Steve Gibson invents broken SYNcookies

[InfoSec News <isn () c4i org>]

http://www.theregister.co.uk/content/55/24189.html

By Thomas C Greene in Washington
Posted: 25/02/2002 at 14:33 GMT

He dares to call it "GENESIS" (Gibson's ENcryption-Enhanced Spoofing
Immunity System). He dares to call it "Beautiful and Perfect." It's
the product of "Three Key Innovations" for which he takes credit and
which culminate in an "Encrypted Token," which is another way of
saying a "SYNcookie", a quite useful thing developed by Dan Bernstein
and Eric Schenk back in 1996.

He dares to claim "immunity" from SYN floods. Only Steve Gibson's lame
knockoff is dangerously broken.

What it is

A SYN flood is a DoS attack in which server resources, not bandwidth,
are stressed. It fakes the initial handshake of a TCP connection with
spoofed IPs which the target machine is unable to answer, so the
target machine allocates system resources in anticipation of a
connection which is never completed. Re-tries and time-outs add up to
perhaps three minutes per bogus SYN. A server's capacity to respond to
legitimate requests can be devoured in a matter of seconds with very
small packets. Only four or five compromised client machines can
cripple a server; in this way it's a fiendishly economical attack.

The handshake is simple: a client initiates with a SYN (synchronize)
packet; the server replies with a SYN/ACK (synchronize/acknowledge)
packet; and the client finalizes with an ACK (acknowledge) packet. If
these steps are followed, a TCP connection is established between the
two.

GENESIS attempts to negotiate the handshake without allocating system
resources until the client's IP can be verified. This is a
common-sense approach, essential to SYNcookies as well. But SYNcookies
were worked out over time by people who, unlike Gibson, have a solid
grasp of TCP/IP and the machines it connects. Even so, it took time
and collaboration, and intellectual modesty, to get all the kinks
ironed out.

Unfortunately Gibson is so infatuated with the self-created myth of
his own genius that he can't be bothered to consult Bernstein and
Schenk, or anyone else for that matter, but goes it alone, inspired
only by his overweening pride and essential incompetence. Of course
his "Beautiful and Perfect" creation is going to be sadly defective.
How could it be otherwise?

One Reg reader who wishes to remain anonymous believes that GENESIS is
more than a mere failure, but actually worse than no SYN protection at
all. It was this person who originally brought the GENESIS project to
our attention, and s/he's offered some very insightful observations.

How it's done

Put simply, authenticating a TCP connection request requires the
server to encrypt some aspect(s) of the client's and the server's
status so as to ensure that the final ACK comes from the same source
as the original SYN (pun fully intended).

Data such as the client's ISN (Initial Sequence Number), originating
IP and port, MSS (Maximum Segment Size), and the server's IP and port,
can be hashed to produce a server ISN which must be available for
decoding in the final ACK packet. If the arithmetic fails, the ACK is
rejected and no resources are devoted to the bogus connection. If it
works out, a connection is made.

Old cookies absolutely need to expire so they can't be reused; and old
ISNs need to be identifiable so that they don't get mixed up with
those belonging to a newer connection. Something unique ('secret')
needs to be plugged into the hash so that cookies valid for one server
can't be used on another, and so that valid ISNs can't be guessed or
bruteforced easily.

Broken

Anyone who reads Bernstein and Schenk's correspondence linked above
will see that authenticating a SYN request is no trivial matter. There
are a number of obstacles, but Gibson manages to overcome only one of
them. Yes, he does manage to deal with the problem of disembodied
sequence numbers, so that out-of-date numbers aren't carried over to
complicate packet reconstruction on a new connection.

But Gibson is silent on the rest of the issues Bernstein and Schenk
have labored to solve.

First, he offers no means to cause a cookie (or "Encrypted Token," as
he prefers to call it), to expire. A valid cookie can be used to
establish a connection. A lot of valid cookies can be used to
establish a lot of connections. Perhaps Gibson is unfamiliar with the
term 'packet sniffer.' Too bad. We'll just sit back and watch the
kiddies gather up zillions of his broken SYNcookies to use against the
fools who trust him.

Second, he ignores MSS. It's hard to achieve decent performance
without knowing it.

Third, he doesn't use a secret, which means that valid ISNs can be
bruteforced and valid ACKs generated -- and abused.

Fourth, he uses RC5, which is slower than MD5 used in SYNcookies --
another performance hit (just in case his gross security sloppiness
didn't already frighten you away).

Pants on fire

Gibson dares to pretend that he'd never heard of SYNcookies when he
set off in quest of beauty and perfection. "Immediately after I posted
the second part of this work to the Web, several participants in the
news groups at grc.com reported that similar work had been done
before. I was unaware of previous work in this area, and consequently
developed my solution independently and without the benefit of any
previous work," Steve claims.

I don't believe a word of it. I think he deliberately set out to
knock-off SYNcookies and simply failed because the work was too
difficult. He's not an übergeek; he just plays one on his Web site.

I did a Google search and turned up more than 7,000 Web pages with the
terms 'SYNcookies' or 'SYN cookies'. This guy is hacking TCP, yet he
never once encountered a single mention of it?

Impossible. No human being could have his head that far up his own ass
-- not even Steve Gibson. 



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