---[ Phrack Magazine Volume 8, Issue 53 July 8, 1998, article 06 of 15
-------------------------[ T/TCP vulnerabilities
--------[ route|daemon9
----[ Introduction and Impetus
T/TCP is TCP for Transactions. It is a backward compatible extension for
TCP to facilitate faster and more efficient client/server transactions. T/TCP
is not in wide deployment but it is in use (see appendix A) and it is supported
by a handful of OS kernels including: FreeBSD, BSDi, Linux, and SunOS. This
article will document the T/TCP protocol in light detail, and then cover some
weaknesses and vulnerabilities.
----[ Background and primer
TCP is a protocol designed for reliability at the expense of expediency
(readers unfamiliar with the TCP protocol are directed to the ancient-but-
still-relevant: http://www.infonexus.com/~daemon9/Misc/TCPIP-primer.txt).
Whenever an application is deemed to require reliability, it is usually built
on top of TCP. This lack of speed is considered a necessary evil. Short lived
client/server interactions desiring more speed (short in terms of time vs.
amount of data flow) are typically built on top of UDP to preserve quick
response times. One exception to this rule, of course, is http. The
architects of http decided to use the reliable TCP transport for ephemeral
connections (indeed a poorly designed protocol).
T/TCP is a small set of extensions to make a faster, more efficient TCP.
It is designed to be a completely backward compatible set of extensions to
speed up TCP connections. T/TCP achieves its speed increase from two major
enhancements over TCP: TAO and TIME_WAIT state truncation. TAO is TCP
Accelerated Open, which introduces new extended options to bypass the 3-way
handshake entirely. Using TAO, a given T/TCP connection can approximate a
UDP connection in terms of speed, while still maintaining the reliability of a
TCP connection. In most single data packet exchanges (such is the case with
transactional-oriented connections like http) the packet count is reduced by a
third.
The second speed up is TIME_WAIT state truncation. TIME_WAIT state
truncation allows a T/TCP client to shorten the TIME_WAIT state by up to a
factor of 20. This can allow a client to make more efficient use of network
socket primitives and system memory.
----[ T/TCP TAO
TCP accelerated open is how T/TCP bypasses the 3-way handshake. Before we
discuss TAO, we need to understand why TCP employs a 3-way handshake.
According to RFC 793, the principal reason for the exchange is the prevention
of old duplicate connection initiations wandering into current connections and
causing confusion. With this in mind, in order to obviate the need for the
3-way handshake, there needs to be a mechanism for the receiver of a SYN to
guarantee that that SYN is in fact new. This is accomplished with a new
extended TCP header option, the connection count (CC).
The CC (referred as tcp_ccgen when on a host) is a simple monotonic
variable that a T/TCP host keeps and increments for every TCP connection
created on that host. Anytime a client host supporting T/TCP wishes to make a
T/TCP connection to a server, it includes (in it's TAO packet) a CC (or CCnew)
header option. If the server supports T/TCP, it will cache that client's
included CC value and respond with a CCecho option (CC values are cached by
T/TCP hosts on a per host basis). If the TAO test succeeds, the 3-way
handshake is bypassed, otherwise the hosts fall back to the older process.
The first time a client host supporting T/TCP and a server host supporting
T/TCP make a connection no CC state exists for that client on that server.
Because of this fact, the 3-way handshake must be done. However, also at that
time, the per host CC cache for that client host is initialized, and all
subsequent connections can use TAO. The TAO test on the server simply checks
to make sure the client's CC is greater then the last received CC from that
client. Consider figure 1 below:
Client Server
T -----------------------------------------------------------------------
i 0 --TAO+data--(CC = 2)--> ClientCC = 1
m 1 2 > 1; TAO test succeeds
e 2 accept data ---> (to application)
[ fig 1 ]
Initially (0) the client sends a TAO encapsulated SYN to the server, with
a CC of 2. Since the CC value on the server for this client is 1 (indicating
they have had previous T/TCP-based communication) the TAO test succeeds (1).
Since the TAO test was successful, the server can pass the data to application
layer immediately (2). If the client's CC had not been greater than the
server's cached value, the TAO test would have failed and forced the 3-way
handshake.
