The child frames in Microsoft Internet Explorer 7 inherit the default charset from the parent window when a charset is not specified in an HTTP Content-Type header or META tag, which allows remote attackers to conduct cross-site scripting (XSS) attacks, as demonstrated using the UTF-7 character set.
http://www.vupen.com/english/advisories/2007/0744
http://www.securityfocus.com/bid/22701
http://www.securityfocus.com/archive/1/461076/100/0/threaded