TCP Maintenance and Minor A. Ramaiah
Extensions Working Group R. Stewart
Internet-Draft M. Dalal
Intended status: Standards Track Cisco Systems
Expires: July 10, 2008 January 7, 2008
Improving TCP's Robustness to Blind In-Window Attacks
draft-ietf-tcpm-tcpsecure-09.txt
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Abstract
TCP has historically been considered protected against spoofed off-
path 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 longer term connections (e.g. H-323 or
Border Gateway Protocol [RFC4271]) have left modern TCP
implementations more vulnerable to these types of spoofed packet
injection attacks.
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
systematically guessing 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
specifies small modifications to the way TCP handles inbound segments
that can reduce the chances of a successful attack.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Applicability Statement . . . . . . . . . . . . . . . . . 4
1.2. Basic Attack Methodology . . . . . . . . . . . . . . . . . 4
1.3. Attack probabilities . . . . . . . . . . . . . . . . . . . 6
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. Security Considerations . . . . . . . . . . . . . . . . . . . 19
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 21
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 22
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
13.1. Normative References . . . . . . . . . . . . . . . . . . . 23
13.2. Informative References . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25
Intellectual Property and Copyright Statements . . . . . . . . . . 26
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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 today's
Internet. Yet when it was standardized over 20 years ago, the
Internet, was a different place, lacking many of the threats that are
now common. The TCP spoofing attacks, which are seen in the Internet
today, fall into this category.
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). The
targeted TCP endpoint will then associate such a packet with an
existing TCP connection. It needs to be noted that, guessing this
4-tuple value is not always easy for an attacker. But there are some
applications (e.g. BGP [RFC4271]) that 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 RST 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. Applicability Statement
The mitigations presented in this document talks about some known in-
window attacks and the solutions to the same. The mitigations
suggested in this draft SHOULD be implemented in devices where the
TCP connections are most vulnerable to the attacks described in this
document. Some examples of such TCP connections are the ones that
tend to be long-lived where the connection end points can be
determined, in cases where no auxiliary anti-spoofing protection
mechanisms like TCP MD5 [RFC2385] can be deployed. These mitigations
MAY be implemented in other cases.
1.2. Basic Attack Methodology
Focusing upon the RST attack, we examine this attack in more detail
to get an overview as to how it works and how this document addresses
the issue. For this attack the goal is for the attacker to cause one
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of the two endpoints of the connection to incorrectly tear down the
connection state, effectively aborting the connection. One of the
important things to note is that, for the attack to succeed the RST
needs to be in the valid receive window. It also needs to be
emphasized that the receive window is independent of the current
congestion window of the TCP connection. The attacker would try to
forge many RST segments to try to cover the space of possible windows
by putting out a packet in each potential window. 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.
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 mischief. 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. The feasibility of
this methodology (without mitigations) was first shown in [SITW].
This is because [RFC0793] specifies that any RST within the current
window is acceptable. Also [I-D.ietf-tcpm-tcp-antispoof] talks about
the probability of a successful attack with varying window sizes and
bandwidth.
A slight enhancement to the TCP's segment processing rules can be
made which makes such an attack much more difficult to accomplish.
If the receiver examines the incoming RST segment and validates that
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the sequence number exactly matches the sequence number that is next
expected, then such an attack becomes much more difficult than
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 segment
processing rules to mitigate all three types of attacks (RST, SYN and
DATA).
1.3. 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]).
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 versus 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 the 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 * 2^32]
packets (assuming a random distribution) [1/2 * (2^32/window)]
packets is required.
Substituting numbers into 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
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receiver. A window size of 65,535 reduces this even further to
32,768 packets. At today's access bandwidths an attack of that size
is feasible.
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. It also needs to be noted that this attack can be
performed in a distributed fashion in order potentially gain access
to more 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 [RFC4271] 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.
For applications that can use the TCP MD5 option [RFC2385], such as
BGP, that option makes the attacks described in this specification
effectively impossible. However, some applications or
implementations may find that option expensive to implement.
