Internet Explorer 6.0, and possibly other versions, allows remote attackers to bypass the same origin security policy and make requests outside of the intended domain by calling open on an XMLHttpRequest object (Microsoft.XMLHTTP) and using tab, newline, and carriage return characters within the first argument (method name), which is supported by some proxy servers that convert tabs to spaces.  NOTE: this issue can be leveraged to conduct referer spoofing, HTTP Request Smuggling, and other attacks.