Windows 10 Nt Heap Exploitation (English version)

Windows Heap Exploitation
Angelboy

Windows Heap Exploitation
• Windows Memory Allocator

• NT heap

• BackEnd

• Exploitation

• FrontEnd

• LowFragmentationHeap

• Exploitation

Windows memory allocator
• Memory allocator in Win10 is very complicated, there are mainly two kind
of memory allocator.

• Nt Heap

• Default memory allocator

• SegmentHeap

• New memory allocator in Win10

• Some system process and UWP will use.

• Nt Heap can be divided into

• Back-End

• Front-End


• In order to prevent memory fragmentation problem, it will enable LFH
after allocating a certain number of the same size of chunk. (e.g.
Continuously allocate same size of chunks 18 times)

• Size <= 0x4000

• The slide based on win10 (1809)

• OS build - 17763.379

• Later versions may not be completely correct

• 1903 is very smimilar

• The structure of heap often change a lot. In the new release version,
you need to trace the structure and workﬂow by yourself.

• Overview
Kernel32.dll
HeapAlloc
HeapFree
msvcrt140.dll
malloc
free
ntdll.dll
RtlAllocateHeap
RtlFreeHeap
ntdll.dll
RtlpAllocateHeap
RtlpFreeHeap
ntdll.dll
RtlpLowFragHeapAlloc
RtlpLowFragHeapFree
Kernel
Front-End
Back-End

• When we call malloc if LFH is disable.
Kernel32.dll
HeapAlloc
HeapFree
msvcrt140.dll
malloc
free
ntdll.dll
RtlAllocateHeap
RtlFreeHeap
ntdll.dll
RtlpAllocateHeap
RtlpFreeHeap
Kernel
Back-End

• When we call malloc at the ﬁrst time or when  
memory pool called userblock of LFH is used  
up if LFH is enable.

• It will use back-end allocator to allocate  
userblock for LFH.
Kernel32.dll
HeapAlloc
HeapFree
msvcrt140.dll
malloc
free
ntdll.dll
RtlAllocateHeap
RtlFreeHeap
ntdll.dll
RtlpAllocateHeap
RtlpFreeHeap
ntdll.dll
RtlpLowFragHeapFree
Kernel
Front-End
Back-End

• When we allocate the same size after enabling LFH, 
it will use front-end allocator and use userblock in  
LFH
Kernel32.dll
HeapAlloc
HeapFree
msvcrt140.dll
malloc
free
ntdll.dll
RtlAllocateHeap
RtlFreeHeap
ntdll.dll
RtlpLowFragHeapFree
Front-End

• HEAP can be divide into

• Process Heap

• Default heap

• The heap shared by the entire process, it will be used when you use
windows API

• Stored in _PEB

• Used in CRT function

• Stored in crt_heap

• HEAP can be divide into

• Private heap

• Create by HeapCreate

• Core data structure

• _HEAP_ENTRY (chunk)

• The basic structure of memory allocator

• It is diﬀerent in front-end and back-end, but it use same name.

data
chunk header
chunk header
data
_HEAP
chunk header
.
.
.
malloc return value

• Core data structure

• _HEAP

• The core structure of memory allocator, it is used to manage the
heap.

• Each heap correspond to a _HEAP structure, usually at the
beginning of the heap

• _HEAP

• EncodeFlagMask

• It will be set to 0x100000 after
heap initialization. It’s used to
determine whether to encode the
header of the chunk in the heap

• Encoding (_HEAP_ENTRY)

• It’s used to do xor with chunk
header
…
EncodeFlagMask0x7c
0x80
0x138
Encoding
…
BlocksIndex
…
FreeList
…
FrontEndHeap
…
0x150
0x198
FrontEndHeapUsageData0x1a8
…

• _HEAP

• BlocksIndex (_HEAP_LIST_LOOKUP_)

• The core structure in back-end
allocator used to manage chunks.

• FreeList (_HEAP_ENTRY)

• A linked list used to collect all freed
chunk in back-end

• Similar as unsorted bin in libc

• Sorted list
…
EncodeFlagMask0x7c
0x80
0x138
Encoding
…
BlocksIndex
…
FreeList
…
FrontEndHeap
…
0x150
0x198
…

• _HEAP

• FrontEndHeap

• A pointer pointed to the structure of
front-end heap

• FrontEndHeapUsageData

• Record the number of chunks used
by various sizes.

• When it reaches a certain level, it
will enable the Front-End allocator
of the corresponding chunk.
…
EncodeFlagMask0x7c
0x80
0x138
Encoding
…
BlocksIndex
…
FreeList
…
FrontEndHeap
…
0x150
0x198
…

• Nt Heap

• Back-End

• Front-End

Nt heap
• Nt Heap

• Back-End

• Data structure

• Memory allocation mechanism


• Divided into

• Allocated chunk

• Freed chunk

• VirtualAlloc chunk

Nt heap

• PreviousBlockPrivateData

• The data of the previous chunk,
because chunk must be aligned to a
multiple of 0x10

• Size

• The size of chunk.

• The value is right shift by 4 bits.
User Data
PreviousBlockPrivateData
(8byte)
Size (2byte)
Flag (1byte)
SmallTagIndex (1byte)
PreviousSize (2byte)
SegmentOffset (1byte)
Unusedbyte (1byte)
Inused

Nt heap

• Flag

• Indicates whether the chunk is busy

• SmallTagIndex

• Checksum is the xor of the ﬁrst three
bytes

• It will be used to verify the header of
chunk
User Data
(8byte)
Size (2byte)
Flag (1byte)
Unusedbyte (1byte)
Inused
xor

Nt heap

• PreviousSize

• The size of previous chunk.

• The value is right shift by 4 bits.
User Data
(8byte)
Size (2byte)
Flag (1byte)
Unusedbyte (1byte)
Inused

Nt heap

• Unusedbyte

• The remaining size of chunk after
allocation

• It can be used to determine the status
of chunk (front-end or back-end)

• User Data

• The data used by user
User Data
(8byte)
Size (2byte)
Flag (1byte)
Unusedbyte (1byte)
Inused

Nt heap

• Flink

• Point to the next chunk in the linked list

• Blink

• Point to the previous chunk in the
linked list

• Unusedbyte

• Must be zero
User Data
(8byte)
Size (2byte)
Flag (1byte)
Unusedbyte (1byte)
freed
Flink
Blink

Nt heap
• _HEAP_VIRTUAL_ALLOC_ENTRY (mmap
chunk)

• Flink

• Point to the next chunk of VirtualAlloc

• Blink

• Point to the previous chunk of
VirtualAlloc
User Data
Size (2byte)
Flag (1byte)
Unusedbyte (1byte)
inused
Flink
Blink
…

Nt heap
• _HEAP_VIRTUAL_ALLOC_ENTRY (mmap
chunk)

• Size

• The size here refers to unused size
without shifting

• Unusedbyte

• Must be 4
User Data
Size (2byte)
Flag (1byte)
Unusedbyte (1byte)
inused
Flink
Blink
…

Nt heap
• FreeLists (_HEAP_ENTRY)

• After free a chunk, it will be placed in FreeLists and will be inserted in
FreeLists according to the size.
Flink
Blink
Chunk header (0x70)
Flink
Blink
Chunk header (0x110)
Flink
Blink

Nt heap
Flink
Blink
Chunk header (0x70)
Flink
Blink
Flink
Blink
PreviousBlockPrivateDat
…
EncodeFlagMask0x7c
0x80
0x138
Encoding
…
BlocksIndex
…
FreeList
…
FrontEndHeap
…
0x150
0x198
…
Flink
Blink
_HEAP

Nt heap
• Remark

• About header encoding

• Every chunk header will be xor with _HEAP->Encoding

• It will be verified when it is decoded

• The verification method is to check whether the checksum is
correct.

