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

The plugin 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.

PoC

As a contributor, create a blank form and add custom html field with the following content in the “Text” tab of the field editor:

Some description about this section

Do not decode the payload. And please ensure that payload is added when editor has Text tab selected. Save the form, it will trigger xss payload.