Ungapped Widgets <= 1 - Authenticated (Contributor+) Stored Cross-Site Scripting via Shortcode

The Ungapped Widgets plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the 'prefillvalues' parameter in the ungapped-form shortcode in all versions up to, and including, 1. This is due to insufficient input sanitization and output escaping on user-supplied attributes. This makes it possible for authenticated attackers, with contributor-level access and above, to inject arbitrary web scripts in pages that will execute when a user accesses an injected page.