Joomla! 3.4.6 is now available. This is a security release for the 3.x series of Joomla which addresses a critical security vulnerability. We strongly recommend that you update your sites immediately.

Source: Joomla! 3.4.6 Released

Here is a link to the Official Announcement at Joomla.org.
If you are running Joomla 2.5.28 (which is end of life) YOU CAN STILL BE HACKED – you need to apply a community supplied patch which is going to be made available on this page

If you are running Joomla 1.5.26 (which is end of life) YOU CAN STILL BE HACKED – you need to apply a community supplied patch which is going to be made available on this page

We recommend users of Joomla consider a myJoomla.com subscription where you can find out loads about your site and its security issues.