CVE-2012-4792

A new year has arrived and, although a little late, the time has come for me to unpack the present that Santa gave to the Council on Foreign Relations this Christmas. Quite a few blogs have already been written in this issue that has gotten CVE-2012-4792, including one by Microsoft, but that didnt stop me from doing my own analysis. I tried to document all the steps I took and write down how I came to my conclusions so readers can follow and maybe repeat this process. All my work has been done on IE8 running on Windows XP, but most of it also applies to Window 7 with the exception of ASLR issues. The mshtml version I worked with is 8.0.6001.19393

• Analysis
• Exploitation
• Conclusion

Analysis

The first thing I did was grab the Metasploit version of the exploit and remove all heapspay and other items to get a clean poc. This resulted in the following html data.

The next step was to turn on pageheap and user stack trace for internet explorer and just run the poc and see what happened.

This resulted in the following windbg log:

Just from this initial run we can already draw some conclusions. The freed object was a CButton object, as can be deducted from the stack trace of the freed memory “mshtml!CButton::`vector deleting destructor'”. And the reuse of the freed memory seem to occur when the onload handler is completely finished: mshtml!CMarkup::OnLoadStatusDone+0x4ef.

When we look back at the HTML code some of it makes sense

This is most likely the code that created the object that is freed later on. Let see if we can find out at what point the object is being freed and when it is being reused. For that we change the javascript to include some log messages that we can use to determine when things are happening. We will also add 2 breakpoints the keep track of the creation and deletion of CButton objects. Creating a CButton object will go through “CButton::CreateElement“.

If we set a breakpoint just after the call to HeapAlloc we know the address of the CButton that has been created. We already know the function responsible for deleting a CButton object and we will set a breakpoint there as well.

By adding javascript log messages between all the call we can easily keep track of the progress of the poc while it runs.

We now run the poc again.

We break when jscript.dll is loaded (sxe ld:jscript) and then set the breakpoints to print out the log messages and CButton creation and deletions. The CButton object is deleted during the call to CollectGarbage but is not reused until after that call is finished. So we can easily take control over the freed objects memory by creating some data of the right size, but more about that later.
The next step is trying to figure out why this use-after-free is actually happening. Microsoft already gave us some hints on the root cause of the issue with their blog post.

Lets go back to the crash and see where edi (pointing to the freed memory) comes from.

Apparently the CElement::FindDefaultElem function returns the CButton element after it already has been freed. This is the function that Microsoft patched out with their Fix it Shim so we are on the right track. This function is called a few times before the process crashes so to make our life easier we wont break on this function but rather on the call to this function in the CMarkup::OnLoadStatusDone function. As a side note: it is also clear that it is realy easy to get EIP control through this freed object since we straight up grab the vftable from the freed object (mov eax, [edi]) and then call a function (call dword ptr [eax+0DCh]) from the vftable. Anyway, here is the windbg log with breakpoints at CButton create and delete so we know what the address of the CButton object was and a breakpoint in the CMarkup::OnLoadStatusDone function before the call to CElement::FindDefaultElem.

The log has been edited to make it a little bit more readable, but what we gain from this is that our CButton object is still referenced in a CDoc element. The next step is to run the poc again (yes, we’ll be doing this a lot) and see why and when the reference is planted there. For this we will break on the mshtml!CDoc::operator new function and then set a memory breakpoint on CDoc Object + 0x1A8 to see which functions write to this location.

It looks like the CElement::SetDefaultElem ‘forgets’ to call AddRef on an object before it adds a reference to the object to the main CDoc object. As such the object can be freed by removing all other references to the object and will still be accessible through the Default Element reference in the CDoc object.

Now that we have a rough idea of what is going on we can try to simplify the PoC a bit more. After I did my own reduction I read the BinVul.com blogpost by @h4ckmp who came to mostly the same conclusions as I did.

Lets start by reading and commenting the POC. First of all, we have a html document with an empty form element and a dfn element. When the document is loaded we start our evil code.

Get a reference to the form object

Get a reference to the dfn object

Create a ‘Q’ element

Set the Q element as the parent of the DFN object. Our (partial) DOM Tree looks like this: Q->DFN

We added a Button element to the DFN Element and our DOM Tree now looks like this: Q->DFN->BUTTON

We squeeze the FORM element in between the Q and the DFN element by setting the FORM element as the parent of the DFN element and now we have this DOM Tree: Q->FORM->DFN->BUTTON

And we just deleted everything …. our (partial) DOM Tree now only holds the Q element and all the references we had to the CButton object have been released again.

