From the announcement:
Development has moved on to some exciting new features for the next major release, but an important security issue was brought to our attention which required an update for our users. The problem is not yet public but you should update your blog as soon as possible to 18.104.22.168. If you are unable to do upgrade in the short-term you may protect yourself by deleting the xmlrpc.php file from your WordPress directory.
This is a security release. I suggest you apply it or you might be the next to suffer the consequences. Don’t be a lazy ass like I was.