Network Working Group R. Stewart
Internet-Draft M. Dalal
Expires: August 17, 2006 Editor
February 13, 2006
Improving TCP's Robustness to Blind In-Window Attacks
draft-ietf-tcpm-tcpsecure-04.txt
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Copyright (C) The Internet Society (2006).
Abstract
A recent study indicates that some types of TCP connections have an
increased vulnerability to spoofed packet injection attacks than
previously believed [SITW]. TCP has historically been considered
protected against spoofed packet injection attacks by relying on the
fact that it is difficult to guess the 4-tuple (the source and
destination IP addresses and the source and destination ports) in
combination with the 32 bit sequence number(s). A combination of
increasing window sizes and applications using a longer term
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connections (e.g. H-323 or Border Gateway Protocol [RFC1771]) have
left modern TCP implementation more vulnerable to these types of
spoofed packet injection attacks.
Note: Both [SITW] and [DTASA] provide charts which can give the
reader an idea as to the time it takes to penetrate an unprotected
system.
Many of these long term TCP applications tend to have predictable IP
addresses and ports which makes it far easier for the 4-tuple to be
guessed. Having guessed the 4-tuple correctly, an attacker can
inject a RST, SYN or DATA segment into a TCP connection by carefly
crafting the sequence number of the spoofed segment to be in the
current receive window. This can cause the connection to either
abort or possibly cause data corruption. This document proposes
small modifications to the way TCP handles inbound segments that can
reduce the probability of such an attack.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. The RESET attack . . . . . . . . . . . . . . . . . . . . . 4
1.2. Attack probabilities . . . . . . . . . . . . . . . . . . . 5
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 8
3. Blind reset attack using the RST bit . . . . . . . . . . . . . 9
3.1. Description of the attack . . . . . . . . . . . . . . . . 9
3.2. Mitigation . . . . . . . . . . . . . . . . . . . . . . . . 9
4. Blind reset attack using the SYN bit . . . . . . . . . . . . . 11
4.1. Description of the attack . . . . . . . . . . . . . . . . 11
4.2. Mitigation . . . . . . . . . . . . . . . . . . . . . . . . 11
5. Blind data injection attack . . . . . . . . . . . . . . . . . 13
5.1. Description of the attack . . . . . . . . . . . . . . . . 13
5.2. Mitigation . . . . . . . . . . . . . . . . . . . . . . . . 13
6. ACK throttling . . . . . . . . . . . . . . . . . . . . . . . . 15
7. Backward Compatibility and Other considerations . . . . . . . 16
8. Middlebox considerations . . . . . . . . . . . . . . . . . . . 17
8.1. Middlebox that resend RST's . . . . . . . . . . . . . . . 17
8.2. Middleboxes that advance sequence numbers . . . . . . . . 17
9. Interoperability Testing . . . . . . . . . . . . . . . . . . . 19
10. Security Considerations . . . . . . . . . . . . . . . . . . . 21
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 23
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
14.1. Normative References . . . . . . . . . . . . . . . . . . . 25
14.2. Informative References . . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26
Intellectual Property and Copyright Statements . . . . . . . . . . 27
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1. Introduction
TCP [RFC0793] is widely deployed and the most common reliable end to
end transport protocol used for data communication in todays
Internet. Yet when it was defined over 20 years ago the Internet, as
we know it, was a different place lacking many of the threats that
are now common. TCP spoofing attacks are one such attack that are
seen on the Internet today.
In a TCP spoofing attack, an off-path attacker crafts TCP packets by
forging the IP source and destination addresses as well as the source
and destination ports (commonly referred to as a 4-tuple value) so
that a target TCP endpoint can associate such a packet with an
existing TCP connection. Note that in and of itself guessing this
4-tuple value is not always easy for an attacker. But there are some
applications (e.g. BGP [RFC1771]) that may have a tendency to use
the same set(s) of ports on either endpoint making the odds of
guessing correctly the 4-tuple value much easier. When an attacker
is successful in guessing the 4-tuple value, one of three types of
injection attacks may be waged against a long-lived connection.
