A vulnerability in Next.js, an open-source React framework, could allow attackers to bypass authorization mechanisms, exposing web applications to security risks. This flaw could have widespread implications, as Next.js is widely used by businesses and developers worldwide.
CVE-2025-29927: Authorization Bypass in Next.js
The vulnerability, tracked as CVE-2025-29927, allows attackers to send requests that reach specific paths without undergoing essential security checks. This flaw poses a severe risk to applications relying on Next.js for authentication and authorization.
Next.js is widely used in web development, with over 9 million weekly downloads on npm. It enables developers to build full-stack applications efficiently. Major companies, including TikTok, Twitch, Hulu, Netflix, Uber, and Nike, integrate Next.js into their platforms.
How the Next.js Authorization Bypass Works
Next.js employs middleware components that process requests before they reach an application’s routing system. These middleware functions manage authentication, authorization, error handling, logging, geo-blocking, rate limiting, and user redirection.
To prevent middleware from continuously re-executing itself, Next.js utilizes the ‘x-middleware-subrequest’ header. This header instructs the system whether to apply middleware functions to a request. When a request contains this header with a specific value, middleware execution is skipped, allowing the request to proceed directly to its destination.
Security researchers Allam Rachid and Allam Yasser (inzo_) discovered that attackers can exploit this mechanism by manually crafting requests containing the appropriate ‘x-middleware-subrequest’ header value. This effectively bypasses the security controls implemented through middleware, granting unauthorized access to restricted areas.
This vulnerability highlights the risks of improper validation in web frameworks. Attackers exploiting this flaw could gain unauthorized access to sensitive data, escalate privileges, or deploy further attacks within a compromised application.
Who Is Affected?
The vulnerability affects all Next.js versions before:
15.2.3
14.2.25
13.5.9
12.3.5
Developers using outdated versions are strongly advised to update immediately, as technical details about this security issue have already been made public, increasing the likelihood of exploitation.
Potential Consequences of Exploitation
If an attacker successfully bypasses authentication mechanisms, they could:
Access restricted user data or administrative functions.
Inject malicious code or alter application behavior.
Use compromised applications as entry points to broader network infrastructures.
Leak sensitive customer information, leading to compliance violations and reputational damage.
These risks emphasize the need for rapid patching and adopting additional security controls to mitigate potential threats.
Next.js Security Bulletin and Impact
According to Next.js’ security advisory, CVE-2025-29927 impacts only self-hosted instances running ‘next start’ with the ‘output: standalone’ configuration. Applications hosted on Vercel and Netlify or deployed as static exports remain unaffected.
However, environments that rely on middleware for authorization and lack subsequent validation in the application itself remain vulnerable. Attackers exploiting this flaw can gain unauthorized access to restricted areas of an application, potentially leading to data leaks, privilege escalation, or further system compromises
Mitigation and Recommended Actions
Developers and organizations using affected versions of Next.js should take the following steps to mitigate the risk:
Upgrade to the Latest Version: Update Next.js to at least version 15.2.3 (or the latest available version) to patch the vulnerability.
Restrict Malicious Requests: If immediate patching is not possible, block external user requests containing the ‘x-middleware-subrequest’ header.
Implement Additional Security Layers: Introduce secondary authorization checks beyond middleware to ensure robust security.
Enable Web Application Firewalls (WAFs): Configure security solutions to detect and block unauthorized request patterns.
Monitor Applications for Suspicious Activity: Keep an eye on unauthorized access attempts and take action against any unusual request patterns.
Conduct Regular Security Audits: Periodic security assessments can help identify weaknesses before attackers exploit them.
Lessons for Developers and Security Teams
This vulnerability underscores the importance of robust security measures in web application development. Developers should:
Avoid relying solely on middleware for security and implement multiple layers of validation.
Regularly update dependencies to prevent known vulnerabilities from being exploited.
Test their applications for security flaws using penetration testing and code reviews.
Stay informed about emerging threats and security advisories in the web development ecosystem.
Conclusion
CVE-2025-29927 presents a significant security risk to Next.js applications, especially those that are self-hosted and rely on middleware for access control. With proof-of-concept details available publicly, it is crucial for developers to patch their systems immediately or implement temporary security measures to mitigate the threat.
Staying proactive with updates and additional security layers can help prevent unauthorized access and safeguard sensitive data from potential exploitation. If you’re running a self-hosted Next.js app, now is the time to take action and secure your application against this critical vulnerability.
By addressing these risks with strong security measures, developers can ensure their applications remain resilient against emerging threats in the cybersecurity landscape.
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