Cross-site scripting (XSS) vulnerability in js/wp-seo-metabox.js in the WordPress SEO by Yoast plugin before 2.2 for WordPress allows remote attackers to inject arbitrary web script or HTML via the post_title parameter to wp-admin/post-new.php, which is not properly handled in the snippet preview functionality.
https://yoast.com/wordpress-seo-2-2/
https://wordpress.org/plugins/wordpress-seo/changelog/