Brave Popup Builder < 0.8.8 - Unauthenticated Arbitrary Shortcode Execution via UTM Parameter

Description

The plugin does not prevent a URL parameter used to pre-fill a form field from being passed to WordPress's shortcode engine, allowing unauthenticated attackers to have arbitrary shortcodes registered on the site executed server-side.

Reaching it requires the site to have published a popup containing a hidden form field configured to take its default value from a URL parameter.