RST - Where an attacker injects a reset segment hoping to cause the
connection to be torn down.
SYN - Where an attacker injects a 'SYN' hoping to cause the receiver
to believe the peer has restarted and so tear down the connection
state.
DATA - Where an attacker tries to inject a "DATA" segment to corrupt
the contents of the transmission.
1.1. The RESET attack
Focusing upon the RESET attack, let's examine this attack in more
detail to get an overview as to how it works and how this document
proposes addressing the issue. For this attack the goal is to cause
one of the two endpoints of the connection to incorrectly tear down
the connection state, effectively closing the connection. To do this
the attacker needs to have or guess several pieces of information
(namely):
1) The 4-tuple value containing the IP address and TCP port number of
both ends of the connection. For one side (usually the server)
guessing the port number is a trivial exercise. The client side
may or may not be easy for an attacker to guess depending on a
number of factors most notably the operating system and
application involved.
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2) A sequence number that will be used in the RST. This sequence
number will be a starting point for a series of guesses to attempt
to present a RST segment to a connection endpoint that would be
acceptable to it. Any random value may be used to guess the
initial sequence number.
3) The window size that the two endpoints are using. This value does
NOT have to be the exact window size since a smaller value used in
lieu of the correct one will just cause the attacker to generate
more segments before succeeding in his mischieve. Most modern
operating systems have a default window size which usually is
applied to most connections. Some applications however may change
the window size to better suit the needs of the application. So
often times the attacker, with a fair degree of certainty (knowing
the application that is under attack), can come up with a very
close approximation as to the actual window size in use on the
connection.
After assembling the above set of information the attacker begins
sending spoofed TCP segments with the RST bit set and a guessed TCP
sequence number. Each time a new RST segment is sent, the sequence
number guess is incremented by the window size. Without mitigation
[SITW] has shown that such an attack is much easier to accomplish
then previously assumed. This is because RFC793 [RFC0793] specifies
that any RST within the current window is acceptable.
A slight modification to the TCP state machine can be made which
makes such an attack much more difficult to accomplish. If the
receiver examines the incoming RST segment and validates that the
sequence number exactly matches the sequence number that is next
expected, then such an attack becomes much more difficult then
outlined in [SITW] (i.e. the attacker would have to generate 1/2 the
entire sequence space, on average). This document will discuss the
exact details of what needs to be changed within TCP's state machine
to mitigate all three types of attacks (RST, SYN and DATA).
1.2. Attack probabilities
Every application has control of a number of factors that effect
drastically the probability of a successful spoofing attack. These
factors include such things as:
Window Size - Normally settable by the application but often times
defaulting to 32,768 or 65,535 depending upon the operating system
(Medina05 [Medina05]).
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Server Port number - This value is normally a fixed value so that a
client will know where to connect to the peer at. Thus this value
normally provides no additional protection.
Client Port number - This value may be a random ephemeral value, if
so, this makes a spoofing attack more difficult. There are some
clients, however, that for whatever reason either pick a fixed
client port or have a very guessable one (due to the range of
ephemeral ports available with their operating system or other
application considerations) for such applications a spoofing
attack becomes less difficult.
For the purposes of the rest of this discussion we will assume that
the attacker knows the 4-tuple values. This assumption will help us
focus on the effects of the window size verses the number of TCP
packets an attacker must generate. This assumption will rarely be
true in the real Internet since at least the client port number will
provide us with some amount of randomness (depending on operating
system).
To successfully inject a spoofed packet (RST, SYN or DATA), in the
past, the entire sequence space (i.e. 2^32) was often considered
available to make such an attack unlikely. [SITW] demonstrated that
this assumption was incorrect and that instead of [1/2 X 2^32]
packets (assuming a random distribution) [1/2 X (2^32/window)]
packets is required.
