The setRequestHeader method of the XMLHttpRequest object in Microsoft Internet Explorer 7 does not restrict the dangerous Transfer-Encoding HTTP request header, which allows remote attackers to conduct HTTP request splitting and HTTP request smuggling attacks via a POST containing a "Transfer-Encoding: chunked" header and a request body with an incorrect chunk size.

More... (http://nvd.nist.gov/nvd.cfm?cvename=CVE-2008-1545)