provides built-in protections against common vulnerabilities like cross-site scripting and cross-site request forgery, though these should be regularly tested and verified rather than simply trusted.
Even with these security mechanisms in place, vulnerabilities can still emerge—either from the framework itself or from misconfigurations during development. That’s why it’s crucial to stay updated, follow secure coding practices, and regularly audit your application for potential risks.
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