Placing real numbers on this formula we see that for a window size of
32,768, an average of 65,536 packets would need to be transmitted in
order to "spoof" a TCP segment that would be acceptable to a TCP
receiver. A window size of 65,535 reduces this even further to
32,768 packets. With rises in bandwidth to both the home and office,
it can only be expected that the values for default window sizes will
continue to rise in order to better take advantage of the newly
available bandwidth.
As we can see from the above discussion this weakness lowers the bar
quite considerably for likely attacks. But there is one additional
dependency which is the duration of the TCP connection. A TCP
connection that lasts only a few brief packets, as often is the case
for web traffic, would not be subject to such an attack since the
connection may not be established long enough for an attacker to
generate enough traffic. However there is a set of applications such
as BGP [RFC1771] which is judged to be potentially most affected by
this vulnerability. BGP relies on a persistent TCP session between
BGP peers. Resetting the connection can result in medium term
unavailability due to the need to rebuild routing tables and route
flapping see [NISCC] for further details.
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It should be noted that there are existing alternative protection
against the threats that this document addresses. For further
details regarding the attacks and the existing techniques, please
refer to draft [DTASA]
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2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [RFC2119].
TCP terminology should be interpreted as described in RFC793
[RFC0793].
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3. Blind reset attack using the RST bit
3.1. Description of the attack
As described in the introduction, it is possible for an attacker to
generate a "RST" segment that would be acceptable to a TCP receiver
by guessing a "in-window" sequence numbers. In particulary RFC 793,
p37, states the following:
"In all states except SYN-SENT, all reset (RST) segments are
validated by checking their SEQ-fields [sequence numbers]. A reset
is valid if its sequence number is in the window. In the SYN-SENT
state (a RST received in response to an initial SYN), the RST is
acceptable if the ACK field acknowledges the SYN."
3.2. Mitigation
RFC793 [RFC0793] currently requires handling of a segment with the
RST bit when in a synchronized state to be processed as follows:
1) If the RST bit is set and the sequence number is outside the
current receive window (SEG.SEQ <= RCV.NXT || SEG.SEQ > RCV.NXT+
RCV.WND) , silently drop the segment.
2) If the RST bit is set and the sequence number is acceptable i.e.:
(RCV.NXT <= SEG.SEQ < RCV.NXT+RCV.WND) then reset the connection.
Instead, this document proposes the following changes should be made
to provide protection against such an attack.
A) If the RST bit is set and the sequence number is outside the
current receive window, silently drop the segment.
B) If the RST bit is set and the sequence number exactly matches the
next expected sequence number (RCV.NXT), then TCP MUST reset the
connection.
C) If the RST bit is set and the sequence number does not exactly
match the next expected sequence value, yet is within the current
receive window (RCV.NXT < SEG.SEQ < RCV.NXT+RCV.WND), TCP MUST
send an acknowledgment (challenge ACK):
<SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
After sending the challenge ACK, TCP MUST drop the unacceptable
segment and stop processing the incoming packet further.
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The previous text,quoted from RFC793 pg 37 would thus become:
In all states except SYN-SENT, all reset (RST) segments are
validated by checking their SEQ-fields [sequence numbers]. A
reset is valid if its sequence number exactly matches the next
expected sequence number. If the the RST arrives and its sequence
number field does NOT match the next expected sequence number but
is within the window, then the receiver should generate an ACK.
In all other cases where the SEQ-field does not match and is
outside the window, the receiver MUST silently discard the
segment.
In the SYN-SENT state (a RST received in response to an initial
SYN), the RST is acceptable if the ACK field acknowledges the SYN.
In all other cases the receiver MUST silently discard the segment.
With the above slight change to the TCP state machine, it becomes
much harder for an attacker to generate an acceptable reset
segment.
