The Avada (Fusion) Builder plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the 'tab_index, empty_notice' parameter in all versions up to, and including, 3.16.2 due to insufficient input sanitization and output escaping. This makes it possible for authenticated attackers, with contributor-level access and above, to inject arbitrary web scripts in pages that will execute whenever a user accesses an injected page. wp_kses_post does not strip the payload on save because the dangerous characters reside within shortcode attribute text between square brackets, which kses treats as plain text rather than HTML markup.