Ninja Forms - File Uploads <= 3.3.34 - Unauthenticated Arbitrary File Upload

The Ninja Forms - File Uploads plugin for WordPress is vulnerable to arbitrary file operations in all versions up to, and including, 3.3.34 via the external (Amazon S3) upload flow. The plugin trusts an attacker-supplied file path from the form submission and stores it as the upload's file_path, which is then used without validation to attach a file to the form's notification email (arbitrary file read), to write fetched content (arbitrary file write, leading to remote code execution when the external store is configured), and in a scheduled deletion (arbitrary file deletion). This makes it possible for unauthenticated attackers to read, write, or delete arbitrary files on the server. Exploitation requires the site to use the plugin's External File Upload (Amazon S3) action; the read variant additionally requires a form Email action configured to attach the uploaded file.