Virus Labs & Distribution
VLAD #6 - Win95 tute

                  Infection of Portable Executables
                                by
                      Qark and Quantum [VLAD]


  The portable executable format is used by Win32, Windows NT and Win95,
  which makes it very popular and likely to become the dominant form of
  executable sometime in the future.  The NE header used by Windows 3.11
  is completely different to the PE header and the two should not be
  confused.

  None of the techniques in this document have been tested on Windows NT
  because no virus writer (we know) has access to it.
   
  At the bottom of this document is a copy of the PE format, which is not
  easy to follow but is the only reference publicly available.  Turbo
  Debugger 32 (TD32) is the debugger used during the research of this
  text, but SoftIce'95 also does the job.

  Calling Windows 95 API
  ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

  A legitimate application calls win95 api by the use of an import
  table.  The name of every API that the application wants to call is
  put in the import table.  When the application is loaded, the data
  needed to call the API is filled into the import table.  As was
  explained in the win95 introduction (go read it), we cannot modify
  this table due to Microsoft's foresight.

  The simple solution to this problem is to call the kernel directly.
  We must completely bypass the Win95 calling stucture and go straight
  for the dll entrypoint.

  To get the handle of a dll/exe (called a module) we can use the API
  call GetModuleHandle and there are other functions to get the
  entrypoint of a module - including a function to get the address of an
  API, GetProcAddress.

  But this raises a chicken and egg question.  How do I call an API so I
  can call API's, if I can't call API's ?  The solution is to call api
  that we know are in memory - API that are in KERNEL32.DLL - by calling
  the address that they are always located at.

  Some Code
  ÄÄÄÄÄÄÄÄÄ

  A call to an API in a legitimate application looks like:

        call APIFUNCTIONNAME
eg.     call CreateFileA

  This call gets assembled to:

        db 9ah          ; call
        dd ????         ; offset into jump table

  The code at the jump table looks like:

        jmp far [offset into import table]

  The offset into the import table is filled with the address of the
  function dispatcher for that API function.  This address is obtainable
  with the GetProcAddress API.  The function dispatcher looks like:

        push function value
        call Module Entrypoint

  There are API functions to get the entrypoint for any named module but
  there is no system available to get the value of the function.  If we
  are calling KERNEL32.DLL functions (of which are all the functions
  needed to infect executables) then we need look no further than this
  call.  We simply push the function value and call the module
  entrypoint.

  Snags
  ÄÄÄÄÄ

  In the final stages of Bizatch we beta tested it on many systems.
  After a long run of testing we found that the KERNEL32 module was
  static in memory - exactly as we had predicted - but it was at a
  different location from the "June Test Release" to the "Full August
  Release" so we needed to test for this.  What's more, one function
  (the function used to get the current date/time) had a different
  function number on the June release than it did on the August release.
  To compensate I added code that checks to see if the kernel is at one
  of the 2 possible locations, if the kernel isn't found then the virus
  doesn't execute and control is returned to the host.

  Addresses and Function Numbers
  ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

  For the June Test Release the kernel is found at 0BFF93B95h
  and for the August Release the kernel is found at 0BFF93C1Dh

  Function              June            August
  ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
  GetCurrentDir       BFF77744         BFF77744
  SetCurrentDir       BFF7771D         BFF7771D
  GetTime             BFF9D0B6         BFF9D14E
  MessageBox          BFF638D9         BFF638D9
  FindFile            BFF77893         BFF77893
  FindNext            BFF778CB         BFF778CB
  CreateFile          BFF77817         BFF77817
  SetFilePointer      BFF76FA0         BFF76FA0
  ReadFile            BFF75806         BFF75806
  WriteFile           BFF7580D         BFF7580D
  CloseFile           BFF7BC72         BFF7BC72


  Using a debugger like Turbo Debugger 32bit found in Tasm 4.0, other
  function values can be found.

