Next.js is a React framework for building full-stack web applications. Prior to versions 14.2.32 and 15.4.7, when next() was used without explicitly passing the request object, it could lead to SSRF in self-hosted applications that incorrectly forwarded user-supplied headers. This vulnerability has been fixed in Next.js versions 14.2.32 and 15.4.7. All users implementing custom middleware logic in self-hosted environments are strongly encouraged to upgrade and verify correct usage of the next() function.
The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination.