Technical Analysis of CVE-2013-3147

In July, Microsoft released a patch for
a memory-corruption vulnerability in the Internet Explorer (IE) Web
browser. The vulnerability enabled remote attackers to execute
arbitrary code or cause a denial of service through a crafted or
compromised website — also known as a drive-by download.

This vulnerability is particularly
interesting because it is so simple that even some tutorials on the
web can trigger it, and it is unique to IE. The vulnerability exists
in onbeforeeditfocus event. Only Internet Explorer
supports this event (detailed information can be found at


The vulnerability triggers when the document markup is manipulated
inside the handler of onbeforeeditfocus event. It leads to a
crash in mshtml!CElement::BubbleBecomeCurrent when the
instruction tries to access the freed pointer.

Following is the state of registers at
the time of crash.

0:008> r

eax=00000004 ebx=00000000 ecx=06034f88 edx=80000035 esi=06082fb0

edi=048466a8 eip=636b31fe esp=037cf9b8 ebp=037cf9d4 iopl=0 nv up ei
pl zr na pe nc

cs=001b ss=0023 ds=0023 es=0023 fs=003b gs=0000 efl=00010246


636b31fe 8b7604 mov esi,dword ptr [esi+4] ds:0023:06082fb4=????????

At this stage ESI holds the freed pointer.

0:008> !heap -p -a esi

address 06082fb0 found in

_DPH_HEAP_ROOT @ 141000

in free-ed allocation ( DPH_HEAP_BLOCK: VirtAddr VirtSize)

5802a58: 6082000 2000

7c927553 ntdll!RtlFreeHeap+0x000000f9

63625834 mshtml!CTreeNode::Release+0x0000002d

63640cea mshtml!PlainRelease+0x00000033

63662fec mshtml!CTreeNode::NodeRelease+0x0000002a

635bd2d4 mshtml!CElement::BecomeCurrent+0x00000145

636b31eb mshtml!CElement::BubbleBecomeCurrent+0x00000094

637eeb74 mshtml!CElement::BubbleBecomeCurrent+0x0000004c

636b5dd5 mshtml!CDoc::PumpMessage+0x0000096a

635f1fea mshtml!CDoc::OnMouseMessage+0x0000055d

635ef664 mshtml!CDoc::OnWindowMessage+0x000007af

6364207a mshtml!CServer::WndProc+0x00000078

7e418734 USER32!InternalCallWinProc+0x00000028

7e418816 USER32!UserCallWinProcCheckWow+0x00000150

7e4189cd USER32!DispatchMessageWorker+0x00000306

7e418a10 USER32!DispatchMessageW+0x0000000f

02562ec9 IEFRAME!CTabWindow::_TabWindowThreadProc+0x00000461

Following was the size of allocation where ESI is pointing:

ESI: 6082fb0

address 06082fb0 found in

_DPH_HEAP_ROOT @ 141000

in busy allocation ( DPH_HEAP_BLOCK: UserAddr UserSize – VirtAddr VirtSize)

5802a58: 6082fb0 4c – 6082000 2000

7c918f01 ntdll!RtlAllocateHeap+0x00000e64

635a8225 mshtml!CHtmRootParseCtx::BeginElement+0x00000030

635bc1e9 mshtml!CHtmTextParseCtx::BeginElement+0x0000006c

635a8420 mshtml!CHtmParse::BeginElement+0x000000b7

635a93b4 mshtml!CHtmParse::ParseBeginTag+0x000000fe

635a6bb6 mshtml!CHtmParse::ParseToken+0x00000082

635a7ff4 mshtml!CHtmPost::ProcessTokens+0x00000237

635a734c mshtml!CHtmPost::Exec+0x00000221

635ac2b8 mshtml!CHtmPost::Run+0x00000015

635ac21b mshtml!PostManExecute+0x000001fd

635cece9 mshtml!CPostManager::PostManOnTimer+0x00000134

6364de62 mshtml!GlobalWndOnMethodCall+0x000000fb

6363c3c5 mshtml!GlobalWndProc+0x00000183

7e418734 USER32!InternalCallWinProc+0x00000028

7e418816 USER32!UserCallWinProcCheckWow+0x00000150

7e4189cd USER32!DispatchMessageWorker+0x00000306

But the story doesn’t end here. The
instruction on which we are getting an exception is moving the pointer
at [ESI+4] to ESI. So looking to the pointer, which is at [ESI+4], is
important. If we monitor the ESI, then we will see the following
sequence (note: the value of ESI is changed from the previous one
because I collected this information at the second run):

ESI: 48bafb0 [ESI+4]: 6050fb0 [[ESI+4]]: 5ff0fd0 [[[ESI+4]]]: 635ba8c0

ESI: 48bafb0 [ESI+4]: 0 <---------------------- Exception

The exception instruction is expecting
6050fb0 at [ESI+4]. But because the pointer is freed,
we hit an exception. The address 6050fb0 is the
pointer to object 5ff0fd0 of type
635ba8c0 as we can see in the above lines and can
also verify with the below windbg command.

0:008> ln 635ba8c0

(635ba8c0) mshtml!CBodyElement::`vftable’ | (637812c0) mshtml!CStackDataAry

<unsigned short,128>::`vftable’

Exact matches:

mshtml!CBodyElement::`vftable’ = <no type information>

Possible Code Execution Path:

This situation can lead to code
execution because the value at [ESI+4] propagates to  the
CElement::BecomeCurrent method which again calls
the CElement::Doc method to access the vftable and
call a function.

Following is the disassembly of the
faulty function:

Our program crashes here:






Here, a few checks are getting the value
of EBX. But EBX is also in control, and the value of EBX remains the
same as it was in the beginning of the function call.






This isn’t the first IE vulnerability in
which the CElement::Doc function played a role — in at
least one other past vulnerability
function played a key role in the code execution.

(Thanks to my colleagues Abhishek Singh
and Yichong Lin for their suggestions.)

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