In cases where the remote peer did generate a RST but it fails to
meet the above criteria (the RST sequence number was within the
window but NOT the exact expected sequence number) when the
challenge ACK is sent back, it will no longer have the
transmission control block (TCB) related to this connection and
hence as per RFC793 [RFC0793], the remote peer will send a second
RST back. The sequence number of the second RST is derived from
the acknowledgment number of the incoming ACK. This second RST if
it reaches the sender will cause the connection to be aborted
since the sequence number would now be an exact match.
Note that the above mitigation may cause a non-amplification ACK
exchange. This concern is detailed in
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4. Blind reset attack using the SYN bit
4.1. Description of the attack
Analysis of the reset attack using the RST bit, highlights another
possible avenue for a blind attacker using a similar set of sequence
number guessing. Instead of using the RST bit an attacker can use
the SYN bit with the exact same symantics to tear down a connection.
4.2. Mitigation
RFC793 [RFC0793] currently requires handling of a segment with the
SYN bit set in the synchronized state to be as follows:
1) If the SYN bit is set and the sequence number is outside the
expected window, send an ACK back to the sender.
2) If the SYN bit is set and the sequence number is acceptable i.e.:
(RCV.NXT <= SEG.SEQ <= RCV.NXT+RCV.WND) then send a RST segment to
the sender.
Instead, changing the handling of the SYN in the synchronized state
to the following will mitigate this attack:
A) If the SYN bit is set, irrespective of the sequence number, TCP
MUST send an ACK (also referred to as challenge ACK) to the remote
peer:
<SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
After sending the acknowledgment, TCP MUST drop the unacceptable
segment and stop processing further.
By sending an ACK, the remote end sender is challenged to confirm the
loss of the previous connection and the request to start a new
connection. A legitimate peer, after restart, would not have a TCB
in the synchronized state. Thus when the ACK arrives the peer should
send a RST segment back with the sequence number derived from the ACK
field that caused the RST.
This RST will confirm that the remote TCP endpoint has indeed closed
the previous connection. Upon receipt of a valid RST, the local TCP
endpoint MUST terminate its connection. The local TCP endpoint
should then rely on SYN retransmission from the remote end to re-
establish the connection.
A spoofed SYN, on the other hand, will then have generated an
additional ACK which the peer will discarded as a duplicate ACK and
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will not affect the established connection.
Note that this mitigation does leave one corner case un-handled which
will prevent the reset of a connection when it should be reset (i.e.
it is a non spoofed SYN wherein a peer really did restart). This
problem occurs when the restarting host chooses the exact same IP
address and port number that it was using prior to its restart. By
chance the restarted host must also choose an initial sequence number
of exactly (RCV.NXT - 1) of the remote TCP endpoint that is still in
the established state. Such a case would cause the receiver to
generate a "challenge" ack as described above. But since the ACK
would be within the outgoing connections window the inbound ACK would
be acceptable, and the sender of the SYN will do nothing with the
response ACK. This sequence will continue as the SYN sender
continually times out and retransmits the SYN until such time as the
connection attempt fails.
This corner case is a result of the RFC793 [RFC0793] specification
and is not introduced by the proposed mitigations.
Note that the above mitigation may cause a non-amplification ACK
exchange. This concern is detailed in Section 10
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5. Blind data injection attack
5.1. Description of the attack
A third type of attack is also highlighted by both the RST and SYN
attacks. It is also possible to inject data into a TCP connection by
simply guessing the a sequence number within the current receive
window of the victim. The ACK value of any data segment is
considered valid as long as it does not acknowledge data ahead of the
next segment to send. In other words an ACK value is acceptable if
it is (SND.UNA-(2^31-1)) <= SEG.ACK <= SND.NXT). This means that an
attacker has to guess two ACK values with every guessed sequence
number so that the chances successfully injecting data into a
connection are 1 in ((2^32 / RCV.WND) * 2).
When an attacker successfully injects data into a connection the data
will sit in the receiver's re-assembly queue until the peer sends
enough data to bridge the gap between the RCV.NXT value and the
injected data. At that point one of two things will occur either:
a) An packet war will ensue with the receiver indicating that it has
received data up until RCV.NXT (which includes the attackers data)
and the sender sending an ACK with an acknowledgment number less
than RCV.NXT.
b) The sender will send enough data to the peer which will move
RCV.NXT even further along past the injected data.
