Internet Engineering Task Force (IETF) A. Ramaiah
Request for Comments: 5961 Cisco
Category: Standards Track R. Stewart
ISSN: 2070-1721 Huawei
August 2010 Improving TCP's Robustness to Blind In-Window Attacks
TCP has historically been considered to be 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 (BGP) [RFC4271]) have left modern TCP
implementations more vulnerable to these types of spoofed packet
Many of these long-term TCP applications tend to have predictable IP
addresses and ports that makes it far easier for the 4-tuple (4-tuple
is the same as the socket pair mentioned in RFC 793) to be guessed.
Having guessed the 4-tuple correctly, an attacker can inject a TCP
segment with the RST bit set, the SYN bit set or data 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 abort or cause data corruption. This document
specifies small modifications to the way TCP handles inbound segments
that can reduce the chances of a successful attack.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction ....................................................31.1. Applicability Statement ....................................31.2. Basic Attack Methodology ...................................41.3. Attack probabilities .......................................52. Terminology .....................................................73. Blind Reset Attack Using the RST Bit ............................73.1. Description of the Attack ..................................73.2. Mitigation .................................................74. Blind Reset Attack Using the SYN Bit ............................94.1. Description of the Attack ..................................94.2. Mitigation .................................................95. Blind Data Injection Attack ....................................105.1. Description of the Attack .................................105.2. Mitigation ................................................116. Suggested Mitigation Strengths .................................127. ACK Throttling .................................................128. Backward Compatibility and Other Considerations ................139. Middlebox Considerations .......................................149.1. Middlebox That Resend RSTs ................................149.2. Middleboxes That Advance Sequence Numbers .................159.3. Middleboxes That Drop the Challenge ACK ...................1510. Security Considerations .......................................1611. Contributors ..................................................1712. Acknowledgments ...............................................1713. References ....................................................1713.1. Normative References .....................................1713.2. Informative References ...................................17
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 off-path 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 (referred to as a 4-tuple value in this
document). 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 correctly guessing 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
RST - Where an attacker injects a RST segment hoping to cause the
connection to be torn down. "RST segment" here refers to a TCP
segment with the RST bit set.
SYN - Where an attacker injects a SYN hoping to cause the receiver
to believe the peer has restarted and therefore tear down the
connection state. "SYN segment" here refers to a TCP segment with
SYN bit set.
DATA - Where an attacker tries to inject a DATA segment to corrupt
the contents of the transmission. "DATA segment" here refers to
any TCP segment containing data.
1.1. Applicability Statement
This document talks about some known in-window attacks and suitable
defenses against these. The mitigations suggested in this document
SHOULD be implemented in devices that regularly need to maintain TCP
connections of the kind most vulnerable to the attacks described in
this document. Examples of such TCP connections are the ones that
tend to be long-lived and where the connection endpoints 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 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
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
starting 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 that 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
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, [RFC4953] talks about the probability of
a successful attack with varying window sizes and bandwidth.
A slight enhancement to 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 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,
1.3. Attack probabilities
Every application has control of a number of factors that drastically
affect 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
(see Figure 6 of [Medina05]).
Server Port number - This value is normally a fixed value so that a
client will know where to connect to the peer. 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
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 are required. In other words, the mean number of tries
needed to inject a RST segment is (2^31/window) rather than the 2^31
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
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
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 that 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], that 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 term-
medium 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 [RFC4953]. It also needs to
be emphasized that, as suggested in [TSVWG-PORT] and [RFC1948], port
randomization and initial sequence number (ISN) randomization would
help improve the robustness of the TCP connection against off-path
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].
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], page
37, 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.
[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, implementations SHOULD implement the following steps in
place of those specified in [RFC0793] (as listed above).
1) If the RST bit is set and the sequence number is outside the
current receive window, silently drop the segment.
2) If the RST bit is set and the sequence number exactly matches the
next expected sequence number (RCV.NXT), then TCP MUST reset the
3) 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):
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 modified RST segment processing 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 that 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 10.
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.
[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
Instead, the handling of the SYN in the synchronized state SHOULD be
performed as follows:
1) 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
After sending the acknowledgment, TCP MUST drop the unacceptable
segment and stop processing further.
By sending an ACK, the remote peer 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 peer 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 that the peer will discard as a duplicate ACK and 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 peer 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 10.
