The vulnerability stems from improper CSRF protection in the OAuth response handling. The primary function involved in processing authorization responses in auth0-js is WebAuth.parseHash, which would be responsible for validating the state parameter. In vulnerable versions (<9.3), this function allowed CSRF checks to be bypassed when the state parameter was missing. The security bulletin explicitly states the state parameter validation was missing in authorization response handling, and parseHash is the core function that would perform this validation in Auth0.js implementations. While no direct patch code is shown, the advisory confirms the fix required adding proper state validation checks in this critical authentication flow function.