Middleware Mishap: A Deep Dive into Next.js CVE-2025-29927

On March 21, 2025, a critical security vulnerability identified as CVE-2025-29927 was disclosed in Next.js, a widely-used React framework for building web applications. This vulnerability allows attackers to bypass authorization checks implemented in middleware, potentially granting unauthorized access to sensitive areas of applications. Given its high severity, with a CVSS score of 9.1, it’s imperative for developers and organizations utilizing Next.js to understand the implications of this flaw and take immediate action to mitigate its risks.

Table of Contents

Understanding the Vulnerability

Next.js applications often employ middleware functions to process incoming requests, handling tasks such as authentication, authorization, redirects, and response modifications. The vulnerability arises from improper handling of the x-middleware-subrequest HTTP header, which is internally used by Next.js to prevent infinite middleware execution loops. By crafting requests that include this header, an attacker can effectively bypass middleware-based authorization checks, gaining unauthorized access to protected routes and resources.

Affected Versions

The vulnerability affects the following versions of Next.js

11.x: All versions from 11.1.4 up to 13.5.6 are affected.
12.x: Versions prior to 12.3.5 are vulnerable.
13.x: Versions from 13.0.0 up to 13.5.8 are affected.
14.x: Versions from 14.0.1 up to 14.2.24 are vulnerable.
15.x: Versions from 15.0.1 up to 15.2.2 are affected.

Notably, applications hosted on Vercel and Netlify are not affected due to infrastructure-level protections implemented by these platforms. However, self-hosted deployments using next start with output: 'standalone' are vulnerable and should be addressed promptly.

Potential Impact

Exploiting this vulnerability can lead to several severe consequences

Unauthorized Access: Attackers may gain access to restricted areas of an application, such as administrative dashboards or user data.
Data Exposure: Sensitive information could be exposed, leading to data breaches and compliance violations.
Privilege Escalation: Attackers might escalate their privileges within the application, leading to further exploitation.

Given the ease of exploitation and the potential for significant damage, addressing this vulnerability is of utmost importance.

Mitigation Steps

To protect your Next.js application from CVE-2025-29927, consider the following steps:

Update Next.js: Upgrade to a patched version of Next.js immediately. The versions containing fixes are
- 15.x: Update to 15.2.3 or later
- 14.x: Update to 14.2.25 or later
- 13.x: Update to 13.5.9 or later
- 12.x: Update to 12.3.5 or later
For version 11.x, which is no longer supported, it’s recommended to upgrade to a supported version or implement the workaround mentioned below.
Implement Workarounds: If immediate updating is not feasible, implement the following workaround
- Block the x-middleware-subrequest Header: Configure your server or reverse proxy to block incoming requests containing the x-middleware-subrequest header. This can prevent unauthorized requests from bypassing middleware.
Monitor and Audit: Regularly monitor your application logs for unusual activities, particularly unauthorized access attempts. Conduct security audits to ensure no unauthorized changes have been made.
Review Middleware Implementations: Evaluate your application’s middleware to ensure that authorization checks are robust and not solely reliant on headers that can be manipulated by clients.

Technical Details

The root cause of CVE-2025-29927 lies in the way Next.js processes the x-middleware-subrequest header. This header is intended for internal use to prevent recursive middleware calls. However, due to insufficient validation, external requests containing this header can trick the application into treating them as internal subrequests, thereby bypassing middleware-based authorization checks.

In practical terms, an attacker can craft an HTTP request with the x-middleware-subrequest header set, allowing them to access routes that should be protected by middleware. This flaw underscores the importance of validating and sanitizing all inputs, including headers, to prevent unauthorized access.

Conclusion

CVE-2025-29927 presents a significant security risk to Next.js applications, particularly those that are self-hosted and rely on middleware for authorization. Immediate action is required to update affected systems, implement workarounds where necessary, and review security practices to prevent exploitation. Staying informed and proactive is crucial in safeguarding applications against such vulnerabilities.