The Pie Forms for WP plugin for WordPress is vulnerable to Arbitrary File Upload in all versions up to, and including, 1.6 via the format_classic function. This is due to insufficient file type validation where the validate_classic method validates file extensions and sets error messages but does not prevent the file upload process from continuing. This makes it possible for unauthenticated attackers to upload files with dangerous extensions such as PHP, which makes remote code execution possible. In order to exploit this vulnerability, the attacker needs to guess the directory in which the file is placed (which is a somewhat predictable hash). In addition to that, the file name is generated using a secure hash method, limiting the exploitability of this vulnerability.