source: https://www.securityfocus.com/bid/9286/info

my little forum is prone to a cross-site scripting vulnerability in the 'email.php' script. The source of the problem is that HTML and script code are not adequately sanitized from input supplied via the URI parameters. A remote attacker could exploit this issue by embedding hostile HTML and script code in a malicious link to the vulnerable script. 

http://www.example.com/forum/email.php?forum_contact="><script>alert(document.domain);</script>