----[ T/TCP TIME_WAIT truncation
Before we can see why it is ok to shorten the TIME_WAIT state, we need to
cover exactly what it is and why it exists.
Normally, when a client performs an active close on a TCP connection, it
must hold onto state information for twice the maximum segment lifetime (2MSL)
which is usually between 60 - 240 seconds (during this time, the socket pair
that describes the connection cannot be reused). It is thought that any
packet from this connection would be expired (due to IP TTL constraints) from
the network. TCP must be consistent with its behavior across all contingencies
and the TIME_WAIT state guarantees this consistency during the last phase of
connection closedown. It keeps old network segments from wandering into a
connection and causing problems and it helps implement the 4-way closedown
procedure. For example, if a wandering packet happens to be a retransmission
of the servers FIN (presumably due to the clients ACK being lost), the client
must be sure to retransmit the final ACK, rather then a RST (which it would do
if it had torn down all the state).
T/TCP allows for the truncation of the TIME_WAIT state. If a T/TCP
connection only lasts for MSL seconds or less (which is usually the case with
transactional-oriented connections) the TIME_WAIT state is truncated to as
little as 12 seconds (8 times the retranmission timeout - RTO). This is
allowable from a protocol standpoint because of two things: CC number
protection against old duplicates and the fact that the 4-way closedown
procedure packet loss scenario (see above) can be handled by waiting for the
RTO (multiplied by a constant) as opposed to waiting for a whole 2MSL.
As long as the connection didn't last any longer then MSL, the CC number
in the next connection will prevent old packets with an older CC number from
being accepted. This will protect connections from old wandering packets
(if the connection did last longer, it is possible for the CC values to wrap
and potentially be erroneously delivered to a new incarnation of a connection).
----[ Dominance of TAO
It is easy for an attacker to ensure the success or failure of the TAO
test. There are two methods. The first relies on the second oldest hacking
tool in the book. The second is more of a brutish technique, but is just as
effective.
--[ Packet sniffing
If we are on the local network with one of the hosts, we can snoop the
current CC value in use for a particular connection. Since the tcp_ccgen is
incremented monotonically we can precisely spoof the next expected value by
incrementing the snooped number. Not only will this ensure the success of our
TAO test, but it will ensure the failure of the next TAO test for the client
we are spoofing.
--[ The numbers game
The other method of TAO dominance is a bit rougher, but works almost as
well. The CC is an unsigned 32-bit number (ranging in value from 0 -
4,294,967,295). Under all observed implementations, the tcp_ccgen is
initialized to 1. If an attacker needs to ensure the success of a TAO
connection, but is not in a position where s/he can sniff on a local network,
they should simply choose a large value for the spoofed CC. The chances that
one given T/TCP host will burn through even half the tcp_ccgen space with
another given host is highly unlikely. Simple statistics tells us that the
larger the chosen tcp_ccgen is, the greater the odds that the TAO test will
succeed. When in doubt, aim high.
----[ T/TCP and SYN flooding
TCP SYN flooding hasn't changed much under TCP for Transactions. The
actual attack is the same; a series of TCP SYNs spoofed from unreachable IP
addresses. However, there are 2 major considerations to keep in mind when
the target host supports T/TCP:
1) SYN cookie invalidation: A host supporting T/TCP cannot, at the same
time, implement SYN cookies. TCP SYN cookies are a SYN flood defense
technique that works by sending a secure cookie as the sequence number
in the second packet of the 3-way handshake, then discarding all state
for that connection. Any TCP options sent would be lost. If the final
ACK comes in, only then will the host create the kernel socket data
structures. TAO obviously cannot be used with SYN cookies.
2) Failed TAO processing result in queued data: If the TAO test fails, any
data included with that packet will be queued pending the completion of
the connection processing (the 3-way handshake). During a SYN flood,
this can make the attack more severe as memory buffers fill up holding
this data. In this case, the attacker would want to ensure the failure
of the TAO test for each spoofed packet.
In a previous Phrack Magazine article, the author erroneously reported that
T/TCP would help to alleviate SYN flood vulnerability. This obviously
incorrect statement was made before copious T/TCP research was done and is
hereby rescinded. My bad.
