A few hours ago, the WordPress team was made aware of a cross-site scripting vulnerability, which could enable commenters to compromise a site, and issued a security patch with version number: "4.2.1". I highly recommend people to update to the latest version a.s.a.p. as usual.
To update Wordpress to version 4.2.1 go to your "Wordpress Dashboard" and hit the refresh plugins button and install any available updates, or download the patch/zip from the website and install it manually.