Windows Executable Infection
by
Qark and Quantum [VLAD]

     This document attempts to explain technically NewExe infection for the
     virus writer.  This isn't for the novice coder and you'll need to
     understand DOS EXE infection before being able to use any of it.

     The infection described in detail here is the same as used by the
     accompanying WinSurfer virus, there are other methods that can be done
     but they aren't our concern (it's up to you to develop new code!)

     You will want a copy of the New Exe header information which is available
     in Ralf Browns interrupt listings under Int21h AH=4Bh, a copy of which is
     included at the end of this article.

     This is a map of what we are trying to do:

           Before infection                  After Infection

         0h---------------------           0h---------------------
           |                   |             |                   |
           |  DOS EXE header   |             |  DOS EXE header   |
           |                   |             |                   |
           ---------------------             ---------------------
           |                   |             |                   |
           |  DOS Code         |             |  DOS Code         |
           |                   |             |                   |
           |                   |         3F8h---------------------
       400h---------------------             |                   |
           |                   | <- Move     |  New EXE Header   |
           |  New EXE Header   |     this    |  and some tables  |
           |  and some tables  |      up     |                   |
           |                   |             | ----------------- |
           | ----------------- |             |   Segment         |
           |   Segment         |             |   Table           |
           |   Table           |             |                   |
           |                   |             |                   |
           |                   |   Insert -> | ----------------- | Virus segment
           | ----------------- |    this     | ----------------- | entry
           |   Misc Other      |             |   Misc Other      |
           |   Tables          |             |   Tables          |
           |                   |             |                   |
           x                   x             x                   x
           x                   x             x                   x
           x                   x             x                   x
     CS:IP ---------------------             ---------------------
           |                   |             |                   |
           |   Windows Code    |             |   Windows Code    |
           |   and Misc        |             |   and Misc        |
           |   Segments        |             |   Segments        |
           |                   |             |                   |
       End ---------------------       CS:IP ---------------------
                                             | Virus segment     |
                                 Append all  | with code etc     |
                                    this ->  |                   |
                                             --------------------- Virus
                                             |                   | Relocation
                                         End --------------------- Entry

     Infection Theory:
     -----------------
     Test the EXE to make sure its windows and that the NE is at offset 1024.
     Reduce the NE pointer at 3ch by eight.
     Reduce DOS SP by eight.
     Any NE offsets which are larger than the segment table offset must be
      increased by eight.
     Increment the segment counter.
     Save the CS:IP.
     Point the CS:IP to the new segment entry
     Calculate the position of the end of the segment table, move all data up to
      and including the segment table up by eight bytes.
     Add another segment entry
     Write the virus to the end of the file
     Write relocation entry to the end of the file.
     ---

     That is a very basic overview of what is wanted.

     The main idea of it all is moving the NE header forward to add a new
     segment table entry, and changing the CS:IP to point to it.

     We'll go through all the stages step by step.

     Testing for a Windows executable.
     ---------------------------------

     A NewEXE file always begins with a DOS header and some DOS executable
     code.  With a windows executable:
            The word at offset 0 is 'MZ'.
            The word at offset 18h is 40h or greater.
     assuming that these conditions are met then we also want the pointer to the
     NE header (3ch) to be equal to 400h.  The reason for this is that room is
     needed to move the NE header forward by 8.

     Rewriting the DOS header.
     -------------------------

     Before rewriting the DOS header, reduce the DOS SP field (10h) by 8, because
     if it was pointing to the end of the DOS code section it would point to
     the start of the NE header. (No real reason for doing it really)
     The NE header pointer at 3ch must be reduced by 8, because we intend on
     moving most of the NE header forward to that position.

     Modifications to the NE header.
     -------------------------------

     From this point onwards we are referring to the offsets in relation to the
     NE header.

     The pointer to the segment table is a word at 22h.  If any of the pointers
     at 4,24h,26h,28h,2ah are larger than [22h], then increase that
     pointer by eight.  The reason for this is that since we are inserting a
     new segment table entry, all tables behind the segment table will be
     eight bytes further from the start of the header.

     The segment counter is a word at offset 1ch.  Increment it by one because
     since we are adding another segment, we need to indicate this in the
     header.

