FluentForms < 4.3.25 - Contributor+ Stored XSS via Custom HTML Form Field

The Contact Form Plugin WordPress plugin before 4.3.25 does not properly sanitize and escape the srcdoc attribute in iframes in it's custom HTML field type, allowing a logged in user with roles as low as contributor to inject arbitrary javascript into a form which will trigger for any visitor to the form or admins previewing or editing the form.