Reflected XSS from the callback handler's error query parameter

The Auth0 Next.js SDK is a library for implementing user authentication in Next.js applications. Versions before and including 1.4.1 are vulnerable to reflected XSS. An attacker can execute arbitrary code by providing an XSS payload in the error query parameter which is then processed by the callback handler as an error message. You are affected by this vulnerability if you are using @auth0/nextjs-auth0 version 1.4.1 or lower unless you are using custom error handling that does not return the error message in an HTML response. Upgrade to version 1.4.1 to resolve. The fix adds basic HTML escaping to the error message and it should not impact your users.