This code is not really necessary to cause the use-after-free but it does make it easier to trigger. I tried to dig up why bit couldn’t come up with an easy explanation.

Just looking at this makes me wonder if we can make this a little bit cleaner. Maybe we don’t even need the DFN and the Q objects at all and just adding a Button to a document and then assigning it to a FORM object might be enough to trigger this issue.

To test this I created the following POC

And yes, this causes the same problem. After running this through windbg with some log messages we get the following result

You can even clean it up further by removing the DIV element and adding the Button directly to the document.body but that does change things a little bit and make the exploitation a bit less straightforward

Exploitation

Anyway, we now know enough to start writing and exploit. We know the size of the freed object and we know when it is being freed, so it is pretty easy to replace the freed memory with something we control. First we want to make sure that the memory being used by the CButton object is allocated by the Low Fragmentation Heap. This will make replacing the freed memory much more reliable because the LFH does not merge coalescent free blocks and will happily reuse the last free block within a certain block range. The freed CButton object has a size of 0x58 (see CButton::CreateElement) so all we need to do is create an allocation of that size and we will refill the freed memory space.
To make sure the memory occupied by the CButton object will be LFH memory we need to enable the LFH for this memory size. I quote Valasek: “The most common trigger for enabling the LFH is 16 consecutive allocations of the same size.”

Of course we need to make sure that we disable pageheap and just to be sure we will also not use the debugheap when running the process with windbg attached.

We added some code to enable the LFH for the CButton element and the added code to replace the freed memory.

running this results in the following crash

Well that it is pretty obvious we should be able to turn this into a full blown exploit. But wouldnt it be nice if we could actually control EIP and not just a call from a controlled register + offfset? To make this reliable most exploit writers go for heapspray and then go from there, but that is not really necessary for IE8 exploits. Assuming we don’t need to force memory disclosure to bypass ASLR and we can rely on a module that doesn’t opt in to ASLR being present in the process, there are other options availble for the casual exploit writer. As far as I know this is a new technique but since it doesn’t apply to IE9 I dont mind dropping it here.

Internet Explorer 8 has support for HTML+TIME which is based on the Synchronized Multimedia Integration Language (SMIL) something nobady cares about anymore I think. Support for this has been removed in IE9 and higher, but we can still do some funny things with this in IE8. More precisely, it allows us to create an arbitrary sized array containing pointers to strings that we control. With this we can take control over the freed 0x58 sized memory and then have the vftable point to a string that is completely under our control, and thus we control where call [eax+0xDC] would go without using a heapspray. at which point we have control over the memory in eax and we have edi point to a list of pointers where we control the data as well. All in all that should be enough to write a poc that does not need heapsprays at all.

For this trick to work we need to add some funny things to the HTML or the HTML+TIME stuff doesn’t work as expected.

by setting the ‘values‘ property of the t:ANIMATECOLOR element to a semicolon separated string we can create an array of pointers that point to the individual elements of the string. So we need to use a string that has 0x58/4 == 0x22 values.

Then we can set the values property to this string and voila, we control EIP directly

Because the values are suposed to be legit colors you need to do this in a try-except construct so you don’t throw an error and stop the execution of the script. Also, doing this will create some ‘noise’ with additional allocations being made, but nothing that isn’t controllable.

Adding this we get the following POC:

Download the POC here.
Running this results in:

From here you can probably use the default ROP chains for Windows XP but I didnt bother trying that.

Conclusion

This is just another Internet Explorer use-after-free bug which was actually relatively easy to analyse and exploit. I used some new and/or non public techniques to get a reliable exploit that doesn’t require heapspray, but all in all this bug can be exploited quite reliably.
If you are interested in analyzing vulnerabilities and writing exploits for them you can take a look at our training, which will focus on IE9.
It is not easy to defend against these type of attacks, but by getting as many details on as many exploitable vulnerabilities as possible we believe we can provide our clients with additional tools and information to strengthen their defenses. If you want more information on this you can contact us at info@exodusintel.com.

– Peter Vreugdenhil
Exodus Intelligence