• The xor of the first three bytes and compare with fourth byte.

Nt heap
• BlocksIndex (_HEAP_LIST_LOOKUP)

• It is mainly used to manage freed chunks of various sizes, so that it can
quickly ﬁnd suitable chunks.

Nt heap

• ExtendedLookup (_HEAP_LIST_LOOKUP)

• Point to next ExtendedLookup

• The next BlocksIndex will manage larger
chunks

• ArraySize

• The max chunk size that will be managed by
the BlocksIndex.

• The ﬁrst BlocksIndex ArraySize will by 0x80
(Actually is 0x800)
BlocksIndex
ExtendedLookup0x0
ArraySize0x8
…
ItemCount (4 bytes)
OutofRangeItems (4 bytes)
0x10
0x14
BaseIndex0x18
ListHead
ListsInUseUlong
ListHint
0x20
0x28
0x30

Nt heap

• ItemCount

• The number of chunks in the
BlocksIndex

• OutofRangeItems

• The number of chunks that exceed
the size managed by this
BlocksIndex
BlocksIndex
ExtendedLookup0x0
ArraySize0x8
…
ItemCount (4 bytes)
0x10
0x14
BaseIndex0x18
ListHead
ListsInUseUlong
ListHint
0x20
0x28
0x30

Nt heap

• BaseIndex

• The starting index of the chunk in the
Blocksindex.

• It is used to ﬁnd a suitable freed chunk in
ListHint

• The next BaseIndex of next BlocksIndex is the
maximum value of current BaseIndex.

• ListHead (_HEAP_ENTRY)

• FreeList 的 Head
BlocksIndex
ExtendedLookup0x0
ArraySize0x8
…
ItemCount (4 bytes)
0x10
0x14
BaseIndex0x18
ListHead
ListsInUseUlong
ListHint
0x20
0x28
0x30

Nt heap

• ListsInUseUlong

• Used to determine whether there is a suitable
chunk in ListHint, which is a bitmap

• ListHint

• Important structure in back-end.

• A pointer array which point to the corresponding
size of chunk array.

• The goal is to ﬁnd the suitable chunk faster.

• The interval of chunk size is 0x10
BlocksIndex
ExtendedLookup0x0
ArraySize0x8
…
ItemCount (4 bytes)
0x10
0x14
BaseIndex0x18
ListHead
ListsInUseUlong
ListHint
0x20
0x28
0x30

…
EncodeFlagMask
Encoding
…
BlocksIndex
…
FreeList
…
FrontEndHeap
…
FrontEndHeapUsageDat
…
_HEAP
BlocksIndex
ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
Flink
Blink
Flink
Blink
Chunk header (0x70)
PreviousBlockPrivate
Flink
Blink
Flink
Blink
Chunk header(0x160)
PreviousBlockPrivat
ListHint
Flink
Flink
Flink
ListHint[7]
ListHint[0x11]
ListHint[0x16]
000001…1…1000000000
ListsInUseUlong

Nt heap
• Allocate (RtlpAllocateHeap)

• It can be divided into

• Size <= 0x4000

• 0x4000 < size <= 0xﬀ000

• Size > 0xﬀ000

Nt heap

• Size <= 0x4000

• The memory allocation is implemented in RtlpAllocateHeap

• First, it will check whether FrontEndHeapStatusBitmap corresponds to the
size is set (which indicates if LFH is enabled)

• If not, add 0x21 to the corresponding FrontEndHeapUsageData

• And check if the value exceeds 0xﬀ00 or (& 0x1f) is larger than 0x10

• If true, it will enable LFH.

Nt heap

• Size <= 0x4000

• Next, it will check whether ListHint corresponds to the size has a value. If true, it will take
the chunk in ListHint ﬁrst.

• If there is a suitable chunk on the ListHint, remove the ListHint, and see whether the size
of chunk's Flink is the same size.

• If true, replace the ListHint with Flink

• If not, clear the ListHint.

• Finally, unlink the chunk to remove the chunk from the linked list and return it to user.

Nt heap

• Size <= 0x4000

• If there is no suitable chunk

• Search from the larger ListHint. If it ﬁnds a large one, remove it
from the ListHint.

• Then split the chunk and insert remaining chunk into freelist and
put into ListHint.

Nt heap

• Size <= 0x4000

• If there is no suitable chunk in FreeList

• Try ExtendHeap to increase heap space, then take the chunk from
it

Nt heap

• 0x4000 < size <= 0xﬀ000

• Everything is similar except without LFH-related operations

Nt heap

• Size > 0xﬀ000 (VirtualMemoryThreshold << 4)

• Use ZwAllocateVirtualMemory directly

• Similar to mmap, it will allocate large memory block , and will be
inserted into _HEAP->VirtualAllocdBlocks

• _HEAP->VirtualAllocdBlocks is linked list used to collect all
VirtualAllocate chunk in back-end

Nt heap
• Free (RtlpFreeHeap)

• Divide into

• Size <= 0xﬀ000

• Size > 0xﬀ000

Nt heap

• Size <= 0xﬀ000

• It will check the alignment ﬁrst and use the unused byte to determine the
chunk state

• If LFH is disable, decrease 1 to the corresponding
FrontEndHeapUsageData

• Then it will check whether the previous chunk or next chunk is freed. If it is
freed, then coalesce them.

• Unlink the coalesced chunk and remove from ListHint.

Nt heap

• Size <= 0xﬀ000

• After the coalescence, update size & prevsize, and check whether it
is the front or last of FreeList. If true, then insert into FreeList. If not,
insert into ListHint and update it.

• It will check the linked list integrity when inserting

• But this check won’t terminate the process.

ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
Flink
Blink
Flink
Blink
Chunk header (0x70)
Chunk header(0x110)
PreviousBlockPrivat
FlinkListHint[7]
Flink
Blink
Chunk header(0x70)
PreviousBlockPrivat
merge chunk
P
Q
R
S
Free(Q)
Size (0x11)
Flag (0x1)
SmallTagIndex (0x10)
Prevsize (0x7)

ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
Flink
Blink
Flink
Blink
Chunk header (0x70)
Chunk header(0x110)
PreviousBlockPrivat
FlinkListHint[7]
Flink
Blink
Chunk header(0x70)
PreviousBlockPrivat
merge chunk
P
Q
R
S
Find Prevchunk
Size (0x11)
Flag (0x1)
Prevsize (0x7)
Prevchunk =
Chunk address
-
(prevsize << 4)

ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
Flink
Blink
Flink
Blink
Chunk header (0x70)
Chunk header(0x110)
PreviousBlockPrivat
FlinkListHint[7]
Flink
Blink
Chunk header(0x70)
PreviousBlockPrivat
merge chunk
P
Q
R
S
Find Prevchunk
Size (0x11)
Flag (0x1)
Prevsize (0x7)
Prevchunk =
chunk addr - 0x70

ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
Flink
Blink
Flink
Blink
Chunk header (0x70)
Chunk header(0x110)
PreviousBlockPrivat
FlinkListHint[7]
Flink
Blink
Chunk header(0x70)
PreviousBlockPrivat
merge chunk
P
Q
R
S
Decode prevchunk 
header
Size (0x11)
Flag (0x1)
Prevsize (0x7)
Size (0x7)
Flag (0x0)
SmallTagIndex (0x7)

ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
Flink
Blink
Flink
Blink
Chunk header (0x70)
Chunk header(0x110)
PreviousBlockPrivat
FlinkListHint[7]
Flink
Blink
Chunk header(0x70)
PreviousBlockPrivat
merge chunk
P
Q
R
S
Check checksum
Size (0x11)
Flag (0x1)
Prevsize (0x7)
Size (0x7)
Flag (0x0)
SmallTagIndex (0x7)
SmallTagInex == 
(size & 0xﬀ)
xor
(size >> 8)
xor
Flag

ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
Flink
Blink
Flink
Blink
Chunk header (0x70)
Chunk header(0x110)
PreviousBlockPrivat
FlinkListHint[7]
Flink
Blink
Chunk header(0x70)
PreviousBlockPrivat
merge chunk
P
Q
R
S
Check checksum
Size (0x11)
Flag (0x1)
Prevsize (0x7)
Size (0x7)
Flag (0x0)
SmallTagIndex (0x7)
7 == 
0x7^0x0^0x0

ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
Flink
Blink
Flink
Blink
Chunk header (0x70)
Chunk header(0x110)
PreviousBlockPrivat
FlinkListHint[7]
Flink
Blink
Chunk header(0x70)
PreviousBlockPrivat
merge chunk
P
Q
R
S
Check 
linked list
Size (0x11)
Flag (0x1)
Prevsize (0x7)
Size (0x7)
Flag (0x0)
SmallTagIndex (0x7)
P->Flink->Blink ==  
P->Blink->Flink ==  
P

ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
Flink
Blink
Flink
Blink
Chunk header (0x70)
Chunk header(0x110)
PreviousBlockPrivat
FlinkListHint[7]
Flink
Blink
Chunk header(0x70)
PreviousBlockPrivat
merge chunk
P
Q
R
S
Find  
BlocksIndex
Size (0x11)
Flag (0x1)
Prevsize (0x7)
Size (0x7)
Flag (0x0)
SmallTagIndex (0x7)

ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
Flink
Blink
Flink
Blink
Chunk header (0x70)
Chunk header(0x110)
PreviousBlockPrivat
FlinkListHint[7]
Flink
Blink
Chunk header(0x70)
PreviousBlockPrivat
merge chunk
P
Q
R
S
Find suitable  
BlocksIndex
Size (0x11)
Flag (0x1)
Prevsize (0x7)
Size (0x7)
Flag (0x0)
SmallTagIndex (0x7)
Check
P->size < ArraySize

ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
Flink
Blink
Flink
Blink
Chunk header (0x70)
Chunk header(0x110)
PreviousBlockPrivat
FlinkListHint[7]
Flink
Blink
Chunk header(0x70)
PreviousBlockPrivat
merge chunk
P
Q
R
S
Check 
ListHint
Size (0x11)
Flag (0x1)
Prevsize (0x7)
Size (0x7)
Flag (0x0)
SmallTagIndex (0x7)
Check
ListHint[7] ==
Prevchunk

ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
Flink
Blink
Flink
Blink
Chunk header (0x70)
Chunk header(0x110)
PreviousBlockPrivat
FlinkListHint[7]
Flink
Blink
Chunk header(0x70)
PreviousBlockPrivat
merge chunk
P
Q
R
S
Check 
ListHint
Size (0x11)
Flag (0x1)
Prevsize (0x7)
Size (0x7)
Flag (0x0)
SmallTagIndex (0x7)
Check
prevchunk->Flink ==
ListHead
If true, update 
ListHint

ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
Flink
Blink
Flink
Blink
Chunk header (0x70)
Chunk header(0x110)
PreviousBlockPrivat
FlinkListHint[7]
Flink
Blink
Chunk header(0x70)
PreviousBlockPrivat
merge chunk
P
Q
R
S
Check 
ListHint[7]->Flink
Size (0x11)
Flag (0x1)
Prevsize (0x7)
Size (0x7)
Flag (0x0)
SmallTagIndex (0x7)
decode
prevchunk->Flink
And check
the checksum
Because the previous
slide is not true, it 
will check the size 
of Flink

ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
Flink
Blink
Flink
Blink
Chunk header (0x70)
Chunk header(0x110)
PreviousBlockPrivat
FlinkListHint[7]
Flink
Blink
Chunk header(0x70)
PreviousBlockPrivat
merge chunk
P
Q
R
S
Check 
ListHint[7]->Flink
Size (0x11)
Flag (0x1)
Prevsize (0x7)
Size (0x7)
Flag (0x0)
SmallTagIndex (0x7)
Size ==  
prevchunk->Flink->size
If true, replace ListHint 
with Flink

ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
Flink
Blink
Flink
Blink
Chunk header (0x70)
Chunk header(0x110)
PreviousBlockPrivat
FlinkListHint[7]
Flink
Blink
Chunk header(0x70)
PreviousBlockPrivat
merge chunk
P
Q
R
S
Update 
ListHint[7]
Size (0x11)
Flag (0x1)
Prevsize (0x7)
Size (0x7)
Flag (0x0)
SmallTagIndex (0x7)
ListHint[7] = 
prevchunk->Flink

ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
Flink
Blink
Flink
Blink
Chunk header (0x70)
Chunk header(0x110)
PreviousBlockPrivat
FlinkListHint[7]
Flink
Blink
Chunk header(0x70)
PreviousBlockPrivat
merge chunk
P
Q
R
S
Unlink 
prevchunk
Size (0x11)
Flag (0x1)
Prevsize (0x7)
Size (0x7)
Flag (0x0)
SmallTagIndex (0x7)
Prevchunk->Blink->Flink 
= Prevchunk->Flink 
Prevchunk->Flink->Blink
= Prevchunk->Blink

ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
Flink
Blink
Chunk header(0x110)
PreviousBlockPrivat
FlinkListHint[7]
Flink
Blink
Chunk header(0x70)
PreviousBlockPrivat
merge chunk
P
Q
R
S
Update 
prevchunk 
and next chunk
Size (0x11)
Flag (0x1)
Prevsize (0x7)
Size (0x18)
Flag (0x0)
SmallTagIndex (0x7)
Prevchunk->size = 
0x11 + Prevchunk->size(7)
R->Prevsize = 0x18
P
Flink
Blink

ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
Flink
Blink
Chunk header(0x110)
PreviousBlockPrivat
FlinkListHint[7]
Flink
Blink
Chunk header(0x70)
PreviousBlockPrivat
merge chunk
P
Q
R
S
Check 
next chunk
Size (0x11)
Flag (0x1)
Prevsize (0x7)
Size (0x18)
Flag (0x0)
SmallTagIndex (0x7)
Check if next chunk 
is freed
P
Flink
Blink

ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
Flink
Blink
Chunk header(0x110)
PreviousBlockPrivat
FlinkListHint[7]
Flink
Blink
Chunk header(0x70)
PreviousBlockPrivat
merge chunk
P
Q
R
S
Find suitable 
BlocksIndex 
For P
Size (0x11)
Flag (0x1)
Prevsize (0x7)
Size (0x18)
Flag (0x0)
SmallTagIndex (0x7)
Check 
P->Size < ArraySize
P
Flink
Blink

ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
Flink
Blink
Chunk header(0x110)
PreviousBlockPrivat
FlinkListHint[7]
Flink
Blink
Chunk header(0x70)
PreviousBlockPrivat
merge chunk
P
Q
R
S
Search 
insert 
point
Size (0x11)
Flag (0x1)
Prevsize (0x7)
Size (0x18)
Flag (0x0)
SmallTagIndex (0x7)
Check 
P->size <
ListHead->Blink->Size 
P->size <
ListHead->Flink->Size
P
Flink
Blink

ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
Flink
Blink
Chunk header(0x110)
PreviousBlockPrivat
FlinkListHint[7]
Flink
Blink
Chunk header(0x70)
PreviousBlockPrivat
merge chunk
P
Q
R
S
Insert 
linked list
Size (0x11)
Flag (0x1)
Prevsize (0x7)
Size (0x18)
Flag (0x0)
SmallTagIndex (0x7)
Check 
S->Blink->Flink 
== S
If not pass it will not  
abort 
But it will not unlink
P
Flink
Blink

ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
Flink
Blink
Chunk header(0x110)
PreviousBlockPrivat
FlinkListHint[7]
Flink
Blink
Chunk header(0x70)
PreviousBlockPrivat
merge chunk
P
Q
R
S
Update 
ListHint
Size (0x11)
Flag (0x1)
Prevsize (0x7)
Size (0x18)
Flag (0x0)
SmallTagIndex (0x7)
If LintHint[0x18] == NULL 
LintHint[0x18] = P
P
Flink
Blink
FlinkListHint[0x18]

ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
Flink
Blink
Chunk header(0x110)
PreviousBlockPrivat
FlinkListHint[7]
Flink
Blink
Chunk header(0x70)
PreviousBlockPrivat
merge chunk
P
Q
R
S
Update 
header
Size (0x11)
Flag (0x1)
Prevsize (0x7)
Size (0x18)
Flag (0x0)
SmallTagIndex =  
(size >> 8) ^ 
(size & 0xﬀ) ^ 
Flag
P
Flink
Blink
FlinkListHint[0x18]

ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
Flink
Blink
Chunk header(0x110)
PreviousBlockPrivat
FlinkListHint[7]
Flink
Blink
Chunk header(0x70)
PreviousBlockPrivat
merge chunk
P
Q
R
S
Encode 
header
Size (0x11)
Flag (0x1)
Prevsize (0x7)
Size (0xda)
Flag (0xab)
P->header ^ 
_HEAP->Encoding
P
Flink
Blink
FlinkListHint[0x18]

Nt heap

• Size > 0xﬀ000

• Check the linked list integrity and remove from _HEAP-
>VirtualAllocdBlocks

• Then use RtlpSecMemFreeVirtualMemory to munmap the chunk

Back-End Exploitation
• Unlink

• Similar bypass method in Linux.

• In short, use the behavior of removing nodes from linked list.

• It should be noted that the checksum must correct when decoding
chunk.

• The good thing is that Flink and Blink are directly point to UserData,
that is, you don’t have to forge chunk header

• Unlink

• Q->Blink->Flink = Q->Flink

• Q->Flink->Blink = Q->Blink
Flink
Blink
Chunk header (0x70)
Flink
Blink
Flink
Blink
QP R

• Unlink

• Q->Blink->Flink = Q->Flink

• Q->Flink->Blink = Q->Blink
Flink
Blink
Chunk header (0x70)
Flink
Blink
QP R

A
A
Chunk header(0x50)
PreviousBlockPrivat
Chunk header(0x110)
PreviousBlockPrivat
P
Q
R
S
P
Q
R
S
T
ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
BlockIndex
Data Pointer
ListHint
UAF
T
A

A
A
Chunk header(0x50)
PreviousBlockPrivat
Chunk header(0x110)
PreviousBlockPrivat
P
Q
R
S
P
Q
R
S
T
ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
BlockIndex
Data Pointer
ListHint
Free(Q)
T
A

Q
A
Q
Chunk header(0x50)
PreviousBlockPrivat
Chunk header(0x110)
PreviousBlockPrivat
P
Q
R
S
P
Q
R
S
T
ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
BlockIndex
Data Pointer
ListHint
T
A
A
ListHead
QListHint[0x11]
free(S)
Update ListHint

S
A
Q
Chunk header(0x50)
PreviousBlockPrivat
S
Chunk header(0x110)
PreviousBlockPrivat
P
Q
R
S
P
Q
R
S
T
ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
BlockIndex
Data Pointer
ListHint
T
A
A
S
SListHint[0x11]
Q
ListHead

S
A
Q
Chunk header(0x50)
PreviousBlockPrivat
S
Chunk header(0x110)
PreviousBlockPrivat
P
Q
R
S
P
Q
R
S
T
ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
BlockIndex
Data Pointer
ListHint
T
A
A
S
SListHint[0x11]
Q
ListHead
Let’s corrupt the heap !

