The Opt-In Downloads plugin for WordPress is vulnerable to arbitrary file uploads due to missing file type validation in the admin_upload() function in all versions up to, and including, 4.07. This makes it possible for authenticated attackers, with Subscriber-level access and above, to upload arbitrary files on the affected site's server which may make remote code execution possible. Due to the presence of an .htaccess file, this can only be exploited to achieve RCE on NGINX servers, unless another vulnerability is present.