After the last release of Joomla 1.5.15 on November 4, 2009, The Joomla Project has released a major security release Joomla 1.5.16.

It is highly recommended to upgrade immediately to JOOMLA! 1.5.16.

Please see the official announcements:

http://www.joomla.org/announcements/release-news/5259-joomla-1516-released.html