The Flipbox Addon for Elementor plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the Flipbox widget's button URL `custom_attributes` field in all versions up to, and including, 2.1.1 due to insufficient validation of custom attribute names. Specifically, the plugin uses `esc_html()` on the attribute name which does not prevent event handler attributes (e.g., `onmouseover`, `onclick`). This makes it possible for authenticated attackers, with author-level access and above, to inject arbitrary web scripts in pages that will execute whenever a user accesses an injected page.