The Isolated Heap

While working on some IE 11 related UAF vulnerabilities, I noticed that there were significant changes in the new IDB.

The initialization function of MSHTML now has some additional code:

push    0               ; dwMaximumSize
push    0               ; dwInitialSize
push    0               ; flOptions
call    ds:__imp__HeapCreate@12 ; HeapCreate(x,x,x)
mov     _g_hIsolatedHeap, eax
test    eax, eax
jnz     short loc_63C94EC5
xor     esi, esi
jmp     loc_6358112F

A new heap is created and stored in a globally accessible way in _g_hIsolatedHeap.

A low fragmentation heap is then created with the new heap:

mov     ecx, eax
call    HeapSetInformation_LowFragmentation_Downlevel
jmp     loc_63A246BA

Object Allocation

Allocations are currently done from two functions:

  • __MemIsolatedAlloc
  • __MemIsolatedAllocClear


This simple wrapper around HeapAlloc will allocate a heap block with the memory zero'd out:

LPVOID _MemIsolatedAllocClear(SIZE_T dwBytes)
    return HeapAlloc(g_hIsolatedHeap, 0x8, dwBytes);


LPVOID _MemIsolatedAlloc(SIZE_T dwBytes)
    return HeapAlloc(g_hIsolatedHeap, 0, dwBytes);

Object Freeing

Everytime an object that was created in this heap is freed, it is now first memset to zero and then freed. Here is an example of an object of size 0x34 being freed:

push    34h             ; Size
push    0               ; Val
push    esi             ; Dst
call    _memset
push    esi             ; lpMem
push    0               ; dwFlags
push    _g_hIsolatedHeap ; hHeap
call    _HeapFree@12    ; HeapFree(x,x,x)


Based on a cursory look, here are a few objects allocated on the Isolated Heap:

Object Size
CBodyElement 40h
CHeadElement 34h
CSelectElement 0E4h
COptionElement 4Ch
CNoShowElement 38h
CObjectElement 170h
CParamElement 3Ch
CPluginSite 198h
CMapElement 3Ch
CAreaElement 64h
CMetaElement 34h
CParaElement 34h
CDivElement 34h
CDocumentType 34h
CTitleElement 40h
CHeaderElement 38h
CStyleElement 5Ch
CCommentElement 44h
CLinkElement 68h
CImgElement 5Ch
CLabelElement 48h
COListElement 38h
CHtmlElement 34h
CBRElement 34h
CScriptElement 0C4h
CFormElement 70h
CFontElement 34h
CAnchorElement 64h
CInput 0C0h
CSpanElement 34h
CPhraseElement 34h
CLIElement 3Ch
CProgressElement 5Ch
CSemanticElement 34h
CBaseElement 3Ch
CBlockElement 34h
CTable 58h
CTableSection 4Ch
CTableCell 48h
CTableRow 54h
CUListElement 38h
CHRElement 3Ch
CButton 58h
CDListElement 38h
CDTElement 34h
CDDElement 34h
CIFrameElement 68h
CHTMLCanvasElement 0ACh
CGenericElement 50h
CTextArea 74h
CSVGFilterElement 50h
CSVGDefsElement 4Ch
CSVGFEColorMatrixElement 50h
CSVGSVGElement 70h
CTitleElement 40h
CUnknownElement 3Ch
CAudioElement 0F8h
CLegendElement 3Ch
CFieldSetElement 40h
CTableCaption 4Ch
CTableCol 3Ch
CVideoElement 190h
CSVGGElement 4Ch
CSVGPathElement 70h
CSVGScriptElement 0D8h
CSVGPolygonElement 70h
CSVGRectElement 50h
CSVGEllipseElement 50h
CSVGPolylineElement 70h
CSVGTextElement 4Ch
CIsIndexElement 34h
CNextIdElement 34h
CMarquee 74h
CWndSelectElement 118h
CWndOptionElement 40h
CTextElement 34h
CFrameElement 5Ch
CSVGElement 4Ch
CSVGTitleElement 54h
CSVGAElement 60h
CSVGCircleElement 50h
CSVGClipPathElement 98h
CSVGDescElement 4Ch
CSVGFEBlendElement 50h
CSVGFEComponentTransferElement 50h
CSVGFECompositeElement 50h
CSVGFEConvolveMatrixElement 50h
CSVGFEDiffuseLightingElement 50h
CSVGFEDisplacementMapElement 50h
CSVGFEDistantLightElement 4Ch
CSVGFEFloodElement 50h
CSVGFEFuncAElement 4Ch
CSVGFEFuncBElement 4Ch
CSVGFEFuncGElement 4Ch
CSVGFEFuncRElement 4Ch
CSVGFEGaussianBlurElement 50h
CSVGFEImageElement 70h
CSVGFEMergeElement 4Ch
CSVGFEMergeNodeElement 4Ch
CSVGFEMorphologyElement 50h
CSVGFEOffsetElement 58h
CSVGFEPointLightElement 4Ch
CSVGFESpecularLightingElement 50h
CSVGFESpotLightElement 4Ch
CSVGFETileElement 50h
CSVGFETurbulenceElement 70h
CSVGImageElement 50h
CSVGLineElement 74h
CSVGLinearGradientElement 0E8h
CSVGMarkerElement 4Ch
CSVGMaskElement 0C0h
CSVGMetadataElement 4Ch
CSVGPatternElement 0B0h
CSVGRadialGradientElement 0FCh
CSVGStopElement 4Ch
CSVGStyleElement 74h
CSVGSwitchElement 54h
CSVGSymbolElement 4Ch
CSVGTextPathElement 60h
CSVGTSpanElement 4Ch
CSVGUseElement 60h
CSVGViewElement 4Ch
CMSHTMLWebViewElement 58h
CSourceElement 34h
CTrackElement 44h
CProcessingInstruction 70h
CBaseFontElement 34h
CBGsound 40h
CDataListElement 3Ch
CFrameSetSite 78h
CFrameSetSite 3Ch

There are more objects that are created on the Isolated Heap, but not listed here