     At offset 14h is the dword pointer to the program entry point, CS:IP
     where the CS is actually the segment table entry number.  Save this pointer
     for use in your relocation entry (that part will be explained in detail
     later on).  Now point the CS:IP instead to the virus segment.  Do this
     by writing the necessary starting IP value to 14h (offset of the entry into
     your segment) and the segment counter value into 16h (since the virus
     segment is the last in the segment table it will be equal to the segment
     counter value which we have incremented).

     You can work this out for yourself, but what you need to do now is move
     the entire NE header, upto and including the segment table, but not
     the data behind it, forward by eight bytes.  Heres an equation on
     calculating how much to move:
            ((segmentcounter-1)*8)+word ptr [22h]

     Directly after this position is where to insert the new virus segment entry.

     The Virus Segment Entry.
     ------------------------

     Format of new executable segment table record:
      00h    WORD    offset in file (shift left by alignment shift for byte offs)
      02h    WORD    length of image in file (0000h = 64K)
      04h    WORD    segment attributes (see below)
      06h    WORD    number of bytes to allocate for segment (0000h = 64K)

     Offsets 2 and 6 in the segment record are just the virus size. Offset
     4 is the segment attribute, we use 180h, which makes the segment executable
     with relocations.

     Calculating offset 0 is more difficult.  It is necessary to get the file
     length and shift it right by the alignment shift value which was saved
     earlier.  I wouldn't know how to do a dword shift so I convert it to a
     division.

     File length into DX:AX :
            mov     ax,4202h
            xor     cx,cx
            cwd
            int     21h
     Shift right DX:AX by alignment shift:
            mov     cl,byte ptr alignment_shift
            push    bx
            mov     bx,1
            shl     bx,cl
            mov     cx,bx
            pop     bx
            div     cx
         AX=required offset 0 value

     Write this eight byte table behind all the moved header.

     Writing the Virus.
     ------------------

     Lseek to the end of the executable and write the virus.

     Writing the Relocation Entry.
     -----------------------------

     You need a relocation entry so that you can return control to the host
     after the virus has done its dirty work.  The relocation entries follow
     the segment itself.

     Format of relocation entry:
     Offset  Size    Description
      00     WORD    Number of relocation entries
             Offset  Size    Description
              00     BYTE    relocation type
              01     BYTE    relocation flags
              02     WORD    offset within segment
              04     WORD    target address segment
              06     WORD    target address offset

     The first word will be 1 because only 1 relocation entry.
     First byte is 3 to signal a 32bit pointer (a far jump).
     Second byte is 4 to signal additive relocation. (doesn't work without this)
     Word at offset 2 is the offset of the dword after the 'far jump' opcode.
     Word at offset 4 is the CS of the original host CS:IP
     Word at offset 6 is the IP of the original host CS:IP

     To put all this into code:

                    db      0eah    ;Opcode of JMP FAR PTR
        ret_ip      dw      0
                    dw      0ffffh

        relocation  dw      1       ;One relocation entry
                    db      3       ;32bit pointer relocation
                    db      4       ;Additive relocation
                    dw      offset ret_ip   ;Offset of the relocation in segment
        hostcs      dw      0       ;CS:IP of place we want our relocation to
        hostip      dw      0       ; point.

     Note: the relocation address data (ret_ip) _must_ be FFFF:0000 as above
           because windows treats it as a linked list.  If you don't understand
           don't worry, just do it!  It won't work otherwise.  It is important
           that before writing the virus body to the executable that you set
           this address to FFFF:0000 so that executable will be setup correctly.

     Write the relocation entry to the end of the file.

     Windows allows a standard Int 21h call just fine so all file manipulation
     is as simple as ever, with a few minor changes.

     Writting TSR/ISR's under Windows.
     ---------------------------------

     Windows is a protected mode environment and thus to set a vector under it we
     must manipulate the various tables that protected mode requires to be
     updated.  Of these tables one is of interest to us.  The "Interrupt
     Descriptor Table" (IDT). The IDT holds all the Protected Mode interrupt
     vectors that are in the "Interrupt Vector Table" (IVT) - if they are
     supported - and some more.  This is all well and good - all we have to do is
     set a vector in the IDT to point to our code like we do with dos and the
     IVT but - there are complications.

