Ray has written a detailed blog article on the Joomla security hole that is currently causing a lot of problems in the wild. This issue exists in versions 2.5.x and 3.1.x. If you use Joomla - even if you have an old, unused test install somewhere - you really have to upgrade to the latest version, or, if you aren't able to upgrade for whatever reason, apply the fix that Ray lays out in the article.
