According to the news item from joomla.org,
This version has been triggered by reported cross site scripting problems that could occur under the right circumstances. We have also addressed many other general bugs and problems as well.
This release addresses serious security vulnerabilities and you should proceed with testing on a backup copy of your site. Once you have verified that your site works as expected, we recommend upgrade your live site as soon as possible.
So there you have it... upgrade your site as soon as possible. Will do! But how? Learn more about upgrading.