The WP Meta SEO plugin for WordPress is vulnerable to Unauthenticated Stored Cross-Site Scripting via the REQUEST_URI server variable in all versions up to, and including, 4.5.18. When the plugin's wpmsTemplateRedirect() hook detects a 404, it concatenates $_SERVER['HTTP_HOST'] with the raw $_SERVER['REQUEST_URI'] and inserts that value verbatim into the wp_wpms_links.link_url column via $wpdb->insert(). This makes it possible for unauthenticated attackers to inject arbitrary web scripts that execute whenever an administrator views the plugin's 404 & Redirects admin page (/wp-admin/admin.php?page=metaseo_broken_link).