The vulnerability lies in the auth0/auth0-php SDK, specifically within the CookieStore class, which is a dependency of auth0/laravel-auth0. The advisory GHSA-c42h-56wx-h85q for laravel-auth0 points to GHSA-v9m8-9xxp-q492 for auth0-php as the root cause.

The patch for auth0-php (commit 04b1f5daa8bdfebc5e740ec5ca0fb2df1648a715 in auth0/auth0-php, corresponding to version 8.3.1) shows modifications in src/Store/CookieStore.php. The core of the vulnerability is the use of unserialize() on cookie data within the decrypt() method. An attacker could provide a specially crafted cookie containing a serialized PHP object, which, when processed by unserialize(), could lead to arbitrary code execution or other malicious outcomes.

The fix involves replacing the unserialize() call with json_decode(..., true) in the decrypt() method. Correspondingly, the encrypt() method was also updated to use json_encode() instead of serialize() to ensure data compatibility and security.

Therefore, the primary vulnerable function that would appear in a runtime profile during exploitation is Auth0\SDK\Store\CookieStore::decrypt from the auth0/auth0-php package, as it directly handles the deserialization of the potentially malicious cookie data. While laravel-auth0 is the affected package reported by the user, the actual vulnerable code resides in its dependency, auth0-php.