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Re: DNS Cache Dan Kamikaze (Actual Exploit Discussion)
From: "eugaaa () gmail com" <eugaaa () gmail com>
Date: Mon, 14 Jul 2008 00:02:38 -0500

My analysis of the problem is now that the exploitation happens when a
recursive server goes looking for a record, and in doing so opens
connections to query each nameserver it finds along the path to the
authoritative namserver.

me -> my_dns(recursive)
my_dns -> root
my_dns -> almost_auth
my_dns -> auth_dns

During this period, an attacker floods the query port (or ports) of
my_dns with a valid response for the domain (this is made possible by
weak transaction ID's in most nameservers and the pseudo-randomness of
the query port) and in doing so manipulates the cache.

There were so many avenues of exploitation for this I actually
overlooked the glaringly obvious one.

On 7/13/08, eugaaa () gmail com <eugaaa () gmail com> wrote:
Yes, the issue was side tracked a bit. And I'm sure I am
misunderstanding the issue at this point (but I'm also reading
accounts of multiple vulnerabilities so that cannot be avoided)

But normally in DNS operations, slaves and their master are placed in
an authority encapsulated domain for transfers. IE. the slaves will
only axfr zones from the master.

And in the case of recursion, assuming the nameservers are recursive
it will hit the root and fly downward looking for the zone's
authoritative nameserver. The exploitation must happen here - a way to
become the authoritative nameserver. Am I wrong? Because it seems like
the transferring of zones/records is accounted for. Are we
manipulating root hints now? Any input is appreciated.





On 7/13/08, Paul Schmehl <pschmehl_lists () tx rr com> wrote:
--On July 13, 2008 9:44:19 PM -0500 eugaaa () gmail com wrote:

If the nameserver is "down" most likely the resolver is going to try a
different one. Meaning you're back to square one. Which is why I asked
what happens if the resolver recv's a response after it's been told
the nameserver is down. In any case, I'm not even sure how resolvers
handle dest unreachables. And again, I think that avenue is moot.

As for your question about theory versus practicality. 2^16 seems
possible. This exact same problem exist with ASLR implementations as
well as stack protection mechanisms (canary values etc). I think even
vista's current address space randomization is 16-bits. However with
these DNS transaction ID's you're not looking at a random number. It's
scope is limited because you've seen the transaction ID's of each
request you've made. IE my first request was 125, my second was 133,
etc. Meaning you pick a number higher up (180) and try to win the
race.


I think you are fundamentally misunderstanding the problem.  The
vulnerability we're discussing allows you to *poison* a nameserver's
cache.  You *want* the nameserver to answer.  You don't want to answer on
its behalf.  You want it to answer - incorrectly - so that users are
fooled into thinking they've been taken to the real site when in fact they
been taken to a "mirror" of the real site, specially prepared for whatever
nefarious purpose you have in mind.

Paul Schmehl
If it isn't already obvious,
my opinions are my own and not
those of my employer.



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