The ZeM STL plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the [zemstl] shortcode in all versions up to and including 1.0. This is due to insufficient input sanitization and output escaping on user-supplied shortcode attributes, specifically the 'url', 'color', and 'bgcolor' parameters. These attribute values are directly interpolated into HTML attribute context without being passed through esc_attr() or any other escaping function. 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.