source: https://www.securityfocus.com/bid/6757/info

An information disclosure weakness has been reported for Opera 7 browsers on the Microsoft Windows platform.

The weakness is due to the way the history object exposes some properties. Specifically, the properties history.next and history.previous are exposed. Any site can make use of these properties to track a user's web surfing habits.

alert("Last URL: "+history.previous+".\nNext URL: "+history.next+".");