Super Forms <= 6.3.316 - Authenticated (Subscriber+) Arbitrary File Upload via 'extensions' Form Element Attribute

The Super Forms – Drag & Drop Form Builder plugin for WordPress is vulnerable to Unrestricted File Type Upload in all versions up to, and including, 6.3.316 via the upload_files function. This is due to missing file type validation in the upload_files function, which reads and applies an attacker-controlled extensions string from _super_elements post meta verbatim as the allowed MIME type map. This makes it possible for authenticated attackers, with Subscriber-level access and above, to upload files that may be executable, which makes remote code execution possible. The attack requires a preceding step: poisoning the _super_elements post meta via the super_save_form AJAX handler, which lacks a capability and nonce check but requires the attacker to be authenticated as at minimum a Subscriber-level user; the subsequent file upload via super_upload_files requires no authentication at all.