Joomla! has released the latest version of their Open Source Software. In a nutshell, the release contains fixes for 1 critical and 2 moderate vulnerabilities.
The Developers urge all 1.5.x users to upgrade _now_
More details can be found on the official announcement: http://www.joomla.org/announcements/release-news/5212-joomla-157-security-release-now-available.html
Find your update package here: http://joomlacode.org/gf/project/joomla/frs/?action=FrsReleaseBrowse&frs_package_id=3947