Joomla! Core is prone to an open redirect vulnerability because the application fails to properly verify 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 3.x.x ranging from 3.0.0 and up to and including 3.4.1 are vulnerable.
Update to Joomla! Core version 3.4.2 or latest