S
A
Q
Chunk header(0x50)
PreviousBlockPrivat
S
Chunk header(0x110)
PreviousBlockPrivat
P
Q
R
S
P
Q
R
S
T
ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
BlockIndex
Data Pointer
ListHint
T
A
&Q - 8
&Q
SListHint[0x11]
Q
ListHead
free(P)

S
A
Q
Chunk header(0x50)
PreviousBlockPrivat
S
Chunk header(0x110)
PreviousBlockPrivat
P
Q
R
S
P
Q
R
S
T
ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
BlockIndex
Data Pointer
ListHint
T
A
&Q - 8
&Q
SListHint[0x11]
Q
ListHead
Check next chunk  
is free

S
A
Q
Chunk header(0x50)
PreviousBlockPrivat
S
Chunk header(0x110)
PreviousBlockPrivat
P
Q
R
S
P
Q
R
S
T
ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
BlockIndex
Data Pointer
ListHint
T
A
&Q - 8
&Q
SListHint[0x11]
Q
ListHead
Decode  
chunk Q

S
A
Q
Chunk header(0x50)
PreviousBlockPrivat
S
Chunk header(0x110)
PreviousBlockPrivat
P
Q
R
S
P
Q
R
S
T
ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
BlockIndex
Data Pointer
ListHint
T
A
&Q - 8
&Q
SListHint[0x11]
Q
ListHead(&Q-8)->Blink
==
(&Q)->Flink
== Q

S
A
Q
Chunk header(0x50)
PreviousBlockPrivat
S
Chunk header(0x110)
PreviousBlockPrivat
P
Q
R
S
P
Q
R
S
T
ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
BlockIndex
Data Pointer
ListHint
T
A
&Q - 8
&Q
SListHint[0x11]
Q
ListHead(&Q-8)->Blink
==
(&Q)->Flink
== Q
Flink
Blink

S
A
Q
Chunk header(0x50)
PreviousBlockPrivat
S
Chunk header(0x110)
PreviousBlockPrivat
P
Q
R
S
P
Q
R
S
T
ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
BlockIndex
Data Pointer
ListHint
T
A
&Q - 8
&Q
SListHint[0x11]
Q
ListHead*(&Q)
==
*(&Q)
== Q

S
A
Q
Chunk header(0x50)
PreviousBlockPrivat
S
Chunk header(0x110)
PreviousBlockPrivat
P
Q
R
S
P
Q
R
S
T
ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
BlockIndex
Data Pointer
ListHint
T
A
&Q - 8
&Q
SListHint[0x11]
Q
ListHead*(&Q)
==
*(&Q)
== Q
Pass the check !!

S
A
Q
Chunk header(0x50)
PreviousBlockPrivat
S
Chunk header(0x110)
PreviousBlockPrivat
P
Q
R
S
P
Q
R
S
T
ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
BlockIndex
Data Pointer
ListHint
T
A
&Q - 8
&Q
SListHint[0x11]
Q
ListHead
Find  
BlockIndex

S
A
Q
Chunk header(0x50)
PreviousBlockPrivat
S
Chunk header(0x110)
PreviousBlockPrivat
P
Q
R
S
P
Q
R
S
T
ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
BlockIndex
Data Pointer
ListHint
T
A
&Q - 8
&Q
SListHint[0x11]
Q
ListHead
Check 
ListHint[0x11]

S
A
Q
Chunk header(0x50)
PreviousBlockPrivat
S
Chunk header(0x110)
PreviousBlockPrivat
P
Q
R
S
P
Q
R
S
T
ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
BlockIndex
Data Pointer
ListHint
T
A
&Q - 8
&Q
SListHint[0x11]
Q
ListHeadBecause S != Q 
it will not replace 
ListHint with Q

S
A
Q
Chunk header(0x50)
PreviousBlockPrivat
S
Chunk header(0x110)
PreviousBlockPrivat
P
Q
R
S
P
Q
R
S
T
ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
BlockIndex
Data Pointer
ListHint
T
A
&Q - 8
&Q
SListHint[0x11]
Q
ListHead
Unlink !!
Q->Blink->Flink
= Q->Flink
Flink
Blink

S
A
Q
Chunk header(0x50)
PreviousBlockPrivat
S
Chunk header(0x110)
PreviousBlockPrivat
P
Q
R
S
P
&Q - 8
R
S
T
ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
BlockIndex
ListHint
T
A
&Q - 8
&Q
SListHint[0x11]
Q
ListHeadUnlink !!
Q = &Q-8

S
A
Q
Chunk header(0x50)
PreviousBlockPrivat
S
Chunk header(0x110)
PreviousBlockPrivat
P
Q
R
S
P
&Q - 8
R
S
T
ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
BlockIndex
ListHint
T
A
&Q - 8
&Q
SListHint[0x11]
Q
ListHead
Unlink !!
Q->Flink->Blink
= Q->Blink
Flink
Blink

S
A
Q
Chunk header(0x50)
PreviousBlockPrivat
S
Chunk header(0x110)
PreviousBlockPrivat
P
Q
R
S
P
&Q
R
S
T
ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
BlockIndex
ListHint
T
A
&Q - 8
&Q
SListHint[0x11]
Q
ListHeadUnlink !!
Q = &Q

S
A
Q
Chunk header(0x50)
PreviousBlockPrivat
S
Chunk header(0x110)
PreviousBlockPrivat
P
Q
R
S
P
&Q
R
S
T
ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
BlockIndex
ListHint
T
A
&Q - 8
&Q
SListHint[0x11]
Q
ListHeadUpdate  
chunk size

S
A
Chunk header(0x50)
PreviousBlockPrivat
S
Chunk header(0x110)
PreviousBlockPrivat
R
S
P
&Q
R
S
T
ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
BlockIndex
ListHint
T
A
SListHint[0x11]
Q
ListHeadUpdate  
chunk size
P

S
A
Chunk header(0x50)
PreviousBlockPrivat
S
Chunk header(0x110)
PreviousBlockPrivat
R
S
P
&Q
R
S
T
ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
BlockIndex
ListHint
T
A
SListHint[0x11]
Q
ListHead
Search insert point
P

S
A
Chunk header(0x50)
PreviousBlockPrivat
S
Chunk header(0x110)
PreviousBlockPrivat
R
S
P
&Q
R
S
T
ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
BlockIndex
ListHint
T
A
SListHint[0x11]
Q
ListHeadFind the last one
And decode it
P

S
A
Chunk header(0x50)
PreviousBlockPrivat
S
Chunk header(0x110)
PreviousBlockPrivat
R
S
P
&Q
R
S
T
ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
BlockIndex
ListHint
T
A
SListHint[0x11]
Q
ListHead
P
Find the ﬁrst one
And decode it

S
A
Chunk header(0x50)
PreviousBlockPrivat
S
Chunk header(0x110)
PreviousBlockPrivat
R
S
P
&Q
R
S
T
ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
BlockIndex
ListHint
T
A
SListHint[0x11]
Q
ListHeadIt want to insert  
in front of A
P

S
A
Chunk header(0x50)
PreviousBlockPrivat
S
Chunk header(0x110)
PreviousBlockPrivat
R
S
P
&Q
R
S
T
ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
BlockIndex
ListHint
T
A
SListHint[0x11]
Q
ListHead
Check 
A->Blink->Flink
== A
P

S
A
Chunk header(0x50)
PreviousBlockPrivat
S
Chunk header(0x110)
PreviousBlockPrivat
R
S
P
&Q
R
S
T
ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
BlockIndex
ListHint
T
A
SListHint[0x11]
Q
ListHead
Check 
Q->Flink
== A
P

S
A
Chunk header(0x50)
PreviousBlockPrivat
S
Chunk header(0x110)
PreviousBlockPrivat
R
S
P
&Q
R
S
T
ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
BlockIndex
ListHint
T
A
SListHint[0x11]
Q
ListHeadFailed 
but not aborting
It will not insert
P

S
A
Chunk header(0x50)
PreviousBlockPrivat
S
Chunk header(0x110)
PreviousBlockPrivat
R
S
P
&Q
R
S
T
ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
BlockIndex
ListHint
T
A
SListHint[0x11]
Q
ListHead
Update  
ListHint
P

S
A
Chunk header(0x50)
PreviousBlockPrivat
S
Chunk header(0x110)
PreviousBlockPrivat
R
S
P
&Q
R
S
T
ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint
BlockIndex
ListHint
T
A
SListHint[0x11]
Q
ListHead
P
PListHint[0x22]

• Edit Q

• We can overwrite the pointer around
Q

• Finally, we can do arbitrary
memory reading and writing.
P
&Q
R
S
T

• Edit Q

• We can overwrite the pointer around
Q

• Finally, we can do arbitrary
memory reading and writing.

