The vulnerability (CVE-2025-48947 / GHSA-f3fg-mf2q-fj3f) in the auth0/nextjs-auth0 SDK (versions 4.0.1 to 4.6.0) stems from the potential for CDN caching of HTTP responses that set session cookies. This is due to the absence of appropriate Cache-Control headers on these sensitive responses. The commit 12a62ca596db3b0827b39a4b865b882423e7cb1e addresses this by introducing a utility function addCacheControlHeadersForSession (in src/server/cookies.ts) and integrating it into several methods of the AuthClient class (in src/server/auth-client.ts).

The AuthClient class is central to handling authentication and session management operations. The methods handleAuthRouter (specifically its login path), handleLogout, handleCallback, handleMe, and handleAccessToken are directly involved in processes that create, modify, or utilize session cookies, or return session-related data. In the vulnerable versions, these methods generated HTTP responses (e.g., setting cookies, returning user data, redirecting after logout) without the necessary Cache-Control directives (like private, no-cache, no-store, must-revalidate, max-age=0). This omission meant that if a CDN or other shared cache was configured to cache responses even with Set-Cookie headers, these sensitive responses could be stored and inadvertently served to other users, leading to session hijacking or information disclosure.

The patch explicitly adds calls to addCacheControlHeadersForSession(res) within these identified methods. This ensures that any response they generate which pertains to session state is correctly marked to prevent caching by shared caches. Therefore, these methods, in their pre-patch state, are the vulnerable functions as they were responsible for emitting responses that could lead to the caching of session cookies. During exploitation, requests to endpoints managed by these functions would result in responses that a misconfigured CDN could cache, triggering the vulnerability. These function names would appear in a runtime profile or stack trace when such vulnerable operations are performed.