Joomla! Core is prone to an open redirect vulnerability because the application fails to properly validate user-supplied input. Exploiting this issue may allow attackers to redirect users to arbitrary web sites and conduct phishing attacks; other attacks are also possible. Joomla! Core versions 1.5.x ranging from 1.5.0 and up to and including 1.5.6 are vulnerable.
Update to Joomla! Core version 1.5.7 or latest