  Calling Conventions
  ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

  Windows 95 was written in C++ and Assembler, mainly C++.  And although
  C calling conventions are just as easy to implement, Microsoft didn't
  use them.  All API under Win95 are called using the Pascal Calling
  Convention.  For example, an API as listed in Visual C++ help files:

        FARPROC GetProcAddress(
                HMODULE  hModule,   // handle to DLL module
                        LPCSTR  lpszProc        // name of function
        );

  At first it would be thought that all you would need to do is push the
  handle followed by a pointer to the name of the function and call the
  API - but no.  Due to Pascal Calling Convention, the parameters need
  to be pushed in reverse order:

          push offset lpszProc
          push dword ptr [hModule]
          call GetProcAddress

  Using a debugger like Turbo Debugger 32bit we can trace the call (one
  step) and follow it to the kernel call as stated above.  This will
  allow us to get the function number and we can do away with the need
  for an entry in the import table.


  Infection of the PE Format
  ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

  Finding the beginning of the actual PE header is the same as for NE
  files, by checking the DOS relocations for 40h or more, and seeking to
  the dword pointed to by 3ch.  If the header begins with a 'NE' it is a
  Windows 3.11 executable and a 'PE' indicates a Win32/WinNT/Win95 exe.

  Within the PE header is 'the object table', which is the most important
  feature of the format with regards to virus programming.  To append code
  to the host and redirect initial execution to the virus it is necessary to
  add another entry to the 'object table'.  Luckily, Microsoft is obsessed
  with rounding everything off to a 32bit boundary, so there will be room
  for an extra entry in the empty space most of the time, which means it
  isn't necessary to shift any of the tables around.

  
  A basic overview of the PE infection:

        Locate the offset into the file of the PE header
        Read a sufficient amount of the PE header to calculate the full size
        Read in the whole PE header and object table
        Add a new object to the object table
        Point the "Entry Point RVA" to the new object
        Append virus to the executable at the calculated physical offset
        Write the PE header back to the file


  To find the object table:
   The 'Header Size' variable (not to be confused with the 'NT headersize')
   is the size of the DOS header, PE header and object table, combined.
   To read in the object table, read in from the start of the file for
   headersize bytes.

   The object table immediately follows the NT Header. The 'NTheadersize'
   value, indicates how many bytes follow the 'flags' field.  So to work
   out the object table offset, get the NTheaderSize and add the offset
   of the flags field (24).

  Adding an object:
   Get the 'number of objects' and multiply it by 5*8 (the size of an object
   table entry).  This will produce the offset of the space within which
   the new virus object table entry can be placed.
   The data for the virus' object table entry needs to be calculated using
   information in the previous (host) entry.

   RVA             = ((prev RVA + prev Virtual Size)/OBJ Alignment+1)
                                                               *OBJ Alignment
   Virtual Size    = ((size of virus+buffer any space)/OBJ Alignment+1)
                                                               *OBJ Alignment
   Physical Size   = (size of virus/File Alignment+1)*File Alignment
   Physical Offset = prev Physical Offset + prev Physical Size
   Object Flags    = db 40h,0,0,c0h
   Entrypoint RVA  = RVA
   
   Increase the 'number of objects' field by one.

   Write the virus code to the 'physical offset' that was calculated, for
   'physical size' bytes.


  Notes
  ÄÄÄÄÄ

  Microsoft no longer includes the PE header information in their developers
  CDROMs.  It is thought that this might be to make the creation of
  viruses for Win95 less likely.  The information contained in the next
  article was obtained from a Beta of the Win32 SDK CDROM.


  Tools
  ÄÄÄÄÄ

  There are many good books available that supply low level Windows 95
  information.  "Unauthorized Windows 95", although not a particularly
  useful book (it speaks more of DOS/Windows interaction), supplies
  utilities on disk and on their WWW site that are far superior to the
  ones that we wrote to research Win95 infection.