• We can overwrite Target 1 - 4
P
Target 1
Target 2
Target 3
Target 4

• After arbitrary memory reading and writing

• Leak

• ntdll.dll

• PebLdr

• binary address

• Kernel32.dll


• Leak

• kernelbase

• KERNELBASE!BasepFilterInfo

• Stack address


• Leak

• ntdll.dll

• _HEAP_LOCK

• _HEAP->LockVariable.Lock

• CriticalSection->DebugInfo

• Point to ntdll


• Leak

• ntdll!PebLdr

• _PEB_LDR_DATA

• You can find all the location of dll

• However the drawback is that the last two byte may be 0.

• You can find the binary base first and find kernel32 from IAT


• Leak

• BinaryBase

• Find kernel32 from IAT


• Leak

• Kernel32

• Important dll

• Many useful function(CreateFile, ReadFile,WriteFile)

• IAT

• Find kernelbase.dll


• Leak

• kernelbase

• KERNELBASE!BasepFilterInfo

• Point to the structure on heap

• You have high probability to ﬁnd stack pointer in the structure

• https://j00ru.vexillium.org/2016/07/disclosing-stack-data-from-the-default-
heap-on-windows/


• Leak

• If there is no stack address in BasepFilterInfo

• You can ﬁnd stack address in TEB which usually locate one page
before or after PEB


• Write

• Return address

• Control RIP

• ROP to VirtualProtect/VirtualAlloc

• Jmp to shellcode

• Nt Heap

• Front-End

• Commonly use in win10


• Only enable in non-debug mode

• Size < 0x4000

• Nt Heap

• Front-End

• Data Structure

• Memory allocation mechanism

LFH
• FrontEndHeap (_LFH_HEAP)

• HEAP (_HEAP)

• Point to the corresponding _HEAP

• Buckets (_HEAP_BUCKET)

• An array to ﬁnd the malloc size
corresponding to the block size
…
Heap0x18
0x2a4
…
Buckets
SegmentInfoArray
…
…
LocalData
0x4a8
0xcc0

LFH
• FrontEndHeap (_LFH_HEAP)

• SegmentInfoArray
(_HEAP_LOCAL_SEGMENT_INFO)

• _HEAP_LOCAL_SEGMENT_INFO array

• Different sizes correspond to different
Segment_info structures, to manage the
information of the corresponding Subsegment.

• LocalData (_HEAP_LOCAL_DATA)

• One of the fields points to the LFH itself, which
is usually used to retrieve the LFH.
…
Heap0x18
…
Buckets
SegmentInfoArray
…
…
LocalData
0x2a4
0x4a8
0xcc0

LFH
• Buckets (_HEAP_BUCKET)

• BlockUnits

• The block size >> 4

• SizeIndex

• Required size >> 4
BlockUnits
Buckets
SizeIndex
…
BlockUnits
SizeIndex
…
…
…
Buckets[0]
Buckets[1]

LFH

• LocalData (_HEAP_LOCAL_DATA)

• Corresponding to _LFH_HEAP-
>LocalData to retrieve _LFH_HEAP
from SegmentInfo

• BucketIndex

• The index in Buckets[]
LocalData
SegmentInfoArray[x]
ActiveSubsegment
CachedItems
…
BucketIndex
…

LFH

• ActiveSubsegment (_HEAP_SUBSEGMENT)

• Very important structure

• Corresponding to the assigned Subsegment

• To maintain Userblock

• Record number of remaining chunk

• The maximum number of chunk in
Userblock
SegmentInfoArray[x]
LocalData
ActiveSubsegment
CachedItems
…
BucketIndex
…

LFH

• CachedItems (_HEAP_SUBSEGMENT)

• _HEAP_SUBSEGMENT array

• Stored the available Subsegment
correspond to the SegmentInfo

• When the ActiveSubsegment used
up, it will reload from CachedItems.
SegmentInfoArray[x]
LocalData
ActiveSubsegment
CachedItems
…
BucketIndex
…

LFH
…
Heap
…
…
…
LocalData
0x18
0x2a4
0x4a8
0xcc0
Buckets[x]
SegmentInfoArray[x]
BlockUnits
SizeIndex
…
_LFH_HEAP
_HEAP_BUCKET
_HEAP_LOCAL_SEGMENT_INFO
…
EncodeFlagMask
Encoding
…
BlocksIndex
…
FreeList
…
FrontEndHeap
…
…
_HEAP
LocalData
ActiveSubsegment
CachedItems
…
BucketIndex
…

LFH

• LocalInfo

• Point back to corresponding
LocalInfo
ActiveSubsegment
UserBlock
…
AggregateExchg
BlockSzie
BlockCount
…
SizeIndex
…
0x0
0x8
0x20
0x24
0x28
0x2a

LFH

• UserBlock (_HEAP_USERDATA_HEADER)

• Memory pool of LFH

• That is, the location of chunk (Block)

• Some metadata will manage the
chunks at the beginning of UserBlock

• Important !
LocalInfo
ActiveSubsegment
UserBlock
…
AggregateExchg
BlockSzie
BlockCount
…
SizeIndex
…
0x0
0x8
0x20
0x24
0x28
0x2a

LFH

• AggregateExchg (_INTERLOCK_SEQ)

• To indicate remaining amount of
freed chunk in Userblock

• LFH uses it to determine if it should
allocate from this UserBlock
LocalInfo
ActiveSubsegment
UserBlock
…
AggregateExchg
BlockSzie
BlockCount
…
SizeIndex
…
0x0
0x8
0x20
0x24
0x28
0x2a

LFH

• BlockSize

• The size of each block (chunk) in the
UserBlock

• BlockCount

• The number of blocks in the UserBlock

• SizeIndex

• The SizeIndex corresponding to the
UserBlock
LocalInfo
ActiveSubsegment
UserBlock
…
AggregateExchg
BlockSzie
BlockCount
…
SizeIndex
…
0x0
0x8
0x20
0x24
0x28
0x2a

LFH
• AggregateExchg (_INTERLOCK_SEQ)

• Depth

• The remaining amount of freed chunk in
UserBlock

• Lock

• Just a Lock
AggregateExchg
Depth0x0
Hint (15bit)
Lock (1bit)
0x8

LFH

• SubSegment

• Point to corresponding Subsegment

• EncodeOﬀsets

• To verify chunk header integrity

• BusyBitmap

• Indicate which chunk is being used
SubSegment
UserBlock
…
EncodedOffsets
BusyBitmap
0x8
0x18
0x20
chunk header
chunk header
chunk header

