Calculated Fields Form <= 5.5.1.3 - Reflected DOM-Based Cross-Site Scripting via 'x' URL Parameter via setChoices()

The Calculated Fields Form – AI Form Builder for WordPress – Contact, Payment, Quote, Quiz & More plugin for WordPress is vulnerable to Reflected DOM-Based Cross-Site Scripting via the 'x (any URL parameter consumed by the form's calculated equation)' parameter in all versions up to, and including, 5.5.1.3 due to insufficient input sanitization and output escaping. This makes it possible for unauthenticated attackers to inject arbitrary web scripts in pages that execute if they can successfully trick a user into performing an action such as clicking on a link. Exploitation requires the target site to have a public form configured with a Select2-enabled dropdown whose choices are populated via a calculated equation that pipes a URL parameter through GETURLPARAMETER() into setChoices({texts:[...]}); given that configuration, exploitation requires only a single crafted link.