Joomla! Core is prone to a variable injection vulnerability. Exploiting this issue may allow attackers to inject unwanted characters into returned data; other attacks with unspecified impact 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