Perhaps not surprisingly, there appears to be a vulnerability in how
Microsoft Internet Explorer handles (or fails to handle) certain
combinations of nested OBJECT tags. This was tested with MSIE
6.0.2900.2180.xpsp.040806-1825 and mshtml.dll 6.00.2900.2873
xpsp_sp2_gdr.060322-1613.
At first sight, this vulnerability may offer a remote compromise vector,
although not necessarily a reliable one. The error is convoluted and
difficult to debug in absence of sources; as such, I cannot offer a
definitive attack scenario, nor rule out that my initial diagnosis will be
proved wrong [*]. As such, panic, but only slightly.
Probably the easiest way to trigger the problem is as follows:
perl -e '{print "<STYLE></STYLE>\n<OBJECT>\nBork\n"x32}' >test.html
...this will (usually) cause a NULL pointer + fixed offset (eax+0x28)
dereference in mshtml.dll, the pointer being read from allocated but still
zeroed memory region.
The aforementioned condition is not exploitable, but padding the page with
preceeding OBJECT tag (and other tags), increasing the number of nested
OBJECTs, and most importantly, adding bogus 'type=' parameters of various
length to the final sequence of OBJECTs, will cause that dereference to
become non-NULL on many installations; then, a range of other interesting
faults should ensue, including dereferences of variable bogus addresses
close to stack, or crashes later on, when the page is reloaded or closed.
[ In absence of sources, I do not understand the precise underlying
mechanics of the bug, and I am not inclined to spend hours with a
debugger to find out. I'm simply judging by the symptoms, but these
seem to be indicative of an exploitable flaw. ]
Several examples of pages that cause distinct faults in my setup (your
mileage may and probably WILL vary; on three test machines, this worked as
described; on one, all examples behaved in non-exploitable 0x28 way):
http://lcamtuf.coredump.cx/iedie2-1.html (eax=0x0, instant dereference)
http://lcamtuf.coredump.cx/iedie2-2.html (bogus esi on reload/leave)
http://lcamtuf.coredump.cx/iedie2-3.html (page fault on browser close)
http://lcamtuf.coredump.cx/iedie2-4.html (bogus esi on reload/leave)
Well, that's it. Feel free to research this further. This vulnerability,
as requested by customers, is released in strict observance of the Patch
Wednesday & Bug Saturday policy.
[*] The ability of the attacker to document the attack scenario probably
doesn't matter for those who pretend to care; cryptic "hi" to
Secunia and their standards of conduct.
======================
Remote overflow in MSIE script action handlers (mshtml.dll)
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From: Michal Zalewski <lcamtuf_at_dione.ids.pl>
Date: Thu, 16 Mar 2006 20:22:55 +0100 (CET)
Good morning,
This might not come as a surprise, but there appears to be a *very*
interesting and apparently very much exploitable overflow in Microsoft
Internet Explorer (mshtml.dll).
This vulnerability can be triggered by specifying more than a couple
thousand script action handlers (such as onLoad, onMouseMove, etc) for any
single HTML tag. Due to a programming error, MSIE will then attempt to
write memory array out of bounds, at an offset corresponding to the ID of
the script action handler multiplied by 4 (due to 32-bit address clipping,
the result is a small positive integer).
The list of IDs can be found on the Web, and is as follows (values in
parentheses = resulting offsets):
onhelp = 0x8001177d (+0x45df4)
onclick = 0x80011778 (+0x45de0)
ondblclick = 0x80011779 (+0x45de4)
onkeyup = 0x80011776 (+0x45dd8)
onkeydown = 0x80011775 (+0x45dd4)
onkeypress = 0x80011777 (+0x45ddc)
onmouseup = 0x80011773 (+0x45dcc)
onmousedown = 0x80011772 (+0x45dc8)
onmousemove = 0x80011774 (+0x45dd0)
onmouseout = 0x80011771 (+0x45dc4)
onmouseover = 0x80011770 (+0x45dc0)
onreadystatechange = 0x80011789 (+0x45e24)
onafterupdate = 0x80011786 (+0x45e18)
onrowexit = 0x80011782 (+0x45e08)
onrowenter = 0x80011783 (+0x45e0c)
ondragstart = 0x80011793 (+0x45e4c)
onselectstart = 0x80011795 (+0x45e54)
What happens next depends on the structure of the page in which the
malicious tag is embedded, as well as previously visited page and
previously initialized extensions (all these factors can be controlled by
the attacker).
When the offending page contains no additional elements, and the user is
not redirected from elsewhere, the browser will typically crash
immediately, because there is no allocated memory at the resulting offset.
In all other cases, crashes will typically occur later, due to attempted
use of unrelated but corrupted in-memory buffers -for example, when the
user attempts to leave or reload the page. Another good example is coming
from a page that contains Macromedia Flash - this usually causes the Flash
plugin itself to choke on corrupted memory on cleanup.
For non-believers, there's a short but fiery demonstration page available
at http://lcamtuf.coredump.cx/iedie.html (yes, it will probably crash your
browser).
Tested on MSIE 6.0.2900.2180.xpsp2.040806-1825 on Windows XP SP2. As far
as I can tell, other browser makes (Firefox, Opera) are not susceptible to
this attack.
I eagerly await due reprimend from Microsoft for not disclosing this
vulnerability in a manner that benefits them most, not passing start, not
collecting $200 (from iDefense?).
Regards,
/mz