----[ T/TCP and trust relationships
An old attack with a new twist. The attack paradigm is still the same,
(readers unfamiliar with trust relationship exploitation are directed to
P48-14) this time, however, it is easier to wage. Under T/TCP, there is no
need to attempt to predict TCP sequence numbers. Previously, this attack
required the attacker to predict the return sequence number in order to
complete the connection establishment processing and move the connection into
the established state. With T/TCP, a packet's data will be accepted by the
application as soon as the TAO test succeeds. All the attacker needs to do is
ensure that the TAO test will succeed. Consider the figure below.
Attacker Server Trusted
-----------------------------------------------------------------------
0 -spoofed-TAO->
1 TAO test succeeds
T 2 data to application
i 3 ---TAO-response->
m 4 no open socket
e 5 <------RST-------
6 tears down connection
[ fig 2 ]
The attacker first sends a spoofed connection request TAO packet to the
server. The data portion of this packet presumably contains the tried and true
non-interactive backdooring command `echo + + > .rhosts`. At (1) the TAO test
succeeds and the data is accepted (2) and passed to application (where it is
processed). The server then sends its T/TCP response to the trusted host (3).
The trusted host, of course, has no open socket (4) for this connection, and
responds with the expected RST segment (5). This RST will teardown the
attacker's spoofed connection (6) on the server. If everything went according
to plan, and the process executing the command in question didn't take too long
to run, the attacker may now log directly into the server.
To deal with (5) the attacker can, of course, wage some sort of denial of
service attack on the trusted host to keep it from responding to the
unwarranted connection.
----[ T/TCP and duplicate message delivery
Ignoring all the other weaknesses of the protocol, there is one major flaw
that causes the T/TCP to degrade and behave decidedly NONTCP-like, therefore
breaking the protocol entirely. The problem is within the TAO mechanism.
Certain conditions can cause T/TCP to deliver duplicate data to the
application layer. Consider the timeline in figure 3 below:
Client Server
-----------------------------------------------------------------------
0 --TAO-(data)--->
1 TAO test succeeds
T 2 accept data ---> (to application)
i 3 *crash* (reboot)
m 4 timeout (resends) --TAO-(data)--->
e 5 TAO test fails (data is queued)
6 established <-SYN-ACK(SYN)-- fallback to 3WHS
7 --ACK(SYN)-----> established (data --> application)
[ fig 3 ]
At time 0 the client sends its TAO encapsulated data to the server (for
this example, consider that both hosts have had recent communication, and the
server has defined CC values for the client). The TAO test succeeds (1) and
the server passes the data to the application layer for processing (2).
Before the server can send its response however (presumably an ACK) it crashes
(3). The client receives no acknowledgement from the server, so it times out
and resends its packet (4). After the server reboots it receives this
retransmission, this time, however, the TAO test fails and the server queues
the data (5). The TAO test failed and forced a 3-way handshake (6) because the
servers CC cache was invalidated when it rebooted. After completing the 3-way
handshake and establishing a connection, the server then passes the queued data
to the application layer, for a second time. The server cannot tell that it
has already accepted this data because it maintains no state after a reboot.
This violates the basic premise of T/TCP that it must remain completely
backward compatible with TCP.
----[ In closing
T/TCP is a good idea that just wasn't implemented properly. TCP was
not designed to support a connectionless-like paradigm while still
maintaining reliability and security (TCP wasn't even designed with security
in mind at all). T/TCP brings out too many problems and discrete bugs in TCP
to be anything more then a novelty.
----[ Appendix A: Internet hosts supporting RFC 1644
This information is ganked from Richard Steven's T/TCP homepage
(http://www.kohala.com/~rstevens/ttcp.html). It is not verfied to be correct.
- www.ansp.br
- www.elite.net
- www.iqm.unicamp.br
- www.neosoft.com
- www.sbq.org.br
- www.uidaho.edu
- www.yahoo.com
----[ Appendix B: Bibliography
1) Braden, R. T. 1994 "T/TCP - TCP Extensions for Transactions...", 38 p
2) Braden, R. T. 1992 "Extending TCP for Transactions - Concepts...", 38 p
3) Stevens, W. Richard. 1996 "TCP Illustrated volume III", 328 p
4) Smith, Mark. 1996, "Formal verification of Communication...", 15 p
----[ EOF