     V86 mode.
     ---------

     When Windows is running in 386 enhanced mode the processor is locked into
     v86 mode.  In v86 mode each application has its own 1mb memory block.  All
     EMS/XMS is locked away from the application and (usually) each process has
     its own IDT/IVT.  The idea being to allow the application to think it is
     alone on the system.  Any attempt to access any other tasks that may be
     running is a "violation of system integrity" and will cause an exception.
     This is no good for a virus and by now you're thinking we're screwed but
     windows solves the problem for us.  Windows has but one IDT and of course
     only one IVT to shadow it and thus if we are to a hook a vector in the IDT
     it will be reflected in every task.  (Ever seen that OS/2 warp advert where
     they say "... and true multi-tasking" - well this simply means that OS/2
     warp holds a seperate IDT for every task under v86 mode.)

     Setting the Vector.
     -------------------

     So let's get this straight.. the IVT is the one at 0:0 that we all know and
     love and the IDT is a protected mode/v86 shadow of it.  So you're most
     probably asking "why can't we just hook the IVT and let windows reflect it
     into the IDT?" - well there's 2 reasons: firstly windows doesn't "reflect"
     the IVT as we would like to think, actually it copies the IVT into the IDT
     on startup then replaces certain routines with "protected mode call
     structures" which call the real mode routines and some routines it doesn't
     even do this, secondly trying to access the IVT directly is a big nono and
     will cause an exception error.

     Can we use DOS ?  well yes, you can.. most dos functions have been reflected
     up into windows although windows is supposed to be trying to get rid of
     them. So you can call int 21,25/35 to set the vector but remember that this
     will be setting a vector in the GVT and only at the discretion of windows.

     A better way: use DPMI.  Windows is NOT your host under protected mode or
     even v86 mode as microsoft would have you believe.  Windows has an
     overlooker that provides windows time-slicing and exception abilities.  In
     fact all windows does is act as a mediator to DPMI and basically slow things
     down.  So how do we use DPMI to set the vector ?  well.. by using functions
     0204h/0205h  - with the vector in BL  - of the DPMI provider int 31.  This
     way windows has no say over what goes in its IDT and thus we have a LOT of
     control.

     Writing the Interrupt.
     ----------------------

     This is perhaps one of the easier things to do - once you understand the
     principles.  In protected mode/v86 mode your code segment is supposed to be
     read only and thus you're supposed to have a code, stack and data segment.
     Everyone knows this is bad so what we need is a way to write to the code
     segment. You won't find one - writing to the code segment just isn't
     possible.  BUT - if we were to have a data segment that pointed to the same
     code segment then we could do it. (actually we don't have any segments in
     protect mode.. we have a thing called a "selector" which gives access
     parameters to areas of memory.)

     To get an "alias selector" to the code segment we can use another DPMI
     function and bypass windows altogether: function ax=000ah with the code
     selector in bx of int 31h, it returns a read/writeable alias selector to
     the code segment in ax.

     So now Protected mode isn't protected and we're free to write to the code
     segment. One problem arises from this idea.  Qark wanted to put this code
     to generate the alias in the loader part of the virus and store it in the
     code segment for the interrupt routine.  This seemed fine until I found out
     that windows likes to move segments around.  One minute your code might be
     at one address the next it may be at another.  Thus you should always
     regenerate the alias on every execution of the ISR or lock down the segment
     - we chose against this last approach as it can cause exception errors from
     windows (DPMI has no problems with this strategy.)

     For a detailed look at DPMI consult Ralf Brown's interrupt listing under
     int 31h.  These functions haven't been included in this text.

     Staying Resident.
     -----------------

     Finally: don't go resident off something that can be closed.  If you go
     resident off something like mine sweeper or something and the user closes
     the application, your code segment will be deleted and removed from the IDT
     and will point to an empty segment.  This will cause windows to hang.
     Solution: look for a process that will never be closed, direct action
     infect this as soon as you find it and only ever go resident off it.

     In the 'system.ini' file in your windows directory, is a variable 'shell='
     that points to a file that is never closed.  Mostly it will be
     'progman.exe' but sometimes other programs are used, so read it in and
     infect it.

     To find the path of the windows directory, search the environment for a
     variable called 'windir=' (yes, lowercase!).  On entry to any windows
     executable ES will point to the PSP (or something functionally similar).
     The word at 2ch is the segment (selector) of the environment segment.
     Scan through this as you would within a DOS application.  We didn't discover
     this through any documentation, but when Quantum accidentally typed 'set'
     from within a DOS window.

