Date: Thu, 11 Dec 2003 00:28:28 +0100 (CET)
From: Michal Zalewski <lcamtuf@...ttot.org>
To: bugtraq@...urityfocus.com
Cc: full-disclosure@...sys.com
Subject: A new TCP/IP blind data injection technique?


Good morning,

I am not quite sure there was no prior discussion of this problem, but I
could not find anything even remotely related, and so I think it makes
sense to post here. This post roughly describes a thought I had recently -
and I have to admit this is pure theory, even though it should be fairly
easy to turn this into a practical attack.

Blind spoofing, hijacking and data insertion into TCP/IP sessions,
although considered by some folks to be a threat of the past, still has
some impact potential; I have provided some arguments to support this
belief in my followup TCP/IP ISN analysis, in section 2, and I bet there it
is just the top of an iceberg [ http://lcamtuf.coredump.cx/newtcp/#risks ].

Closing all the attack venues by deploying "proper" cryptography is not
always feasible and easy, and even then, the protection is not complete -
the DoS potential remains. Without cryptography, the integrity of TCP/IP
sessions is protected only by a small set of parameters that are -
hopefully - not known to a person not involved in the communications, and
offer enough possible values to make brute-force attacks usually not
feasible. In practice, the Internet largely relies on the correctness and
unpredictability of the initial sequence number generation algorithms used
in TCP/IP stacks on various systems and devices.

I have done some research on the quality of those implementations, as some
of you may recall; so did others, and the situation have greatly improved
in the past 5 years or so, although it is still not quite what we would
like it to be. It is, however, expected that all mainstream operating
systems offer a reasonable ISN strength, and thus are not susceptible to
trivial TCP/IP stream invasion.

There seems to be a more fundamental problem, however, a problem that
renders sequence numbers and their quality practically irrelevant in
certain common scenarios.

Consider the following: Bob sends a TCP/IP ACK packet to Alice, with a
data payload and within an established session, of which session the
attacker is aware (attacker-induced or server to server traffic, perhaps).
Bob's packet exceeds the MTU somewhere en route (be it on some WAN
interface, or on a local PPPoA, PPPoE or VPN interface), a situation not
quite unheard of; the IP packet gets fragmented in order to be delivered
successfully.

The first IP fragment would carry the beginning of the TCP packet,
including port numbers, sequence number, and other information that may be
relatively difficult for a third party (the attacker) to guess otherwise.
The other fragment (fragments) of Bob's packet carry the remaining section
of the TCP/IP payload, and would be put back together with the headers and
previous sections of the packet once received by Alice. Here is where the
attacker strikes: he may spoof the second IP fragment, instead of
attempting to determine the sequence number, and insert data into the TCP
payload.

There are only two problems he would face:

  1. Figuring out the IP ID value. Usually a minor inconvenience, since a
     majority of systems use sequential numbers, and so it is possible to
     guess the next value with no effort.

  2. Sending a fragment that would, after reassembly, still validate against
     TCP/IP checksum in the headers. The only real unknown is the sequence
     (and perhaps acknowledgment) number in there - the remainder can be
     usually either predicted to a degree, or simply overwritten with
     overlapping fragments, but the sequence number cannot be, for obvious
     reasons.

There are two approaches to the latter problem. Since the checksum is only
16 bits, it might be reasonable to simply trust your luck, rinse and
repeat. Also, since it is possible to overwrite the checksum field with an
overlapping IP fragment, and it is possible to control the packet payload,
perhaps it is also trivial to come up with such an input to have the
(trivial!) checksum depend less on the unknown part of the packet, and
more on the known payload.  The TCP/IP checksum is designed to be very
fast and detect accidental bit flips and other snafus, not to provide any
security - so I have a strong feeling it is possible to bias it this way
or another; on the other hand, I do not have time to give it more thought
at the moment (honestly), any thoughts?

To summarise... the attack seems to be fairly practical, at the very least
significally decreasing the search space, at the very best, effectively
disabling any session integrity protection gained from unpredictable ISNs.
There are two major mitigating factors for this kind of attacks:

  1. Path MTU discovery (DF set) prevents fragmentation [*]; some modern
     systems (Linux) default to this mode - although PMTU discovery is
     also known to cause problems in certain setups, so it is not always
     the best way to stop the attack.

     [*] Also note that certain types of routers or tunnels tend to
     ignore DF flag, possibly opening this vector again.

  2. Random IP ID numbers, a feature of some systems (OpenBSD?), although also
     risky (increasing reassembly collission probability), make the attack
     more difficult.

     In the situation when it is necessary to brute-force all bits of the
     checksum, and all bits of the IP ID, the complexity of this data
     injection method starts to be comparable to full 32-bit ISN
     brute-force - usually not feasible.

     In the likely situation it is not necessary to brute force all
     checksum variants, the feature becomes only an inconvenience,
     raising the bar only slightly.

Note that this has nothing to do with old firewall bypassing techniques
and other tricks that used fragmentation to fool IDSes and so on -
mandatory defragmentation of incoming traffic on perimeter devices will
not solve the problem.

Cheers,
-- 
------------------------- bash$ :(){ :|:&};: --
 Michal Zalewski * [http://lcamtuf.coredump.cx]
    Did you know that clones never use mirrors?
--------------------------- 2003-12-11 00:22 --

      http://lcamtuf.coredump.cx/alpha/

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