The Joomla! team have released an IMPORTANT upgrade for Joomla 1.5 which fixes a high level sercurity issue which allows an unauthenticated, unauthorized user to reset the password of the first enabled user (lowest id). Typically, this is the administrator user

You are advised to upgrade to the latest Joomla! 1.5.6 release ASAP to fix the security hole.

For more information visit the offical Joomla! web site.