     Facts and Possible Techniques.
     ------------------------------

     At offset 0eh in the New Executable header is the 'auto data segment index'.
     This is another segment table index, which will be automatically in DS
     on execution entry.  Although this is pure guesswork, if you set that
     'auto data segment' to the same segment as CS, (ie in every way the same
     except the segment attributes) then you could write to your CS without
     using DPMI and would also open the gateway to polymorphic windows viruses.
     (Polymorphism would be pretty useless if you had to put a DPMI call before
     you could write to your code segment.)

     Mark Ludwig's 'Windows Virus #2' uses a different form of infection in that
     it extends the length of the code segment as indicated in the segment
     table, and moves the entire host program starting from the end of the file
     back by virus length until the end of the code segment is reached, at which
     point the virus is written.  The IP is then repointed to the virus.
     There are some advantages to this, such as no need to add relocation entries
     because the jmp is intra-segment but this is greatly outweighed by the
     fact that moving an entire file is very slow and a much larger number of
     pointers in the header need to be modified to account for the change.

     There are DPMI calls for allocating memory (Int 31h ax=501h) but it is
     unknown at this point whether the memory remains allocated after the program
     is closed.  Mark Ludwig makes extensive use of these calls for temporary
     buffers in his direct action 'Windows Virus #2'.

     Mark Ludwig also demonstrates use of the WinAPI calls, which involve adding
     a new relocation entry for each call and pushing some stuff on the stack.
     This may become important when win95 comes out but for the moment it is
     more convenient to simply use int 21h.

     Some food for thought.
     ----------------------

     Microsoft has a dream [here we go], the dream is to control the world.  They
     intend to do this with applications like Windows [scary isn't it ?], and by
     the looks of it they might succeed.  A lot of people think computers should
     be easy, that there should be no skill involved in it and that everyone
     should HAVE to use one.  Microsoft are using this idea to flood the market
     with shit like Windows, in the hope that it will be accepted as a "standard".
     Windows is already this but Microsoft aren't happy with one success, they
     want to be on top of the hill.

     This is where we come in.  If Microsoft is going to succeed in taking over
     the world then we should be there making their lives difficult.  The
     WinSurfer virus created by us is so stable that an average windows
     user wouldn't even notice it.  [Then again the average windows user
     wouldn't notice if his hair caught on fire.]  And thus will stay with us
     for quite some time.  At least until AVers recognise the windows market
     and start releasing scanners.

     The New Executable format.     (From Ralf Browns Interrupt listing 21 AH=4B)
     --------------------------

     new executables begin running with the following register values
            AX = environment segment        (Wrong! AX=0)
            BX = offset of command tail in environment segment
            CX = size of automatic data segment (0000h = 64K)
            ES,BP = 0000h                   (Wrong! ES=PSP)
            DS = automatic data segment
            SS:SP = initial stack
      the command tail corresponds to an old executable's PSP:0081h and
      following, except that the 0Dh is turned into a NUL (00h); new
      format executables have no PSP  (All wrong)

    Format of .EXE file header:
    Offset  Size    Description
     00h  2 BYTEs   .EXE signature, either "MZ" or "ZM" (5A4Dh or 4D5Ah)
     02h    WORD    number of bytes in last 512-byte page of executable
     04h    WORD    total number of 512-byte pages in executable (includes any
                    partial last page)
     06h    WORD    number of relocation entries
     08h    WORD    header size in paragraphs
     0Ah    WORD    minimum paragraphs of memory to allocation in addition to
                    executable's size
     0Ch    WORD    maxi paragraphs to allocate in addition to executable's size
     0Eh    WORD    initial SS relative to start of executable
     10h    WORD    initial SP
     12h    WORD    checksum (one's complement of sum of all words in executable)
     14h    DWORD   initial CS:IP relative to start of executable
     18h    WORD    offset within header of relocation table
                    40h or greater for new-format (NE,LE,LX,PE,etc.) executable
     1Ah    WORD    overlay number (normally 0000h = main program)
    ---new executable---
     1Ch  4 BYTEs   ???
     20h    WORD    behavior bits
     22h 26 BYTEs   reserved for additional behavior info
     3Ch    DWORD   offset of new executable (NE,LE,etc) header within disk file,
                    or 00000000h if plain MZ executable

