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Date: Mon, 30 Nov 2009 11:54:10 -0600
Subject: Some more details on IE STYLE zero-day

Here are a few more details on researching the IE STYLE zero-day
vulnerability.  Microsoft Security Advisory 977981 describes the
vulnerability as follows:

"The vulnerability exists as an invalid pointer reference of Internet
Explorer.  It is possible under certain conditions for a CSS/Style
object to be accessed after the object is deleted.  In a
specially-crafted attack, Internet Explorer attempting to access a
freed object can lead to running attacker-supplied code."

However, I have not found any evidence of accessing freed memory -- as
far as I can tell, the problem is a logic bug.  The CDispNode family
of classes contains a flags field that happens to be located
immediately after the vtable pointer, the lowest four bits of which
I'll refer to as the "extra size index."
CDispNode::SetExpandedClipRect uses the extra size index of a class
instance as an index into CDispNode::_extraSizeTable, a constant array
where each element represents a count of machine words of, I guess,
extra data that precedes the class instance.  (This means that a
CDispNode-family class instance is not expected to snugly occupy its
own heap block.)  CDispNode::SetExpandedClipRect then backs up the
class instance pointer (the 'this' pointer, which of course points to
the vtable pointer initially) by that many machine words and expects
to find a flags field there.  This is only a problem if the extra size
index is 0, because CDispNode::_extraSizeTable[0] == 0, and in
Internet Explorer 6 and 7, that is exactly what happens --
CDispNode::SetExpandedClipRect is called for a CDispScroller class
instance with an extra size index of 0, so SetExpandedClipRect backs
up the 'this' pointer by 0 machine words (i.e., it doesn't move the
pointer at all) and operates on the class instance's vtable pointer as
though it were a flags field.

Although Internet Explorer 8 may call CDispNode::SetExpandedClipRect
during an attempted exploitation, it only does so for CDispContainer
and CDispLeafNode instances with non-zero extra size indices, never
(as far as I can tell) for a CDispScroller instance with an extra size
index of 0 (although such instances are still used).  Presumably IE8
contains a silent fix for the flawed logic that allowed the incorrect
SetExpandedClipRect call to happen.

I'm not planning to release an update to last week's unofficial patch
(, but if anyone wants
to create one that prevents IE from even crashing and/or detects
exploitation attempts, I'd recommend patching
CDispNode::SetExpandedClipRect to first check for an extra size index
of 0.  This doesn't cure the logic, but it will prevent the only known
bad side effect of exploitation attempts.  (Again, I haven't seen any
memory misuse or corruption, so assuming there really isn't any, there
should be no other side effects.  That would also mean that the
mechanism of the vulnerability is entirely reliable.)

I've confirmed that every Internet Explorer 7 x86 MSHTML.DLL is
potentially exploitable -- none of them contain a vtable slot with bit
15 set.  (The virtual function pointers in question all match either
xxxx0xxx, xxxx4xxx, xxxx5xxx, xxxx6xxx, or xxxx7xxx.)

If you'd like to research this vulnerability more for yourself, you
can breakpoint CStyleSheetArray::ReleaseStyleSheet (called during
CElement::put_outerHTML) and watch the fields changed during
CImplArray::Delete and CStyleSheet::ChangeStatus.  These fields are
consulted later when preparing a "fancy format helper"
(CBodyElement::ApplyDefaultFormat =>
CFormatInfo::PrepareFancyFormatHelper, and
CElement::ApplyInnerOuterFormats; also see CMarkup::ApplyStyleSheets)
to create a new CMarkup, which is then examined in
CElement::IsBodySizingForStrictCSS1NeededCore.  The return value from
this function determines whether
CFlowLayout::BodySizingForStrictCSS1Doc =>
CDispNode::SetExpandedClipRect is called during
CFlowLayout::SizeContentDispNode (here a CDispLeafNode's vtable
pointer also gets OR'ed) and then during CFlowLayout::SizeDispNode.

Hope this helps someone,

-- Derek

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