Trojan Horses

[John F. Haugh II]

In article <1990Oct7.155203.13283 at hq.demos.su> avg at hq.demos.su (Vadim G. Antonov) writes:
>	Hi! I think the problem of login trojan horses has a quite simple
>	solution:
>
>	1) There should be UNMASKABLE method of killing all programs
>	   reading this terminal. It may be a predefined sequence
>	   of characters or a special hardware signal like BREAK or
>	   CARRIER DROP. This feature should be hard-coded in TTY
>	   driver.

This is often refered to as "the secure attention key", or SAK for
short.  The idea is that some special key sequence is associated with
the user wanting to validate the path to the system.  By pressing the
SAK sequence the user is certain they are speaking directly to the
system, and not to a trojan horse.  The BREAK key is a typical choice
since it is baud-rate insensitive.  Any key sequence which is
dependent on the correct baud rate being set at any moment is likely
to be circumvented by a clever hacker switching baud rates on you.

>	2) All processes, associated with a TTY should be killed
>	   (as SIGHUP does) andprotected processes should be
>	   RE-ASSOCIATED with an unique TTY-id (which actually
>	   does not exist).

Killed is a tad strong.  The only real requirement is that unauthorized
access to the device be revoked.  You can do this simply by marking a
file table entry which references the device as "stale" and returning
an error on any attempt (other than close, I suppose ...) to use that
descriptor.  In practice it seems that being overly agressive with your
desires to kill processes makes users unhappy.  Suitably privileged
processes would be able to re-open the device, allowing daemons such as
biff and month to avoid losing touch with reality ...

You might want to get a copy of the DoD Orange Book, if you don't
have one already.
-- 
John F. Haugh II                             UUCP: ...!cs.utexas.edu!rpp386!jfh
Ma Bell: (512) 832-8832                           Domain: jfh at rpp386.cactus.org
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