    Format of new executable header:
    Offset  Size    Description
     00h  2 BYTEs   "NE" (4Eh 45h) signature
     02h  2 BYTEs   linker version (major, then minor)
     04h    WORD    offset from start of this header to entry table (see below)
     06h    WORD    length of entry table in bytes
     08h    DWORD   file load CRC (0 in Borland's TPW)
     0Ch    BYTE    program flags (see below)
     0Dh    BYTE    application flags (see below)
     0Eh    WORD    auto data segment index
     10h    WORD    initial local heap size
     12h    WORD    initial stack size (added to data seg, 0000h if SS <> DS)
     14h    DWORD   program entry point (CS:IP), "CS" is index into segment table
     18h    DWORD   initial stack pointer (SS:SP), "SS" is segment index
                    if SS=automatic data segment and SP=0000h, the stack pointer
                    is set to the top of the automatic data segment, just below
                    the local heap
     1Ch    WORD    segment count
     1Eh    WORD    module reference count
     20h    WORD    length of nonresident names table in bytes
     22h    WORD    offset from start of this header to segment table (see below)
     24h    WORD    offset from start of this header to resource table
     26h    WORD    offset from start of this header to resident names table
     28h    WORD    offset from start of this header to module reference table
     2Ah    WORD    offset from start of this header to imported names table
                    (array of counted strings, terminated with a string of length
                    00h)
     2Ch    DWORD   offset from start of file to nonresident names table
     30h    WORD    count of moveable entry point listed in entry table
     32h    WORD    file alignment size shift count
                    0 is equivalent to 9 (default 512-byte pages)
     34h    WORD    number of resource table entries
     36h    BYTE    target operating system
                    00h unknown
                    01h OS/2
                    02h Windows
                    03h European MS-DOS 4.x
                    04h Windows 386
                    05h BOSS (Borland Operating System Services)
     37h    BYTE    other EXE flags
                    bit 0: supports long filenames
                    bit 1: 2.X protected mode
                    bit 2: 2.X proportional font
                    bit 3: gangload area
     38h    WORD    offset to return thunks or start of gangload area
     3Ah    WORD    offset to segment reference thunks or length of gangload area
     3Ch    WORD    minimum code swap area size
     3Eh  2 BYTEs   expected Windows version (minor version first)
    Note:   this header is documented in detail in the Windows 3.1 SDK
            Programmer's Reference, Vol 4.

    Bitfields for new executable program flags:
    Bit(s)  Description
     0-1    DGROUP type
              0 = none
              1 = single shared
              2 = multiple (unshared)
              3 = (null)
     2      global initialization
     3      protected mode only
     4      8086 instructions
     5      80286 instructions
     6      80386 instructions
     7      80x87 instructions

    Bitfields for new executable application flags:
    Bit(s)  Description
     0-2    application type
            001 full screen (not aware of Windows/P.M. API)
            010 compatible with Windows/P.M. API
            011 uses Windows/P.M. API
     3      is a Family Application (OS/2)
     5      0=executable, 1=errors in image
     6      non-conforming program (valid stack is not maintained)
     7      DLL or driver rather than application
            (SS:SP info invalid, CS:IP points at FAR init routine called with
              AX=module handle which returns AX=0000h on failure, AX nonzero on
              successful initialization)

    Format of new executable segment table record:
     00h    WORD    offset in file (shift left by align shift to get byte offs)
     02h    WORD    length of image in file (0000h = 64K)
     04h    WORD    segment attributes (see below)
     06h    WORD    number of bytes to allocate for segment (0000h = 64K)
    Note:   the first segment table entry is entry number 1

    Bitfields for segment attributes:
    Bit(s)  Description
     0      data segment rather than code segment
     1      unused???
     2      real mode
     3      iterated
     4      movable
     5      sharable
     6      preloaded rather than demand-loaded
     7      execute-only (code) or read-only (data)
     8      relocations (directly following code for this segment)
     9      debug info present
     10,11  80286 DPL bits
     12     discardable
     13-15  discard priority

    Format of new executable relocation data (immediately follows segment image):
    Offset  Size    Description
     00h    WORD    number of relocation items
     02h 8N BYTEs   relocation items
                    Offset  Size    Description
                     00h    BYTE    relocation type
                                    00h LOBYTE
                                    02h BASE
                                    03h PTR
                                    05h OFFS
                                    0Bh PTR48
                                    0Dh OFFS32
                     01h    BYTE    flags
                                    bit 2: additive
                     02h    WORD    offset within segment
                     04h    WORD    target address segment
                     06h    WORD    target address offset