The information in this article concerns the backdoors in MS-DOS and
BIOS that can be used and abused by a virus for it's own ends! Most
of them concern the Int21h DOS services interrupt and Int13h Disk
Int40h - The floppy disk handler gets relocated to Int40h by the Hard
disk BIOS and is called by Int 13h. Int40h can be hooked and
used to infect floppy disks. Be cautious when using this
because if no harddisk is present it isn't set.
Int30h - This is not infact an interrupt at all. It is a far jump
to the original interrupt 21h handler that is stored at
the address of int30h. It was originally used for the CP/M
emulation in earlier versions of DOS and remains there today.
If you look at the PSP information, at PSP: is a 'call
to the DOS function dispatcher'. It actually calls the int30h
we are talking about (but due to a microsoft stuff up it misses
by two bytes). There are two ways of using this backdoor, the
first way could be for a really hard method for infection or a
good destructive payload. You can use it directly by calling
it in an unusual fashion. This function could be messed up by
some programs but I have yet to see it not work anywhere.
Thanx go out to John Switzer for supplying me for this
information although I'm sure he wouldn't appreciate it's
You can only use DOS functions AH=0 to 24h with this and
any functions that require AL can't be used.
Int21h Proc Near
;Call this from your code with the same parameters
; as the real DOS int 21h function.
;Truly weird I'm sure you'll agree!
mov cl,ah ;It uses CL.
mov ax,offset return_addr ;Stack is backwards
pushf ;Flags are last!!
db 0eah ;JMP FAR PTR
dw 0c0h ;30h * 4
dw 0 ;Interrupt table.
ret ;Back to user.
This is different in that it uses the segment:offset
address of the Int30h to get the original 'proper' Int21h
that we are all used to. This method is used by the
writers of the MG virus (who also wrote creeping death,
very talented and good researchers!) Anyway you can work
that out yourself, thats why it's called research!
Int2fh - When DOS gets loaded it hooks int13h and saves the original
ah=13h addresses for its own use. When this function is called it
returns two addresses where one is slightly closer to the
original int13h than the other, but I'm not too sure which is
the closer of the two (they are often equal). If you play
with this yourself look it up in Ralf Brown's, you can probably
point the DOS calls to your virus if you do it right.
To grab the original int13h without messing up DOS:
int 2fh ;Get the int13h's
push es ;Save them
int 2fh ;Put them back to what they were.
pop bx ;Now we've got our handlers.
;From here you can either choose to use ES:BX or DS:DX
;as your int13h.
Seg70h - Segment 70h is used by DOS. All DOS disk access passes through
it at sometime. All you have to do is scan through it for
the bytes of the different calls. This method was first
used by the Creeping Death virus and is used in the 1984
(listed as 'ignorant' by CARO) and Daemon viruses. I'd
suggest running through this with a debugger and having a
look to work out what's going on. DOS has been using 70:B4
to store the original Int 13h since DOS 3.3.
dec si ;SI-1
lodsw ;DS:[SI] to AX SI+2
cmp ax,1effh ;FF1E = CALL FAR PTR [xxxx]
cmp word ptr [si],0b4h ;This is just there :)
jmp set_fake_int13 ;We've found it!
mov si,[si] ;SI=Where the address is
;save the int13h into the virus
mov cs:orig_store,word ptr [si]
mov cs:orig_store+2,word ptr [si+2]
;point it to our virus
mov word ptr [si],offset our_int13
mov word ptr [si+2],cs
;ret or whatever...
Int2fh - Have a look at this interrupt in Ralf Browns (a must for every
virus programmer) it can do ALL the interrupt 21h functions!
The only problem is working out the DOS stacks and so
on. It is handy for bypassing AV monitors, but it is much
too huge to go into in any detail.
BIOS - Within BIOS lurk a number of stationary entry points to
entry interrupts. There are a few problems with these, as alot
points of BIOSes are incompatible and QEMM won't work with them
but they can be useful because there isn't ANYTHING that
can be done to stop it.
Here are a list of addresses that are guaranteed not to
work half the time but have a look anyway.
F000:EC59 Floppy disk int 13h
F000:F859 Int 15h, sometimes useful
Int2ah - This is called by Int 21h on every file related function. By
ah=82h modifying the stack or certain registers you can change the
function that was called to whatever you want. DOS stores
the function multiplied by two in BL (eg Int 21h AH=40h will
be BL=80h when the int 2ah is called.). If you change this
BL to another function it should fool most AV monitors. This
may only work for some versions of DOS.
Int21h - If you call this service you can do any DOS function. Have a
ax=5d00h look! All you have to do is set your registers up in a table.
It should be easy to write a basic simulated int21h using
Anymore ? Not that I can think of! If you know any... tell me!!
- VLAD #3 INDEX -