Next.js Auth Bypass – CVE-2025-29927
Key Points
- Research suggests CVE-2025-29927 is a critical authentication bypass vulnerability in Next.js, affecting versions before 14.2.25 and 15.2.3.
- It seems likely that attackers can exploit this by manipulating the
x-middleware-subrequestheader, bypassing middleware authorization checks. - The evidence leans toward updating to Next.js 14.2.25 or 15.2.3 as the primary fix, with filtering the header at the proxy level as a workaround if updates are not possible.
Vulnerability Overview
CVE-2025-29927 is an authentication bypass issue in Next.js, a popular React framework for building web applications. This vulnerability allows attackers to skip authorization checks in the middleware by setting a specific HTTP header, potentially accessing protected resources without authentication.
Impact and Affected Versions
This vulnerability can lead to unauthorized access, data breaches, and system compromise, especially for applications relying on middleware for security. It affects Next.js versions prior to 14.2.25 and 15.2.3, making it crucial for users to verify and update their versions.
Mitigation Steps
To mitigate, update to Next.js 14.2.25 or 15.2.3 for a permanent fix. If updating isn’t feasible, filter the x-middleware-subrequest header at the edge or proxy level, such as using NGINX, to prevent exploitation.
Unexpected Detail
An unexpected aspect is that applications hosted on platforms like Vercel, Netlify, and Cloudflare may already be protected, reducing the immediate risk for some users, though self-hosted deployments remain vulnerable.
Comprehensive Analysis of CVE-2025-29927: Next.js Authentication Bypass Vulnerability
Introduction
In the dynamic field of web development, frameworks like Next.js have become essential for creating efficient, scalable applications. However, security vulnerabilities can pose significant risks, and one such issue, CVE-2025-29927, has recently been identified as a critical authentication bypass exploit in Next.js’s middleware. This report provides a detailed examination of the vulnerability, its implications, and mitigation strategies, tailored for developers, security professionals, and enterprise stakeholders.
Background on Next.js and Middleware
Next.js, developed by Vercel, is a React-based framework that enables full-stack web application development. A key feature is its middleware functionality, which allows developers to execute code before a request is completed. Middleware, defined in files like middleware.js or middleware.ts at the project’s root, is used for tasks such as authentication, authorization, and request modification. For instance, it can redirect users, rewrite URLs, or enforce security headers, making it integral to application security (Routing: Middleware | Next.js).
Vulnerability Details
CVE-2025-29927, disclosed on March 21, 2025, according to the National Vulnerability Database (NVD), is an improper authorization vulnerability affecting Next.js versions prior to 14.2.25 and 15.2.3. The vulnerability arises from the ability to bypass authorization checks within the middleware by manipulating the x-middleware-subrequest header.
Mechanism of Exploitation
Next.js uses the x-middleware-subrequest header internally to manage subrequests initiated by the middleware, preventing recursive loops. Normally, this header indicates that the request is an internal subrequest, exempt from certain checks. However, attackers can exploit this by including the x-middleware-subrequest header in their HTTP requests, tricking the application into treating the request as internal. This bypasses middleware authorization checks, potentially allowing unauthorized access to protected routes or resources.
Research from sources like RunZero Blog highlights that exploitation is trivial, requiring only the addition of this header to bypass security checks, including authentication mechanisms. The vulnerability is rated critical with a CVSS v3 base score of 9.1, underscoring its severity (NVD – CVE-2025-29927).
Impact Assessment
The implications of CVE-2025-29927 are significant, particularly for applications relying on middleware for security:
- Unauthorized Access: Attackers can access sensitive data or perform actions requiring authentication, such as accessing user profiles or administrative functions.
- Data Breach: This could lead to the exposure of confidential information, including personal data, financial records, or intellectual property, posing risks under data protection regulations.
- System Compromise: In severe cases, attackers might manipulate the system, potentially leading to further exploits like remote code execution or denial-of-service attacks.
Given Next.js’s widespread adoption, the vulnerability could impact numerous applications, especially those with self-hosted deployments using next start and output: ‘standalone’ (Next.js Blog Post).