LFH

• Block (chunk)

• The allocated memory return to user.
SubSegment
UserBlock
…
EncodedOffsets
BusyBitmap
0x8
0x18
0x20
chunk header
chunk header
chunk header

LFH

• SubSegmentCode

• Encoded metadata to retrieve the
location of UserBlock

• PreviousSize

• The index of the chunk in UserBlock
User Data
(8byte)
SubSegmentCode (4byte)
Unusedbyte (1byte)
Inused

LFH

• Unusedbyte

• Unusedbyte & 0x80 must be true

• To indicate it is LFH chunk
User Data
(8byte)
Unusedbyte (1byte)
Inused

LFH

• Unusedbytes

• Must be 0x80

• To indicate it is LFH freed chunk
User Data
(8byte)
Unusedbyte (1byte)
Freed

LFH
• Remark

• About EncodedOﬀsets

• EncodedOﬀsets is xor of the following values

• (sizeof(userblock header) | (BlockUnit*0x10 << 16))

• LFHkey

• Userblock address

• _LFH_HEAP address

LFH
• Remark

• About LFH header encoding

• The chunk header will xor with following values when initialization

• _HEAP address

• LFHkey

• Chunk address >> 4

• ((chunk address) - (UserBlock address)) << 12

LFH
…
Heap
…
…
…
LocalData
Buckets[x]
SegmentInfoArray[x]
BlockUnits
SizeIndex
…
_LFH_HEAP
_HEAP_BUCKET
LocalInfo
UserBlock
…
AggregateExchg
BlockSzie
BlockCount
…
SizeIndex
…
Depth
Hint (15bit)
Lock (1bit)
_HEAP_SUBSEGMENT
SubSegment
…
EncodedOffsets
BusyBitmap
chunk header
chunk header
chunk header
_INTERLOCK_SEQ
_HEAP_USERDATA_HEADER
LocalData
ActiveSubsegment
CachedItems
…
BucketIndex
…

• Nt Heap

• Front-End

• Initialization

• If FrontEndHeapUsageData[x] & 0x1f > 0x10, LFH will be initialized in next allocation

• It will ExtendFrontEndUsageData and create new BlocksIndex (0x80-0x400)

• Create FrontEndHeap

• Initialize SegmentInfoArrays[idx]

• It will start using front-end allocator from next allocation with same size

• LFH(Initialization)

• malloc(0x40) * 16
…
0x210
The range of LFH is index
from 0x0 to 0x80


• malloc(0x40) (17th)
…
0x231 0x231 & 0x1f > 0x10

…
0x231
heap->CompatibilityFlag |= 0x20000000
After setting this ﬂag, it will initialize 
LFH in the next allocation

…
EncodeFlagMask
Encoding
…
BlocksIndex
…
FreeList
…
FrontEndHeap
…
…
_HEAP
0x231
BlocksIndex
ExtendedLookup
ArraySize
…
ItemCount (4 bytes)
BaseIndex
ListHead
ListsInUseUlong
ListHint



• ExtendFrontEndUsageData and create new BlocksIndex
(0x80-0x400)

• The range of LFH is modiﬁed to index from 0x0 to
0x400

• Create and initialize FrontEndHeap (mmap)

• initialize SegmentInfoArrays[idx]

• Assign SegmentInfoArrays[BucketIndex] to
segmentInfo
…
0x4

…
EncodeFlagMask
Encoding
…
BlocksIndex
…
FreeList
…
FrontEndHeap
…
…
_HEAP
0x251 0x4
BlocksIndex
ExtendedLookup
ArraySize(0x80)
…
ItemCount (4 bytes)
BaseIndex (0x0)
ListHead
ListsInUseUlong
ListHint
BlocksIndex
ExtendedLookup
ArraySize (0x400)
…
ItemCount (4 bytes)
BaseIndex (0x80)
ListHead
ListsInUseUlong
ListHint

…
Heap
…
…
…
LocalData
Buckets[x]
SegmentInfoArray[x]
BlockUnits
SizeIndex
…
_LFH_HEAP
_HEAP_BUCKET
LocalData
ActiveSubsegment
CachedItems
…
BucketIndex
…
…
EncodeFlagMask
Encoding
…
BlocksIndex
…
FreeList
…
FrontEndHeap
…
…
_HEAP
0x251 0x4



• Allocate Userblock and initialize every
chunk.

• Set the corresponding ActiveSubsegment
to the UserBlock

• Randomly return chunk to user
Userblock metadata
Chunk 00
Chunk 01
…
Chunk MaxBlockCount

…
Heap
…
…
…
LocalData
Buckets[x]
SegmentInfoArray[x]
BlockUnits
SizeIndex
…
_LFH_HEAP
_HEAP_BUCKET
LocalInfo
UserBlock
…
AggregateExchg
BlockSzie
BlockCount
…
SizeIndex
…
Depth
Hint (15bit)
Lock (1bit)
_HEAP_SUBSEGMENT
SubSegment
…
EncodedOffsets
BusyBitmap
chunk header
chunk header
chunk header
_INTERLOCK_SEQ
LocalData
ActiveSubsegment
CachedItems
…
BucketIndex
…

• LFH

• Allocate (RtlpLowFragHeapAllocFromContext)

• It will check whether ActiveSubsegment has chunk available.

• Check ActiveSubsegment->depth

• If not, it will search from CachedItem, and replace ActiveSubsegment
with CachedItem’s SubSegment if available.

…
Heap
…
…
…
LocalData
Buckets[x]
SegmentInfoArray[x]
BlockUnits
SizeIndex
…
_LFH_HEAP
_HEAP_BUCKET
LocalInfo
UserBlock
…
AggregateExchg
BlockSzie
BlockCount
…
SizeIndex
…
Depth
Hint (15bit)
Lock (1bit)
_HEAP_SUBSEGMENT
SubSegment
…
EncodedOffsets
BusyBitmap
chunk header
chunk header
chunk header
_INTERLOCK_SEQ
LocalData
ActiveSubsegment
CachedItems
…
BucketIndex
…
Find SegementIfoArray by bucket->SizeIndex

…
Heap
…
…
…
LocalData
Buckets[x]
SegmentInfoArray[x]
BlockUnits
SizeIndex
…
_LFH_HEAP
_HEAP_BUCKET
LocalInfo
UserBlock
…
AggregateExchg
BlockSzie
BlockCount
…
SizeIndex
…
Depth
Hint (15bit)
Lock (1bit)
_HEAP_SUBSEGMENT
SubSegment
…
EncodedOffsets
BusyBitmap
chunk header
chunk header
chunk header
_INTERLOCK_SEQ
LocalData
ActiveSubsegment
CachedItems
…
BucketIndex
…
Check ActiveSubsegment