Depending upon the TCP implementation in question and the TCP traffic
characteristics at that time, data corruption may result. In case
(a) the connection will eventually be reset by one of the sides
unless the sender produces more data that will transform the ACK war
into case (b). The reset will usually occur via User Time Out (UTO)
(see section 4.2.3.5 of [RFC1122]).
Note that the protections illustrated in this section neither cause
an ACK war nor prevent one from occurring if data is actually
injected into a connection. The ACK war is a product of the attack
itself and cannot be prevented (other than by preventing the data
from being injected).
5.2. Mitigation
An additional input check should be added to any incoming segment.
The ACK value MUST be acceptable only if it is in the range of
((SND.UNA - MAX.SND.WND) <= SEG.ACK <= SND.NXT). MAX.SND.WND is
defined as the largest window that the local receiver has ever
advertised to its peer. This window may be a scaled value i.e. the
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value may be larger than 65,535 bytes (RFC1323 [RFC1323]). This
small check will greatly reduce the vulnerability to an attacker
guessing a valid sequence number since not only must he/she guess the
sequence number in window, but must also guess a proper ACK value
within a scoped range. This mitigation reduces but does not
eliminate the ability to generate false segments. It does however
reduce the probability that invalid data will be injected.
Note that the above mitigation may cause a non-amplification ACK
exchange. This concern is detailed in
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6. ACK throttling
In order to alleviate multiple RSTs/SYNs from triggering multiple
challenge ACKs, an ACK throttling mechanism SHOULD be implemented.
Suggested values are to send no more than 10 challenge ACKs in a 5
second window. These numbers are empirical in nature and have been
obtained from the RST throttling mechanism implemented in some OS's.
These value MUST be tunable by a system administrator to accommodate
different preceived threats.
An alternative mechanism may also be used that does not involve an
additional timer. In such an implementation a sender would only send
X acks between any window advancment. Note that such a limitation
will not require a timer but must be implemented with care to avoid a
deadlock in the face of ack loss.
An implementation SHOULD include a ACK throttling mechanism to be
conservative. Currently there is no known bad behavior that can be
attributed to the lack of ACK throttling, but as a general principle,
if ever invoked, something incorrect is occuring and such a mechanisn
will act as a failsafe that protects both the sender and the network.
An administrator who is more concerned about protecting his bandwidth
and CPU utilization may set smaller ACK thottling values whereas an
administrator who is more interested in faster cleanup of stale
connections (i.e. concerned about excess TCP state) may decide to set
a higher value thus allowing more RST's to be processed in any given
time period.
The time limit SHOULD be tunable to help timeout brute force attacks
faster than a potential legitimate flood of RSTs.
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7. Backward Compatibility and Other considerations
1) All of the proposed mitigation techniques in this document are
totally compatible with existing (RFC793 [RFC0793]) compliant TCP
implementations as this document introduces no new assumptions or
conditions.
2) There is a corner scenario in the above proposed mitigations which
will require more than one round trip time to successfully abort
the connection as per the figure below. This scenario is similar
to the one in which the original RST was lost in the network.
TCP A TCP B
1.a. ESTAB <-- <SEQ=300><ACK=101><CTL=ACK><DATA> <-- ESTAB
b. (delayed) ... <SEQ=400><ACK=101><CTL=ACK><DATA> <-- ESTAB
c. (in flight) ... <SEQ=500><ACK=101><CTL=RST> <-- CLOSED
2. ESTAB --> <SEQ=101><ACK=400><CTL=ACK> --> CLOSED
(ACK for 1.a)
... <SEQ=400><ACK=0><CTL=RST> <-- CLOSED
3. CHALLENGE --> <SEQ=101><ACK=400><CTL=ACK> --> CLOSED
(for 1.c)