There are alternative protections against the threats that this
document addresses. For further details regarding the attacks and
the existing techniques, please refer to draft
[I-D.ietf-tcpm-tcp-antispoof]
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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]. TCP
terminology should be interpreted as described in [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 "in-window" sequence numbers. In particular [RFC0793],
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
[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 requires that implementations SHOULD implement
the following steps in place of those specified in [RFC0793] (as
listed above).
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>
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After sending the challenge ACK, TCP MUST drop the unacceptable
segment and stop processing the incoming packet further. Further
segments destined to this connection will be processed as normal.
The previous text, quoted from [RFC0793] 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 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 [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.
A valid RST received out-of-order would still generate a challenge
ACK in response. If this RST happens to be a genuine one, the other
end would send an RST with an exact sequence number match which would
cause the connection to be dropped.
Note that the above mitigation may cause a non-amplification ACK
exchange. This concern is discussed in Section 9.
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4. Blind reset attack using the SYN bit
4.1. Description of the attack
The 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 semantics to tear down a connection.
4.2. Mitigation
[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, the handling of the SYN in the synchronized state SHOULD be
performed as follows:
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 discard 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 [RFC0793] specification and is
not introduced by these new requirements.
Note that the above mitigation may cause a non-amplification ACK
exchange. This concern is discussed in Section 9.
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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 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). The (2^31 - 1) in the above
inequality takes into account the fact that comparisons on TCP
sequence and acknowledgement numbers is done using the modulo 32 bit
arithmetic to accommodate the number wraparound. This means that an
attacker has to guess two ACK values with every guessed sequence
number so that the chances of 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 :
a) A 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
All TCP stacks SHOULD implement the following mitigation. TCP stacks
which implement this mitigation MUST add an additional input check to
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any incoming segment. The ACK value is considered acceptable only if
it is in the range of ((SND.UNA - MAX.SND.WND) <= SEG.ACK <=
SND.NXT). All incoming segments whose ACK value doesn't satisfy the
above condition MUST be discarded silently. A new state variable
MAX.SND.WND is defined as the largest window that the local sender
has ever received from its peer. This window may be scaled to a
value larger than 65,535 bytes ([RFC1323]). This small check will
reduce the vulnerability to an attacker guessing a valid sequence
number, since he/she not only must guess the in-window sequence
number, but 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.
Implementations can also chose to hard code the MAX.SND.WND value to
the maximum permissible window size i.e., 65535 in the absence of
window scaling. In presence of the window scaling option the value
becomes (MAX.SND.WND << Snd.Wind.Scale).
This mitigation also helps in improving robustness on accepting
spoofed FIN segments (FIN attacks). Among other things, this
mitigation requires that the attacker also needs to get the
acknowledgment number to fall in the range mentioned above in order
to successfully spoof a FIN segment leading to the closure of the
connection. Thus, this mitigation greatly improves the robustness to
spoofed FIN segments.
Note that the above mitigation may cause a non-amplification ACK
exchange. This concern is discussed in Section 9.
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6. ACK throttling
In order to alleviate multiple RSTs/SYNs from triggering multiple
challenge ACKs, an ACK throttling mechanism is suggested as follows :
1) The system administrator can configure the number of challenge
ACKs that can be sent out in a given interval. For example, in
any 5 second window, no more than 10 challenge ACKs should be
sent.
2) The values for both the time and number of ACKs SHOULD be tunable
by the system administrator to accommodate different perceived
levels of threat and/or system resources.
It should be noted that these numbers are empirical in nature and
have been obtained from the RST throttling mechanisms existing in
some implementations. Also note that no timer is needed to implement
the above mechanism, instead a timestamp and a counter can be used.
An implementation SHOULD include an 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 occurring and such a
mechanism 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 throttling 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
All of the new required mitigation techniques in this document are
totally compatible with existing ([RFC0793]) compliant TCP
implementations as this document introduces no new assumptions or
conditions.
