The Pie Register Premium plugin for WordPress is vulnerable to Unrestricted File Type Upload in all versions up to, and including, 3.8.4.0 via the pieregfileupload function. This is due to the plugin performing file type validation via wpcheckfiletype and wpcheckfiletypeandext but never checking whether those validation calls produced errors before moveuploadedfile() is called, combined with no nonce or capability check on the AJAX handler. This makes it possible for unauthenticated attackers to upload files of dangerous types (e.g. PHP scripts) that may be executable, which makes remote code execution possible. The server returns the generated destination filename in the response, giving the attacker the exact path needed to trigger the uploaded file.