WordPress 4.2.1 is now available. This is a critical security release for all previous versions and we strongly recommend you update your sites immediately.
A few hours ago, the WordPress team was made aware of a cross-site scripting vulnerability, which could enable commenters to compromise a site. The vulnerability was discovered by Jouko Pynnönen.
WordPress 4.2.1 has begun to roll out as an automatic background update, for sites that support those.
If your site does not support automatic background updates, or you aren’t sure, log into your site and head to Dashboard → Updates and simply click “Update Now”.