XSS attacks account for 12.75% of all of today’s Web-based attacks, and almost 70% of all reported vulnerabilities are XSS-related. Almost 90% of all websites have at least one XSS vulnerability.
http://news.softpedia.com/news/infographic-how-xss-attacks-work-490686.shtml
origin - http://www.pipiscrew.com/?p=1812 how-xss-attacks-work