Stumbleupon Cross-Site Scripting Vulnerability

While I have previously identified XSS and/or CSRF vulnerabilities in both Digg and Reddit, Stumbleupon has largely remained innocuous to these types of attacks for multiple reasons. First, the primary method of user-login and authentication is through the toolbar, which makes it substantially harder for malicious javascript intercept. Furthermore, because many of the many valuable user functions are triggered through the individual’s personal subdomain (user.stumbleupon.com) and www.stumbleupon.com, it becomes quite difficult to execute complex functions such as auto-voting or friend-adding. That being said, there are still work arounds that exist. In the proof of concept I was able to execute, the vector of attack was the invitefriends.php file which does...