Re: DNS Cache Dan Kamikaze (Actual Exploit Discussion)

[Paul Schmehl <pschmehl_lists () tx rr com>]

--On July 13, 2008 2:50:26 PM -0500 eugaaa () gmail com wrote:

> http://blogs.zdnet.com/security/?p=1466
> Can someone clarify what they meant by "non-reversible patch" ?

The patch changes the default behavior of dns so that queries are 
responded to from random ports rather than always from the same port 
(usually 53.)  Reversing the patch merely returns you to the previous 
default behavior.  It does not get you to the vulnerability that, in 
conjunction with non-random port responses, would allow you to spoof dns 
queries.  (This is speculation on my part.  Dan hasn't shared the details 
me with.)  IOW, there is a separate vulnerability in dns, which Dan has 
not yet revealed, that allows you to take advantage of the non-random 
nature of query responses.

Readers should note that if you override the patch behavior by specifying 
a query response port (the syntax is available to do that), you negate the 
patch.

> http://www.debian.org/security/2008/dsa-1603
> Are these .deb patches automagical?

If you mean, is bind patched after you've followed the directions in the 
advisory (i.e. run apt-get update and then apt-get install bind9), then 
yes, it "automagical" as you put it.

> *scratches head*
> I'm not interested in discussing the hype or scene-war aspect of this
> vulnerability.
>
> Has anyone actually verified the impact of this vulnerability? Any code,
> anything?
> (http://www.milw0rm.com/exploits/4266)
No because the real vulnerability has not yet been released.  (Again, this 
is my speculation.)  The patches will make bind much more resistant to a 
spoofing attack that would have been easy to do once the real 
vulnerability is revealed publicly.

BTW, if you want to check your name server (so long as it's not doing 
forwarding), you can run this command:
# dig @yourserver +short porttest.dns-oarc.net TXT

A patched server will respond like this:
# dig @ns1.stovebolt.com +short porttest.dns-oarc.net TXT
z.y.x.w.v.u.t.s.r.q.p.o.n.m.l.k.j.i.h.g.f.e.d.c.b.a.pt.dns-oarc.net.
"66.221.101.249 is GOOD: 26 queries in 1.3 seconds from 26 ports with std 
dev 18347.40"

An unpatched server will return POOR: 26 queries in 1.3.seconds from 1 
port.

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

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