Internet Explorer 7 ActiveX bgColor property NULL pointer dereference (DoS)

[Alexander Sotirov <asotirov@determina.com>]

I thought that after the success of MoBB last year, fuzzing browsers will be
pointless, since all vendors would take care of the easily-found bugs before a
release. It turns out that I was wrong. I ran a very simple ActiveX fuzzer
against Vista and found a NULL pointer dereference bug in no time. The
vulnerable ActiveX control is on the pre-approved list in IE7, which makes the
bug easy to trigger with no security warnings and no user interaction.

Try this:

<script language="JavaScript">
    obj = new ActiveXObject("giffile");
    obj.bgColor;
</script>

MSRC said that this is a reliability bug and not a security issue, and it will
be fixed at some point in the future. I agree that DoS bugs against IE are not
very important (as long as skape doesn't drop any more vulns like MS06-051 :-),
but it's interesting that such a simple bug in such an obvious part of the IE7
attack surface was not discovered and fixed before the release.

See the full technical details at
http://www.determina.com/security.research/vulnerabilities/activex-bgcolor.html

More about fuzzers and ActiveX at
http://determina.blogspot.com/2007/01/fuzzing-shouldnt-work.html


Alexander Sotirov
Determina Security Research