There was recently reports of a security issue with Joomla! 1.5.26
Support for the Joomla 1.5 series ended in September of 2012.
If you still have a web site that's built on Joomla! 1.5, I recommend that you read:
Please contact us for a quote on migrating your old Joomla web site to the latest version (Joomla! 3.2).