An urgent, high-level security threat has been identified in Joomla 1.5. This exploit allows someone to gain full access to your joomla web site with little or no knowledge. The official announcement can be found on Joomla.org. All users of Joomla 1.5 should IMMEDIATELY upgrade to 1.5.6 or apply a simple patch.

More information about the upgrade can be found here http://joomla.org/announcements/release-news/5199-joomla-156-released.html