full-disclosure - NAT router inbound network traffic subversion

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Message-ID: <1106892739.9371.26.camel@localhost.localdomain>
From: khermansen at ht-technology.com (Kristian Hermansen)
Subject: NAT router inbound network traffic subversion

I have Googled around and asked a highly-respected Professor at my
University whether it is possible to direct packets behind a NAT router
without the internal 192.168.x.x clients first requesting a connection
to the specific host outside.  The answer I received is "not possible".
I also asked if this can be thought of as a security feature, to which
the reply was again "yes".

Now, I wouldn't place all my bets on his answer and I am calling on
someone out there to clear up my question.  If NAT really does only
allow inbound connections with a preliminary request as he suggests, it
seems that the only way to get an "unauthorized" packet behind the
router is by some flaw in the firmware of the device.

How about if the client has requested a connection to Google.com from
behind his Linksys home NAT router: would it be possible for an outside
attacker to spoof packets from Google's IP to get packets into the
network?  Or do we need to know the sequence numbers as well?  Or is
there an even more devious way to get packets on the inside without a
client's initiative?

Has there been any research into this?  Are there statistics on worm
propagation and exploited network hosts in relation to those individuals
that did not own routers (and instead connected directly to their
modem)?  If *all* home users on the Internet had NAT routers during the
summer of 2003, would we have significantly slowed the spread of
Blaster?  I believe these all to be very important questions and the
security aspects of the ability to route packets behind NAT really
interests me...maybe some of you can elaborate :-)
-- 
Kristian Hermansen <khermansen@...technology.com>
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