The Ninja Forms WordPress plugin before 3.14.10 does not prevent user-supplied query-string input, used to pre-populate a form field's default value, from being processed as a shortcode, allowing unauthenticated attackers to execute arbitrary shortcodes registered on the site when a form so configured is embedded on a public page.
https://wpscan.com/vulnerability/9c96f977-c105-4b45-a3cd-4751fb4aa5a1/