GreyMagic Security Advisory GM#009-IE

By GreyMagic Software, Israel.
23 Aug 2002.

Available in HTML format at http://security.greymagic.com/adv/gm009-ie/.

Topic: Accessing remote/local content in IE.

Discovery date: 18 Feb 2002.

Affected applications:

All tested versions of Microsoft Internet Explorer (IE5+); prior versions
may be vulnerable as well.

Note that any other application that uses Internet Explorer's engine
(WebBrowser control) is affected as well (Outlook, MSN Explorer, etc.).

Introduction:

Back in 1997, when Internet Explorer 4 was first released, XML was just
starting to become popular. The popularity of XML prompted Microsoft to
devise the early prototype of XML data-islands, using the <script> element
in conjunction with the "language" attribute set to "XML".

When Internet Explorer 5 was released the <script> data-island had been
replaced with the more intuitive <xml> data-island. But even though the
<script> syntax was now deprecated, Microsoft kept it for the sake of
backwards compatibility.

Discussion:

By default the <script> data-island only allows URLs from the same domain as
the document to be assigned to its "src" attribute. Unfortunately, it fails
to correctly validate its "src" attribute against a redirection, which
potentially allows any web page to do the following:

• Read XML files from any URL.
• Read portions of non-XML files from any URL.
• Read local XML files, in some setups.
• Read portions of local non-XML files, in some setups.

Any of the exploitations above may expose sensitive and private information
belonging to the user.

The <script> data-island can be exploited by assigning a URL to the "src"
attribute, which will redirect to a local or remote location. The XML engine
will then process the content of that location.

In case the location does not contain valid XML content, parts of it can
still be read by using XMLDOM's error mechanism. The "srcText" property of
the "parseError" object will contain the line of text that could not be
parsed by the engine. If the content does not contain line breaks at all
then the entire content may be exposed.

Exploit:

This example reads the contents of a remote URL, "getFile.asp" redirects to
the desired location, allowing us to read it:

<script language="xml" src="getFile.asp" id="oFile"></script>
<script language="jscript">
onload=function () {
    var oXD=oFile.XMLDocument,
        oPE=oXD.parseError;

alert(
        oXD.firstChild || oPE.line>0 ?
            "File found!\n"+
            "Details:\n\n"+
            (oXD.xml || "Line "+oPE.line+" contains '"+oPE.srcText+"'")
        :
            "File does not exist or could not be retrieved."
    );
}
</script>

Solution:

Microsoft was first informed on 18 Feb 2002, a patch was finally released on
22 Aug 2002 after a long investigation and testing period.

The patch may be downloaded from:
http://www.microsoft.com/technet/security/bulletin/ms02-047.asp.

Tested on:

IE5 NT4, remote locations only.
IE5.5 Win98, remote locations only.
IE5.5 NT4, both remote and local locations.
IE6 Win2000, remote locations only.
IE6 WinXP, both remote and local locations.

Demonstration:

A fully dynamic proof-of-concept demonstration of this issues is available
at http://security.greymagic.com/adv/gm009-ie/.

Feedback:

Please mail any questions or comments to [email protected].

• Copyright © 2002 GreyMagic Software.