Re: [fw-wiz] Vulnerability Response (was: BGP TCP RST Attacks)

From: Marcus J. Ranum (
Date: 06/01/04

  • Next message: Brian Ford: "Re:[fw-wiz] Vulnerability Response (was: BGP TCP RST Attacks)"
    To: "M. Dodge Mumford" <>, "Paul D. Robertson" <>
    Date: Tue, 01 Jun 2004 13:13:20 -0400

    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.


    firewall-wizards mailing list

  • Next message: Brian Ford: "Re:[fw-wiz] Vulnerability Response (was: BGP TCP RST Attacks)"