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.