The vulnerability chain involves two key components:

1. In users.queries.php, the 'newValue' parameter from the POST request is stored without validation, enabling attackers to set malicious paths.
2. In core.php, the require_once statement directly uses the tainted $_SESSION['user_language'] value to include files, enabling arbitrary PHP execution. The lack of input validation in users.queries.php and the unsafe file inclusion in core.php together create the path traversal vulnerability.