🚨 CVE-2025-29927: Critical Authorization Bypass in Next.js Middleware
On March 21, 2025, a critical vulnerability—CVE-2025-29927—was disclosed in Next.js, a widely adopted React framework. This flaw allows attackers to bypass middleware-based security checks, potentially leading to unauthorized access, cache poisoning, and denial of service (DoS) attacks.
🔍 Understanding the Vulnerability
📌 What Is CVE-2025-29927?
CVE-2025-29927 is an authorization bypass vulnerability in Next.js that exploits the x-middleware-subrequest HTTP header. By crafting requests with specific header values, attackers can trick Next.js into skipping middleware execution, thereby bypassing critical security checks like authentication and authorization.
🛠️ How Does It Work?
Next.js uses middleware functions defined in files like middleware.js or middleware.ts to process incoming HTTP requests. These middleware functions often handle essential tasks such as authentication, authorization, and localization.
The framework employs the x-middleware-subrequest header internally to prevent infinite loops during request processing. However, the value of this header is predictable, often based on the middleware file's path. Attackers can exploit this predictability by crafting requests with appropriate header values to bypass middleware execution.
🎯 Affected Versions
The vulnerability affects the following Next.js versions:
- 15.x: 15.0.0 up to and including 15.2.2
- 14.x: 14.0.0 up to and including 14.2.24
- 13.x: 13.0.0 up to and including 13.5.6
- 12.x: 12.0.0 up to and including 12.3.4
- 11.x: 11.1.4 up to and including 11.1.4
Applications hosted on platforms like Vercel and Netlify are not affected, as these platforms have implemented infrastructure-level protections.
🚨 Attack Scenarios
1. Authorization Bypass
Attackers can gain unauthorized access to protected routes by crafting requests that bypass middleware-based authorization checks. This is particularly concerning for applications that rely solely on middleware for access control.
2. Cache Poisoning Leading to DoS
By bypassing middleware responsible for localization or routing, attackers can cause the application to serve incorrect or error pages. If these responses are cached by a CDN or other caching layer, it can lead to widespread service disruptions.
🧪 Proof of Concept (PoC)
Several security researchers have published proof-of-concept exploits demonstrating how this vulnerability can be exploited:
🛡️ Mitigation Strategies
✅ Upgrade to Patched Versions
The Next.js team has released patches addressing this vulnerability. Upgrade to the following versions:
- 15.x: 15.2.3
- 14.x: 14.2.25
- 13.x: 13.5.9
- 12.x: 12.3.5
Upgrading is the most effective way to mitigate this vulnerability.
🔧 Implement Workarounds
If immediate upgrading is not feasible, consider the following workarounds:
Strip the x-middleware-subrequest Header: Configure your web server or proxy to remove this header from incoming requests.
Enhance Authorization Mechanisms: Implement additional server-side authorization checks beyond middleware to validate user permissions.
📈 Observed Exploitation Attempts
Security researchers have observed initial exploit attempts targeting this vulnerability. Attackers are crafting requests with the x-middleware-subrequest header set to values like src/middleware:src/middleware:src/middleware:src/middleware:src/middleware to simulate multiple internal subrequests, triggering Next.js's internal redirect logic and bypassing middleware checks.
🧩 Additional Resources
📝 Conclusion
CVE-2025-29927 is a critical vulnerability that poses significant risks to applications using Next.js middleware for security controls. Given the simplicity of exploitation and the potential impact, it's imperative to upgrade to the patched versions or implement the recommended workarounds immediately. Regularly review your application's security posture and stay informed about emerging threats to ensure robust protection.
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