After hearing of some confusion regarding this vulnerability I thought it
might be useful to post the description from our SF database to the list.
This was written up by Eric Schultze and myself, and we would like to
thank Georgi Guninski, Steve Lipner, and David LeBlanc for their help.
Thanks also to Elias Levy for providing a background into the DHTML
behaviors.

The issue is the following:

The "download behavior" feature of Microsoft's Internet Explorer 5 may
allow a malicious web site operator to read files on an IE5 client
computer or on a computer that is in the client's 'Local Intranet' web
content zone.

IE5 introduced a new feature called DHTML Behaviors. DHTML Behaviors allow
web developers to encapsulate methods, properties and events that can then
be applied to HTML and XML elements. IE5 comes with set of built-in DHTML
behaviors. One of them is the "#default#download" behaviors. This behavior
defines a new Javascript method called "startDownload" that takes two
parameters, the file to download and a function to call once the file has
been downloaded.

By default the "startDownload" method checks that the file to be
downloaded is in the same web content zone as the file calling the method.
When both the file to be downloaded and the file executing the behavior
are in the same security zone, the client will safely download the
requested file and subsequently perform the specified function.

A malicious web site owner may bypass this security restriction and force
an IE5 client to both read and perform a follow-up action on the contents
of a local file or files in other security zones. This action may include
sending the contents of the file back to the malicious web site operator.

 Here's how it works:

1: An IE5 client visits a malicious website and loads a web page
containing a client side scripting that makes use of the
"#default#download" behavior.

2: The client side script calls the "startDownload" method and passes it
the URL of a file to download and a function to call with the contents of
the file once the file is finished downloading.

3. The startDownload method verifies that the URL is in fact in the same
zone as the malicious web server.

4: The startDownload method begins the download, requesting the URL
specified in step 2 from a malicious web server.

5: The malicious web server send an HTTP redirect to some other file in
any security zone including local files on the IE5 client machine (for
example: c:\winnt\repair\sam._).

6: startDownload reads the file and executes the function specified in
step 2 on that file's content.

The malicious web server has now bypassed the security restrictions
outlined earlier by successfully forcing the client to load and act upon a
file that resides in a web content zone different than that of the
malicious web server. This can all be done transparently to the end user.

This vulnerability cannot be used to delete or modify files on the
vulnerable IE5 client. The vulnerability can only retrieve text files or
small parts of binary files.


Ben Greenbaum
Site Content Manager
Security Focus
http://www.securityfocus.com