GreyMagic Security Advisory GM#001-IE

by GreyMagic Software, Israel.
27 Feb 2002.

Topic: Executing arbitrary commands without Active Scripting or ActiveX.

Discovery date: 25 Feb 2002.

Affected applications:

Any application that hosts the WebBrowser control (5.5+) is affected since
this exploit does not require Active Scripting or ActiveX. Some of these
applications are:

• Microsoft Internet Explorer
• Microsoft Outlook
• Microsoft Outlook Express

Introduction:

In an advisory from Jan 10 2002 "The Pull" demonstrated how it is still
possible to use an older bug (initially discovered by Dildog) in the
<object> HTML element to run arbitrary commands.

Although "The Pull"'s findings were interesting, his analysis of the
re-found bug was erroneous, the problem does not lie within the Popup
object, the problem is with dynamically inserted HTML fragments at any point
in the document.

All "createPopup" does is create a (featureless) window containing an empty
HTML document, this does not pose a threat, but later on, that document has
HTML injected to it (using innerHTML), which is the actual problem.

For example, the following code will work just the same:

<span id="oSpan"></span>
<script language="jscript" defer>
oSpan.innerHTML='<object
classid="clsid:11111111-1111-1111-1111-111111111111"
codebase="c:/winnt/system32/calc.exe"></object>';
</script>

(Note: innerHTML is not the only property used to dynamically insert HTML to
any element, it is also possible to use outerHTML, insertAdjacentHTML and
more to gain the same results.)

Discussion:

So now that we identified the origin of the problem we can search for ways
to dynamically insert HTML without using any Active Scripting at all. It
will then become possible to use this bug in more "protected" environments,
such as Microsoft Outlook or Internet Explorer with Active Scripting and
ActiveX disabled.

One of the exciting features that came along in IE4 was Data Binding; it
enables developers to completely separate any application data from the
presentation layer. The data sources (DSO) for Data Binding can be almost
anything, CSV files (with TDC), HTML, XML and many more. Data Binding binds
HTML elements (data consumers) such as div or span to the DSO without need
for a single line of script code.

We found out that when the "dataFormatAs" attribute is set to "HTML" on the
consumer, Data Binding internally uses innerHTML in order to insert the data
into the element (otherwise innerText is used).

So all we need to do now is supply a DSO that contains the offending
<object> element, the rest will be done for us by the Data Binding engine,
no scripting needed.

Exploit:

In the following example we're using an XML data-island as our DSO and a
span element as the data consumer. Using XML is especially comfortable
because it can be embedded within the document, without need for external
requests that may be stopped by the host application.

<span datasrc="#oExec" datafld="exploit" dataformatas="html"></span>
<xml id="oExec">
<security>
<exploit>
<![CDATA[
<object id="oFile"
classid="clsid:11111111-1111-1111-1111-111111111111"
codebase="c:/winnt/system32/calc.exe"></object>
]]>
</exploit>
</security>
</xml>

Solution:

There is no configuration-tweaking workaround for this bug, it will work as
long as the browser parses HTML. The only possible solution must come in the
form of a patch from Microsoft.

Tested on:

IE5.5sp2 Win98, all patches, Active scripting and ActiveX disabled.
IE5.5sp2 NT4 sp6a, all patches, Active scripting and ActiveX disabled.
IE6sp1 Win2000 sp2, all patches, Active scripting and ActiveX disabled.
IE6sp1 WinXP, all patches, Active scripting and ActiveX disabled.

Demonstration:

We put together two proof-of-concept demonstrations:

• Simple: attempts to run "c:/winnt/system32/calc.exe".
• Advanced: lets the user pick what they want to run.

They can both be found at http://security.greymagic.com/adv/gm001-ie/.

Feedback:

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

• Copyright © 2002 GreyMagic Software.

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