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It is possible to bypass authorization checks within a Next.js application, if the authorization check occurs in middleware.

Next.js is a React framework for building full-stack web applications. Versions prior to 14.2.24 and 15.1.6 have a race-condition vulnerability. This issue only affects the Pages Router under certain misconfigurations, causing normal endpoints to serve pageProps data instead of standard HTML. This issue was patched in versions 15.1.6 and 14.2.24 by stripping the x-now-route-matches header from incoming requests. Applications hosted on Vercel's platform are not affected by this issue, as the platform does not cache responses based solely on 200 OK status without explicit cache-control headers. Those who self-host Next.js deployments and are unable to upgrade immediately can mitigate this vulnerability by stripping the x-now-route-matches header from all incoming requests at the content development network and setting cache-control: no-store for all responses under risk. The maintainers of Next.js strongly recommend only caching responses with explicit cache-control headers.

This vulnerability is similar to CVE-2018-14732. When running a Next.js server locally (e.g. through npm run dev), the WebSocket server is vulnerable to the Cross-site WebSocket hijacking (CSWSH) attack. and a bad actor can access the source code of client components, if a user was to visit a malicious link while having the Next.js dev server running.