What is happening to Joomla 1.5?

Joomla 1.5 is being phased out, and will no longer be supported after April 2012. This means that there will be no further security releases after this point, and website using Joomla 1.5 will be vulnerable to attacks from hackers and phishers. Accordingly it is recommended that all Joomla 1.5 sites be migrated to Joomla 2.5 as soon as possible.

Because Joomla is open source, all of the code base is available for the public to view. This means that security vulnerabilities can be found and exploited, but also that they can be patched and prevented very quickly. Joomla 1.5 is a very mature platform, but security releases have still been coming out at regular intervals, and it is almost certain that security issues will become public knowledge after support lapses, and they will not be fixed.