I think the name “cross site” is confusing. It’s easy to hear that and think it involves code on one website attacking code on another website. That’s not what it is. Not to mention its unfortunate “true” acronym.
It simply means: executing abritrary JavaScript code on the page.
This could be JavaScript that is inserted into the URL or through form submissions. If either of those ways of accepting information doesn’t “clean” the information it is getting before outputting it again on the page, then arbitrary JavaScript can run on that page and that’s an XSS vulnerability.
If JavaScript can run on the page, then it can access cookies.
If it can access cookies, then it can access active sessions.
If it can access active sessions, it can log in as you to websites you are logged in to, at least long enough to change passwords or other havoc.
Symantec has said that 80% of internet vulnerabilities are due to XSS.
Read More at : http://css-tricks.com/what-is-xss/
