Joomla 1.5 reaches its end of life in April 2012. After this point, no more security releases for Joomla 1.5 will be available. If a security issue is found in Joomla 1.5, it will not be patched by the project.
Joomla Announcements
Security News
Joomla! Security Center Announced that there is a XSS Vulnerability which effects Joomla! version 1.6.5 and all earlier 1.6.x versions.