Canto <= 3.1.1 - Missing Authorization to Unauthenticated File Upload

Summary

The Canto plugin for WordPress is vulnerable to Missing Authorization in all versions up to, and including, 3.1.1 via the `/wp-content/plugins/canto/includes/lib/copy-media.php` file. This is due to the file being directly accessible without any authentication, authorization, or nonce checks, and the `fbc_flight_domain` and `fbc_app_api` URL components being accepted as user-supplied POST parameters rather than read from admin-configured options. Since the attacker controls both the destination server and the `fbc_app_token` value, the entire fetch-and-upload chain is attacker-controlled — the server never contacts Canto's legitimate API, and the uploaded file originates entirely from the attacker's infrastructure. This makes it possible for unauthenticated attackers to upload arbitrary files (constrained to WordPress-allowed MIME types) to the WordPress uploads directory. Additional endpoints (`detail.php`, `download.php`, `get.php`, `tree.php`) are also directly accessible without authentication and make requests using a user-supplied `app_api` parameter combined with an admin-configured subdomain.