Affected Versions and Scope
The vulnerability affects Next.js versions prior to 14.2.25 (for the 14.x series) and 15.2.3 (for the 15.x series). To determine if an application is vulnerable, users should check their installed version against these thresholds. Notably, research suggests that applications hosted on platforms like Vercel, Netlify, and Cloudflare may already be mitigated, as these platforms have implemented protections, reducing the immediate risk for such deployments (zhero-web-sec Research).
Mitigation Strategies
Mitigating CVE-2025-29927 requires prompt action, with the following approaches recommended:
Permanent Fix
The most effective solution is to update Next.js to a patched version:
- For Next.js 14.x, upgrade to 14.2.25 or later.
- For Next.js 15.x, upgrade to 15.2.3 or later.
These updates address the vulnerability by ensuring proper handling of the x-middleware-subrequest header, preventing bypasses. The Next.js team released these patches on March 22, 2025, as detailed in their blog post (Next.js Blog Post).
Workaround for Delayed Updates
If updating is not immediately feasible, a workaround is to filter the x-middleware-subrequest header at the edge or proxy level before it reaches the Next.js application. This prevents external requests with the header from being processed as internal subrequests. For example, using NGINX, the following configuration can be implemented:
server {
listen 80;
location / {
proxy_pass http://localhost:3000;
proxy_set_header x-middleware-subrequest "";
}
}This configuration removes the header, ensuring that even if an attacker sets it, the application will not treat the request as internal. Similar configurations can be applied using other proxies like Apache or Cloudflare, depending on the deployment environment.
Detection and Monitoring
To detect potential exploitation or verify vulnerability status, the following steps are recommended:
- Version Check: Confirm the installed Next.js version is not affected (prior to 14.2.25 or 15.2.3).
- Testing in Controlled Environment: Send a test request with the
x-middleware-subrequestheader set and observe if authorization checks are bypassed. This should be done in a staging or isolated environment to avoid unintended access.
Additionally, monitoring for unusual access patterns, such as requests with the x-middleware-subrequest header from external IPs, can help detect potential attacks. Tools like intrusion detection systems (IDS) or web application firewalls (WAF) can be configured to block such requests.
Red Team, Blue Team, and Enterprise Perspectives
From a red team perspective, this vulnerability is attractive for exploitation in penetration testing, as it allows bypassing authentication with minimal effort. Attackers could target applications with sensitive middleware logic, such as those handling financial transactions or user data, to gain unauthorized access.
For blue teams, detecting and mitigating this vulnerability involves ensuring timely updates, monitoring for header manipulation, and implementing proxy-level filtering. Regular security audits and vulnerability scans, using tools like Nessus or OpenVAS, can help identify affected systems.
From an enterprise perspective, the implications are significant for large-scale deployments. Organizations using Next.js should prioritize patch management, especially for self-hosted applications, and consider the cost-benefit of hosted solutions like Vercel, which may already mitigate the issue. Enterprises should also review their security policies to ensure middleware is not the sole layer of protection, implementing defense-in-depth strategies.
Conclusion and Best Practices
CVE-2025-29927 underscores the importance of robust security in web frameworks. Developers should prioritize updating to the latest Next.js versions and, where updates are delayed, implement header filtering at the proxy level. Best practices include:
- Regularly updating dependencies to address known vulnerabilities.
- Avoiding reliance on middleware alone for critical security checks, implementing additional layers like API-level authentication.
- Conducting regular security assessments, especially for applications handling sensitive data.
By understanding and addressing this vulnerability, stakeholders can safeguard their applications against potential exploits, ensuring both security and continuity.
Tables
Below is a summary of key details for quick reference:
| Aspect | Details |
|---|---|
| CVE ID | CVE-2025-29927 |
| Affected Versions | Prior to 14.2.25 and 15.2.3 |
| CVSS Score | 9.1 (Critical) |
| Exploitation Method | Manipulate x-middleware-subrequest header |
| Impact | Unauthorized access, data breach, system compromise |
| Fix | Update to 14.2.25 or 15.2.3 |
| Workaround | Filter header at proxy level (e.g., NGINX configuration) |
And a table for mitigation options:
| Mitigation Option | Description | Applicability |
|---|---|---|
| Update to Patched Version | Upgrade to Next.js 14.2.25 or 15.2.3 | All affected versions |
| Proxy-Level Filtering | Remove x-middleware-subrequest header using NGINX or similar | When updates are not immediately feasible |