Formidable Forms < 6.35 - Unauthenticated Arbitrary Shortcode Execution via [entry_key] Custom HTML Token

Description

The plugin does not prevent a request-derived value from reaching the WordPress shortcode parser when it substitutes a supported token into a form's custom HTML, allowing unauthenticated visitors to have arbitrary shortcodes, with attacker-chosen attributes, executed server-side on any page displaying an affected form.

Exploitation requires the site owner to have added the token to one of the form's custom HTML blocks; it is a documented, supported token but is not present in the shipped default.