Critical CVE-2025-29927: Next.js Authorization Bypass Threat and Mitigation Strategies

Executive Summary

The discovery of CVE-2025-29927, a critical vulnerability in Next.js, has raised significant cybersecurity concerns across multiple sectors. Next.js, a prominent open-source web framework from Vercel, has been found vulnerable to an authorization bypass that can potentially be exploited to gain unauthorized access to protected routes. This vulnerability has been identified in versions 11.1.4 through 13.5.6, 14.x before 14.2.25, and 15.x before 15.2.3. Given the widespread adoption of Next.js, this issue demands immediate action from affected organizations to mitigate potential security breaches.

Technical Information

The vulnerability originates from a flaw in the processing of the

x-middleware-subrequest

header by Next.js middleware. This header is designed to prevent infinite middleware execution loops. However, it can be manipulated by attackers, enabling them to bypass authorization checks and access restricted routes without proper credentials. The exploitation mechanism relies on the ability to craft requests with specific header values, effectively rendering middleware controls ineffective. In versions prior to 12.2, the middleware files are named

_middleware.ts

located within the

pages

folder, while in versions 12.2 and later, the middleware file is named

middleware.ts

and resides outside the

pages

folder. The vulnerability persists even in versions 13.2.0 and later, as the header checks occur prior to the introduced recursion depth checks, allowing exploitation to continue.

The impact of this vulnerability can be severe, leading to scenarios such as authorization bypass, CSP (Content Security Policy) bypass, and cache-poisoning denial-of-service (DoS) attacks. Unauthorized parties can potentially execute cross-site scripting (XSS) attacks by circumventing middleware-set CSP headers, and the bypass of cache control headers can result in the unauthorized poisoning of cached content.

Exploitation in the Wild

The simplicity of exploitation, requiring only the addition of a specific HTTP header, exacerbates the threat posed by this vulnerability. The lack of complex techniques needed for exploitation makes it particularly concerning, especially considering the extensive use of Next.js in web applications worldwide. There have been no widely reported cases of exploitation in the wild to date, but the potential for such occurrences is significant, given the straightforward nature of the attack.

APT Groups using this vulnerability

While specific Advanced Persistent Threat (APT) groups exploiting this vulnerability have not been identified, it is crucial for organizations in highly targeted sectors such as finance, healthcare, and government to remain vigilant. These sectors are often primary targets for cyber espionage and cybercriminal activities, and the introduction of this vulnerability could potentially be leveraged by sophisticated threat actors in future campaigns.

Affected Product Versions

The affected versions of Next.js include 11.1.4 through 13.5.6, 14.x versions before 14.2.25, and 15.x versions before 15.2.3. Organizations utilizing these versions should prioritize updates or apply workarounds to mitigate the risk of exploitation.

Workaround and Mitigation

Immediate mitigation can be achieved by updating to the latest patched versions of Next.js, specifically 15.2.3 or 14.2.25, depending on the currently deployed version. For those unable to update immediately, a temporary workaround involves stripping the

x-middleware-subrequest

header using load balancer rules, custom middleware, or web server configurations. Examples include using

proxy_set_header x-middleware-subrequest "";

in Nginx configurations or

RequestHeader unset x-middleware-subrequest

in Apache configurations. Additionally, ProjectDiscovery has released a Nuclei template for detecting this vulnerability, which can be employed to scan Next.js deployments for potential exposure.

References

Further details can be found in the ProjectDiscovery Blog (https://projectdiscovery.io/blog/nextjs-middleware-authorization-bypass), the GitHub Security Advisory (https://github.com/vercel/next.js/security/advisories/GHSA-f82v-jwr5-mffw), and Rachid Allam’s Research (https://zhero-web-sec.github.io/research-and-things/nextjs-and-the-corrupt-middleware).

Rescana is here for you

At Rescana, we are dedicated to assisting our clients in navigating the complexities of cybersecurity through our Third Party Risk Management (TPRM) platform. We provide continuous security assessments and monitoring, ensuring that organizations are equipped to address vulnerabilities such as CVE-2025-29927 effectively. Our team is available to answer any questions regarding this report or other cybersecurity concerns at ops@rescana.com.