The XSS Game by Google
I'm quite surprised that these exploits aren't blocked at the browser level by default with developers having to write code to make the exploits work if they need to.For example, if browsers flatly refused to load code from an external URL unless the address was whitelisted in the page's HTTP response headers then you'd make level 6's exploit impossible without much of an impact on web development.The CORS header Access-Control-Allow-Origin can be used to force a browser to work that way, but only if a site sets it. I'm suggesting we're at the point now where browsers should be secure by default, even if it breaks some old sites.