... <SEQ=400><ACK=0><CTL=RST> <-- RESPONSE
4.a. ESTAB <-- <SEQ=400><ACK=101><CTL=ACK><DATA> 1.b reaches A
b. ESTAB --> <SEQ=101><ACK=500><CTL=ACK>
c. (in flight) ... <SEQ=500><ACK=0><CTL=RST> <-- CLOSED
5. RESPONSE arrives at A, but dropped since its outside of window.
6. ESTAB <-- <SEQ=500><ACK=0><CTL=RST> 4.c reaches A
7. CLOSED CLOSED
3) For the mitigation to be maximally effective against the
vulnerabilities discussed in this document, both ends of the TCP
connection need to have the fix. Although, having the mitagations
at one end might prevent that end from being exposed to the
attack, the connection is still vulnerable at the other end.
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8. Middlebox considerations
8.1. Middlebox that resend RST's
Consider a middlebox M-B tracking connection between two TCP endhosts
E-A and E-C. If E-C sends a RST with a sequence number that is
within the window but not an exact match to reset the connection and
M-B does not have the fix proposed here, it may clear the connection
and forward the RST to E-A saving an incorrect sequence number. If
E-A does not have the fix it will clear the connection and everything
will be fine. However if E-A does have the proposed fix above, it
will send a challenge ACK to E-C. M-B, being a middlebox, may
intercept this ACK and resend the RST on behalf of E-C with the old
sequence number. This RST, will again, not acceptable and may
trigger a challenge ACK.
This may cause a RST/ACK war to occur. However we believe that if
such a case exists in the Internet the middle box design itself is
flawed. Consider a similar scenario where the RST from M-B to E-A
gets lost, E-A will continue to hold the connection and E-A might
send an ACK an arbitrary time later after the connection state was
destroyed at M-B. For this case, M-B will have to cache the RST for
an arbitrary amount of time till until it is confirmed that the
connection has been cleared at E-A. Further, this is not compliant
to RFC793 which dictates that the sequence number of a RST has to be
derived from the acknowledgment number of the incoming ACK segment.
8.2. Middleboxes that advance sequence numbers
Some middleboxes may compute RST sequence numbers at the higher end
of the acceptable window. The scenario is the same as the earlier
case, but in this case instead of sending the cached RST, the
middlebox (M-B) sends a RST that computes its sequence number as a
sum of the ack field in the ACK and the window advertised by the ACK
that was sent by E-A to challenge the RST as depicted below. The
difference in the sequence numbers between step 1 and 2 below is due
to data lost in the network.
TCP A Middlebox
1. ESTABLISHED <-- <SEQ=500><ACK=100><CTL=RST> <-- CLOSED
2. ESTABLISHED --> <SEQ=100><ACK=300><WND=500><CTL=ACK> --> CLOSED
3. ESTABLISHED <-- <SEQ=800><ACK=100><CTL=RST> <-- CLOSED
4. ESTABLISHED --> <SEQ=100><ACK=300><WND=500><CTL=ACK> --> CLOSED
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5. ESTABLISHED <-- <SEQ=800><ACK=100><CTL=RST> <-- CLOSED
Although the authors are not aware of an implementation that does the
above, it could be mitigated by implementing the ACK throttling
mechanism described earlier.
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9. Interoperability Testing
Interoperability testing was performed among the operating systems
from Juniper Networks, WindRiver Systems, QNX Software and Cisco
Systems. The following topology was used:
+---------+ +---------+
|TCP A |----+------|Victim B |
+---------+ | +---------+
|
+---------+ |
|Attacker |----+
+---------+
In the above topology B is the unit under test. TCP A is a remote
peer and the attacker workstation is used to generate malicious
spoofed packets.
First, an unmodifed stack had the following tests performed upon it.
