The vulnerability occurs in the admin/file-manager/attachments/add endpoint, which maps to the add() method in the AttachmentsController. The exploit demonstrates that arbitrary PHP files can be uploaded and executed, indicating insufficient file type validation in the upload handling logic. In CakePHP-based applications like Croogo, controller actions like add() typically process form submissions, including file uploads. The lack of proper file extension/MIME type checks in this method allows dangerous file types to be stored in web-accessible directories, leading to RCE.