Just saw this was released Monday December 14, 2015.
Not 100% what the security hole is, but it seems to be a big one. This time all Joomla Versions from 1.5 to 3.4.5 are affected.
Here is the official download page
https://www.joomla.org/announcements/release-news/5641-joomla-3-4-6-released.html