A TCP connection was brought up between TCP endpoint A and B. The
4-tuple of the connection was manually populated in the brute force
attack script running on the attacker workstation. The script
crafted TCP packets by using the 4-tuple and by generating sequence
numbers that incremented from 0 to the max permissible sequence
number (2^32 -1) in varying increments of window size of 8K to 64K
(the window size agreed by A and B was larger than 8K, but was
ignored to make the test conservative). The time it took to cause
the connection to reset/corrupt was then recorded.
The test was repeated by then generating sequence numbers that were
window (the one agreed between endpoints A and B) size apart. It was
observed that increasing the window size caused the connection to be
reset faster than with a smaller window. Further, it was also
observed that the initial sequence number (ISN) selection also played
a role in how fast a connection was aborted, with ISN selection in
the lower half of the sequence space aborting sooner than an ISN in
the upper half. Results were also found to be influenced by any
active data transfer on the connection.
Active data transfer sometimes caused the connection to be reset
faster and some other times to slow the attack. The window size
selection at the attacker workstation and how it compares to the
actual window size between A and B was also found to be a factor.
The tests were repeated with a stack that did have the fix and as
expected it became difficult as compared to the previous results to
cause the connection to abort.
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[Editors Note: Add time test data gathered at inter-op here!!]
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10. Security Considerations
A reflector attack is possible with the proposed RST/SYN mitigation
techniques. Here an off-path attacker can cause a victim to send an
ACK segment for each spoofed RST/SYN segment that lies within the
current receive window of the victim. This, however, does not cause
any sort of amplification since the attacker must generate a segment
for each one that the victim will generate.
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11. IANA Considerations
This document contains no IANA considerations.
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12. Contributors
Mitesh Dalal and Amol Khare of Cisco Systems came up with the
solution for the RST/SYN attacks. Anantha Ramaiah and Randall
Stewart of Cisco Systems discovered the data injection vulnerability
and together with Patrick Mahan and Peter Lei of Cisco Systems found
solutions for the same. Paul Goyette, Mark Baushke, Frank
Kastenholz, Art Stine and David Wang of Juniper Networks provided the
insight that apart from RSTs, SYNs could also result in formidable
attacks. Shrirang Bage of Cisco Systems, Qing Li and Preety Puri of
Wind River Systems and Xiaodan Tang of QNX Software along with the
folks above helped in ratifying and testing the interoperability of
the suggested solutions.
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13. Acknowledgments
Special thanks to Mark Allman, Ted Faber, Steve Bellovin, Vern
Paxson, Allison Mankin, Sharad Ahlawat, Damir Rajnovic, John Wong and
the tcpm WG members for suggestions and comments. Some of the text
in this document has been derived from
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14. References
14.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
14.2. Informative References
[DTASA] Touch, J., "Defending TCP Against Spoofing Attacks",
draft-touch-tcp-antispoof-00 (work in progress),
July 2004.
[Medina05]
Medina, A., Allman, M., and S. Floyd, "Measuring the
Evolution of Transport Protocols in the Internet. ACM
Computer Communication Review, 35(2), April 2005.
http://www.icir.org/mallman/papers/tcp-evo-ccr05.ps
(figure 6)".
[NISCC] NISCC, "NISCC Vulnerability Advisory 236929 -
Vulnerability Issues in TCP".
[RFC1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
[RFC1323] Jacobson, V., Braden, B., and D. Borman, "TCP Extensions
for High Performance", RFC 1323, May 1992.
[RFC1771] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
(BGP-4)", RFC 1771, March 1995.
[RFC3562] Leech, M., "Key Management Considerations for the TCP MD5
Signature Option", RFC 3562, July 2003.
[SITW] Watson, P., "Slipping in the Window: TCP Reset attacks".
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Authors' Addresses
Randall R. Stewart
Editor
4875 Forest Drive
Suite 200
Columbia, SC 29206
USA
Phone:
Email: rrs@cisco.com
Mitesh Dalal
Editor
170 Tasman Drive
San Jose, CA 95134
USA
Phone: +1-408-853-5257
Email: mdalal@cisco.com
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