There is a corner scenario in the above 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
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 mitigations 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 connections between two TCP end
hosts 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 recommended in this document, 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 the connection would
get cleared as required. However if E-A does have the required fix,
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 be acceptable and may
trigger a challenge ACK.
The above situation may result in a RST/ACK war. However, we believe
that if such a case exists in the Internet, the middle box design
does not comply to [RFC0793]. [RFC0793] dictates that the sequence
number of a RST has to be derived from the acknowledgment number of
the incoming ACK segment. It is outside the scope of this document
to suggest mitigations to the ill-behaved middleboxes.
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.
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 the
sum of the acknowledgement 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.
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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
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. Security Considerations
These changes to the TCP state machine do NOT protect an
implementation from on-path attacks. It also needs to be emphasized
that while mitigations within this document make it harder for off-
path attackers to inject segments, it does NOT make it impossible.
The only way to fully protect a TCP connection from both on and off
path attacks is by using either IPSEC-AH [RFC4302] or IPSEC-ESP
[RFC4303].
Implementers also should be aware that the attacks detailed in this
specification are not the only attacks available to an off-path
attacker and that the counter measures described herein are not a
comprehensive defense against such attacks.
In particular, administrators should be aware that forged ICMP
messages provide off-path attackers the opportunity to disrupt
connections or degrade service. Such attacks may be subject to even
less scrutiny than the TCP attacks addressed here, especially in
stacks not tuned for hostile environments. It is important to note
that some ICMP messages, validated or not, are key to the proper
function of TCP. Those ICMP messages used to properly set the path
maximum transmission unit are the most obvious example. There are a
variety of ways to choose which, if any, ICMP messages to trust in
the presence of off-path attackers and choosing between them depends
on the assumptions and guarantees developers and administrators can
make about their network. This specification does not attempt to do
more than note this and related issues.
In any case, this RFC details only part of a complete strategy to
prevent off-path attackers from disrupting services that use TCP.
Administrators and implementers should consider the other attack
vectors and determine appropriate mitigations in securing their
systems.
Another notable consideration is that a reflector attack is possible
with the required RST/SYN mitigation techniques. In this attack, 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. It should be noted, however, that this does not cause
any amplification since the attacker must generate a segment for each
one that the victim will generate.
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10. IANA Considerations
This document contains no IANA considerations.
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11. 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.
ACK throttling was introduced to this document by combining the
suggestions from the tcpm working group.
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12. Acknowledgments
Special thanks to Mark Allman, Ted Faber, Steve Bellovin, Vern
Paxson, Allison Mankin, Sharad Ahlawat, Damir Rajnovic, John Wong,
Joe Touch, Alfred Hoenes and other members of the tcpm WG for
suggestions and comments.
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13. References
13.1. 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.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
December 2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
13.2. Informative References
[I-D.ietf-tcpm-tcp-antispoof]
Touch, J., "Defending TCP Against Spoofing Attacks",
draft-ietf-tcpm-tcp-antispoof-06 (work in progress),
February 2007.
[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.
[RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC 2385, August 1998.
[RFC3562] Leech, M., "Key Management Considerations for the TCP MD5
Signature Option", RFC 3562, July 2003.
[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
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[SITW] Watson, P., "Slipping in the Window: TCP Reset attacks,
Presentation at 2004 CanSecWest
http://www.cansecwest.com/archives.html".
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Authors' Addresses
Anantha Ramaiah
Cisco Systems
170 Tasman Drive
San Jose, CA 95134
USA
Phone: +1 (408) 525-6486
Email: ananth@cisco.com
Randall R. Stewart
Cisco Systems
4875 Forest Drive
Suite 200
Columbia, SC 29206
USA
Phone: +1 (803) 345-0369
Email: rrs@cisco.com
Mitesh Dalal
Cisco Systems
170 Tasman Drive
San Jose, CA 95134
USA
Phone: +1 (408) 853-5257
Email: mdalal@cisco.com
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