The Prototype (prototypejs) framework before 1.5.1 RC3 exchanges data using JavaScript Object Notation (JSON) without an associated protection scheme, which allows remote attackers to obtain the data via a web page that retrieves the data through a URL in the SRC attribute of a SCRIPT element and captures the data using other JavaScript code, aka "JavaScript Hijacking."
http://www.fortifysoftware.com/servlet/downloads/public/JavaScript_Hijacking.pdf
http://www.debian.org/security/2009/dsa-1952
http://secunia.com/advisories/37677