Just shortly after WordPress 4.2 was released a critical security flaw was discovered. WordPress 4.2.1 is now available for everyone to install. This is a critical security release for all previous versions and we strongly encourage you to 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.