Deserialization of untrusted data

The Welcart e-Commerce plugin for WordPress is vulnerable to PHP Object Injection in all versions up to, and including, 2.12.1 via deserialization of untrusted input in the Telecom EDY payment callback (usces_action_acting_transaction). Unauthenticated attackers can store arbitrary 'reserve' key/value pairs as order metadata during a public checkout, then invoke the callback with an attacker-chosen 'option' parameter to select and unserialize that metadata without any provider signature, source-address, transaction-identity or ownership check. A POP chain is present in the TCPDF library bundled with the plugin itself, so no additional plugin or theme is required. This makes it possible for unauthenticated attackers to delete arbitrary files on the server, including wp-config.php, which can lead to remote code execution when an attacker re-runs the WordPress installer against a database they control. Successful exploitation is contingent on an admin printing an invoice to trigger file deletion.