Re: Vulnerability Response (was: BGP TCP RST Attacks)

["Marcus J. Ranum" <mjr () ranum com>]

M. Dodge Mumford wrote:
> Paul D. Robertson said:
> > If it can't be attacked, then arguably, it doesn't need to be fixed.
>
> That sentiment surprises me a bit. It appears to me to violate the concept
> of defense in depth.

This is Peter Tippett's theory of synergistic controls. If you have
several things that each reduce the likelihood of something bad
happening, then it's really good to do more of them a little bit
because the marginal returns eventually go down.

So, if making your network separated so that "it can't be attacked"
is going to address 95% of the risks (ninjas, nanobots, etc, are still
a problem) and hardening the system is going to address another 95%
you're best off if you do the easiest/cheapest one first. In the case
of using my "perfect firewall" it's usually easier since it's almost
always easier and cheaper to NOT DO SOMETHING than to DO
something. The equipment cost for an air gap is low. ;)

What's interesting is that if you have 2 security controls that each
help block (on average, assuming random distribution of attack
vectors - which is an interesting assumption) 50% of the attacks,
then you've got 75% of the attacks blocked. Again, the assumption
of random distribution is an interesting and important problem
in the theory. If the attacks distribute disproportionately - if you
can whack 50% of the network attacks and 90% of the attacks
are networked - then your air gap is going to show a much higher
value (95% of 90%)    One of the things that makes firewalls
remain attractive is that a disproportion of attacks are networked
AND the effort factor to install them at a perimeter is low.

The concept of defense in depth is to do some pretty basic
stuff in lots of places. And it works. So if you're willing to
assume in Paul's example that "the system cannot be attacked
is ONLY 95% effective - then a 50% effective antivirus system
on the desktop behind the airgap bumps your likelihood of an
attack getting through down to a whopping 2.5%.  But if you
think about it, your first line of defense makes a lot of the
difference and after that it's all diminishing returns.

Hmm... Did I just say that "just doing ANYTHING" is a good
start? I think I did. ;)  Perhaps that's why we find ourselves
on the fence about the host/network - where do I secure it ?
issue - doing *anything* that's not manifestly stupid helps
a great deal. Doing any 2 things that aren't manifestly
stupid gets you most of the rest of the way 100% for all
intents and purposes.  If you accept some of the logic I've
thrown at you above, then it stands to reason that doing
things that help less than 40-50% of the time is probably
a waste of time unless you're doing 3 or more of them.

mjr. 

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