…
Heap
…
…
…
LocalData
Buckets[x]
SegmentInfoArray[x]
BlockUnits
SizeIndex
…
_LFH_HEAP
_HEAP_BUCKET
LocalInfo
UserBlock
…
AggregateExchg
BlockSzie
BlockCount
…
SizeIndex
…
Depth
Hint (15bit)
Lock (1bit)
_HEAP_SUBSEGMENT
SubSegment
…
EncodedOffsets
BusyBitmap
chunk header
chunk header
chunk header
_INTERLOCK_SEQ
LocalData
ActiveSubsegment
CachedItems
…
BucketIndex
…
Check ArgregateExchg.Depth not 0

…
Heap
…
…
…
LocalData
Buckets[x]
SegmentInfoArray[x]
BlockUnits
SizeIndex
…
_LFH_HEAP
_HEAP_BUCKET
LocalInfo
UserBlock
…
AggregateExchg
BlockSzie
BlockCount
…
SizeIndex
…
Depth
Hint (15bit)
Lock (1bit)
_HEAP_SUBSEGMENT
SubSegment
…
EncodedOffsets
BusyBitmap
chunk header
chunk header
chunk header
_INTERLOCK_SEQ
LocalData
ActiveSubsegment
CachedItems
…
BucketIndex
…
Check Userblock

• LFH


• Retrieve RtlpLowFragHeapRandomData[x]

• It will retrieve the value from RtlpLowFragHeapRandomData[x+1] next
round.

• x is 1 byte, x = rand() %256 after 256 rounds

• RtlpLowFragHeapRandomData is a 256-byte array ﬁlled with random value

• The range of random value is 0x0 - 0x7f

• LFH


• Finally, the index of chunk is

• RtlpLowFragHeapRandomData[x]*maxidx >> 7

• Take the nearest chunk if collision

• Check (unused byte & 0x3f) !=0 (indicate chunk is freed)

• Modify index and unused byte in header and return to user

….
….
….
SubSegment
…
EncodedOffsets
BusyBitmap
chunk header
chunk header
chunk header
• Get an index

• RtlpLowFragHeapRandomData[x]*maxidx >> 7

• Check if the BusyBitmap correspond to index is
0

• Return the chunk if true

• Otherwise take the next nearest chunk
chunk header

….
….
….
SubSegment
…
EncodedOffsets
BusyBitmap
chunk header
chunk header
chunk header
• If the index is inused

• Take next nearest chunk

• Set the bitmap to 1
chunk header

….
….
….
SubSegment
…
EncodedOffsets
BusyBitmap
chunk header
chunk header
chunk header
chunk header
• If the index is inused

• Take next nearest chunk

• Set the bitmap to 1

• LFH

• Free (RtlFreeHeap)

• Update unused byte in chunk header

• Find the index of the Chunk and reset Userblock->BusyBitmap

• Update ActiveSubsegment->AggregateExchg

• If the free chunk is not belong to ActiveSubsegment, it will try to put
into cachedItems

LocalInfo
UserBlock
…
AggregateExchg
BlockSzie
BlockCount
…
SizeIndex
…
Depth
Hint (15bit)
Lock (1bit)
_HEAP_SUBSEGMENT
SubSegment
…
EncodedOffsets
BusyBitmap
chunk header
chunk header
chunk header
_INTERLOCK_SEQ
• Free

• Find UserBlock by chunk
header

LocalInfo
UserBlock
…
AggregateExchg
BlockSzie
BlockCount
…
SizeIndex
…
Depth
Hint (15bit)
Lock (1bit)
_HEAP_SUBSEGMENT
SubSegment
…
EncodedOffsets
BusyBitmap
chunk header
chunk header
chunk header
_INTERLOCK_SEQ
• Free

header

• Find corresponding
SubSegment

LocalInfo
UserBlock
…
AggregateExchg
BlockSzie
BlockCount
…
SizeIndex
…
Depth
Hint (15bit)
Lock (1bit)
_HEAP_SUBSEGMENT
SubSegment
…
EncodedOffsets
BusyBitmap
chunk header
chunk header
chunk header
_INTERLOCK_SEQ
• Free

header

• Find corresponding
SubSegment

• Assign unused byte to
0x80

• Clear the bitmap

• Update AggregateExchg

LocalInfo
UserBlock
…
AggregateExchg
BlockSzie
BlockCount
…
SizeIndex
…
_HEAP_SUBSEGMENT
SubSegment
…
EncodedOffsets
BusyBitmap
chunk header
chunk header
chunk header
_HEAP_USERDATA_HEADERLocalData
ActiveSubsegment
CachedItems
…
BucketIndex
…
LocalInfo
UserBlock
…
AggregateExchg
BlockSzie
BlockCount
…
SizeIndex
…
_HEAP_SUBSEGMENT
Depth (1)
Hint (15bit)
Lock (1bit)
_INTERLOCK_SEQ
CachedItems
ActiveSubsegment  
is not equivalent to 
SubSegment of free 
chunk

LocalInfo
UserBlock
…
AggregateExchg
BlockSzie
BlockCount
…
SizeIndex
…
_HEAP_SUBSEGMENT
SubSegment
…
EncodedOffsets
BusyBitmap
chunk header
chunk header
chunk header
_HEAP_USERDATA_HEADERLocalData
ActiveSubsegment
CachedItems
…
BucketIndex
…
LocalInfo
UserBlock
…
AggregateExchg
BlockSzie
BlockCount
…
SizeIndex
…
_HEAP_SUBSEGMENT
Depth (1)
Hint (15bit)
Lock (1bit)
_INTERLOCK_SEQ
CachedItems
CachedItems[0]

LFH Exploitation
• Reuse attack

• If we have Use After Free

• Because of randomness of LFH, we can not predict the next chunk.

• Ｗe can ﬁll up UserBlock and then free one of them, it will return
same chunk in next allocation with same size.

LFH Exploitation
• normal case

• malloc(sizeof(A))
Userblock header

LFH Exploitation
• normal case

• malloc(sizeof(B))
Userblock header
fptr

LFH Exploitation
• normal case

• free(A)
Userblock header B
fptr

LFH Exploitation
• normal case

Userblock header B
fptr

LFH Exploitation
• normal case

Userblock header B
fptr
Hard to use

LFH Exploitation
• Reuse attack

Userblock header

LFH Exploitation
• Reuse attack

• malloc(sizeof(B)) x 6
Userblock header
fptr

LFH Exploitation
• Reuse attack

• free(A)
Userblock header
B
B B B
fptr B B

LFH Exploitation
• Reuse attack

Userblock header
B
B B B
fptr B B

LFH Exploitation
• Reuse attack

• A->fptr
Userblock header
B
B B B
fptr B B

LFH Exploitation
• Reuse attack

• A->fptr
Userblock header
B
B B B
fptr B B
Hĳack the control ﬂow !

Reference
• https://github.com/saaramar/
35C3_Modern_Windows_Userspace_Exploitation

• http://illmatics.com/Understanding_the_LFH.pdf

• https://github.com/saaramar/Deterministic_LFH

Thank you for listening
angelboy@chroot.org @scwuaptx

Windows 10 Nt Heap Exploitation (English version)

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