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 acknowledgment 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 ( 1/2 (2^32 / RCV.WND) * 2). Thus, the mean
number of tries needed to inject data successfully is
1/2 (2*2^32/RWND) = 2^32/RCV.WND.
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:
1) A packet war will ensue with the receiver indicating that it has
received data up until RCV.NXT (which includes the attacker's
data) and the sender sending an ACK with an acknowledgment number
less than RCV.NXT.
2) The sender will send enough data to the peer that 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 22.214.171.124 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).
All TCP stacks MAY implement the following mitigation. TCP stacks
that implement this mitigation MUST add an additional input check to
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 and an ACK sent back. It needs to
be noted that RFC 793 on page 72 (fifth check) says: "If the ACK is a
duplicate (SEG.ACK < SND.UNA), it can be ignored. If the ACK
acknowledges something not yet sent (SEG.ACK > SND.NXT) then send an
ACK, drop the segment, and return". The "ignored" above implies that
the processing of the incoming data segment continues, which means
the ACK value is treated as acceptable. This mitigation makes the
ACK check more stringent since any ACK < SND.UNA wouldn't be
accepted, instead only ACKs that are in the range ((SND.UNA -
MAX.SND.WND) <= SEG.ACK <= SND.NXT) get through.
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, not only one 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 the 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 10.
6. Suggested Mitigation Strengths
As described in the above sections, recommendation levels for RST,
SYN, and DATA are tagged as SHOULD, SHOULD, and MAY, respectively.
The reason that DATA mitigation is tagged as MAY, even though it
increased the TCP robustness in general is because, the DATA
injection is perceived to be more difficult (twice as unlikely) when
compared to RST and SYN counterparts. However, it needs to be noted
that all the suggested mitigations improve TCP's robustness in
general and hence the choice of implementing some or all mitigations
recommended in the document is purely left to the implementer.
7. 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
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
An implementation SHOULD include an ACK throttling mechanism to be
conservative. While we have not encountered a case where the lack of
ACK throttling can be exploited, as a fail-safe mechanism we
recommend its use. An implementation may take an excessive number of
invocations of the throttling mechanism as an indication that network
conditions are unusual or hostile.
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.
8. 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
There is a corner scenario in the above mitigations that 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
... <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.
9. Middlebox Considerations
9.1. Middlebox That Resend RSTs
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 middlebox is
generating packets a conformant TCP endpoint would not generate.
[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 until it is confirmed that the connection has been cleared at
9.2. 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 acknowledgment 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
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.
9.3. Middleboxes That Drop the Challenge ACK
It also needs to be noted that, some middleboxes (Firewalls/NATs)
that don't have the fix recommended in the document, may drop the
challenge ACK. This can happen because, the original RST segment
that was in window had already cleared the flow state pertaining to
the TCP connection in the middlebox. In such cases, the end hosts
that have implemented the RST mitigation described in this document,
will have the TCP connection left open. This is a corner case and
can go away if the middlebox is conformant with the changes proposed
in this document.
10. 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 Authentication Header (AH)
[RFC4302] or IPsec Encapsulating Security Payload (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. Unless implementers address
spoofed ICMP messages [RFC5927], the mitigations specified in this
document may not provide the desired protection level.
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
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.
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.
Special thanks to Mark Allman, Ted Faber, Steve Bellovin, Vern
Paxson, Allison Mankin, Sharad Ahlawat, Damir Rajnovic, John Wong,
Joe Touch, Alfred Hoenes, Andre Oppermann, Fernando Gont, Sandra
Murphy, Brian Carpenter, Cullen Jennings, and other members of the
tcpm WG for suggestions and comments. ACK throttling was introduced
to this document by combining the suggestions from the tcpm working
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.
13.2. Informative References
[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,
[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.
[RFC1948] Bellovin, S., "Defending Against Sequence Number
Attacks", RFC 1948, May 1996.
[RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP
MD5 Signature Option", RFC 2385, August 1998.
[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
[RFC4953] Touch, J., "Defending TCP Against Spoofing Attacks",
RFC 4953, July 2007.
[RFC5927] Gont, F., "ICMP Attacks against TCP", RFC 5927,
[SITW] Watson, P., "Slipping in the Window: TCP Reset
attacks", Presentation at 2004 CanSecWest,
[TSVWG-PORT] Larsen, M. and F. Gont, "Transport Protocol Port
Randomization Recommendations", Work in Progress,