Internet Engineering Task Force (IETF) F. Gont
Request for Comments: 6528 SI6 Networks / UTN-FRH
Obsoletes: 1948 S. Bellovin
Updates: 793 Columbia University
Category: Standards Track February 2012
Defending against Sequence Number Attacks
This document specifies an algorithm for the generation of TCP
Initial Sequence Numbers (ISNs), such that the chances of an off-path
attacker guessing the sequence numbers in use by a target connection
are reduced. This document revises (and formally obsoletes) RFC
1948, and takes the ISN generation algorithm originally proposed in
that document to Standards Track, formally updating RFC 793.
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,
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 22. Generation of Initial Sequence Numbers . . . . . . . . . . . . 33. Proposed Initial Sequence Number Generation Algorithm . . . . 44. Security Considerations . . . . . . . . . . . . . . . . . . . 55. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 66. References . . . . . . . . . . . . . . . . . . . . . . . . . . 66.1. Normative References . . . . . . . . . . . . . . . . . . . 66.2. Informative References . . . . . . . . . . . . . . . . . . 7Appendix A. Address-Based Trust-Relationship Exploitation
Attacks . . . . . . . . . . . . . . . . . . . . . . . 10A.1. Blind TCP Connection-Spoofing . . . . . . . . . . . . . . 10Appendix B. Changes from RFC 1948 . . . . . . . . . . . . . . . . 121. Introduction
For a long time, the Internet has experienced a number of off-path
attacks against TCP connections. These attacks have ranged from
trust-relationship exploitation to denial-of-service attacks
[CPNI-TCP]. Discussion of some of these attacks dates back to at
least 1985, when Morris [Morris1985] described a form of attack based
on guessing what sequence numbers TCP [RFC0793] will use for new
connections between two known end-points.
In 1996, RFC 1948 [RFC1948] proposed an algorithm for the selection
of TCP Initial Sequence Numbers (ISNs), such that the chances of an
off-path attacker guessing valid sequence numbers are reduced. With
the aforementioned algorithm, such attacks would remain possible if
and only if the attacker already has the ability to perform "man-in-
This document revises (and formally obsoletes) RFC 1948, and takes
the ISN generation algorithm originally proposed in that document to
Section 2 provides a brief discussion of the requirements for a good
ISN generation algorithm. Section 3 specifies a good ISN selection
algorithm. Appendix A provides a discussion of the trust-
relationship exploitation attacks that originally motivated the
publication of RFC 1948 [RFC1948]. Finally, Appendix B lists the
differences from RFC 1948 to this document.
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 RFC 2119 [RFC2119].
2. Generation of Initial Sequence Numbers
RFC 793 [RFC0793] suggests that the choice of the ISN of a connection
is not arbitrary, but aims to reduce the chances of a stale segment
from being accepted by a new incarnation of a previous connection.
RFC 793 [RFC0793] suggests the use of a global 32-bit ISN generator
that is incremented by 1 roughly every 4 microseconds.
It is interesting to note that, as a matter of fact, protection
against stale segments from a previous incarnation of the connection
is enforced by preventing the creation of a new incarnation of a
previous connection before 2*MSL have passed since a segment
corresponding to the old incarnation was last seen (where "MSL" is
the "Maximum Segment Lifetime" [RFC0793]). This is accomplished by
the TIME-WAIT state and TCP's "quiet time" concept (see Appendix B of
Based on the assumption that ISNs are monotonically increasing across
connections, many stacks (e.g., 4.2BSD-derived) use the ISN of an
incoming SYN segment to perform "heuristics" that enable the creation
of a new incarnation of a connection while the previous incarnation
is still in the TIME-WAIT state (see p. 945 of [Wright1994]). This
avoids an interoperability problem that may arise when a node
establishes connections to a specific TCP end-point at a high rate
Unfortunately, the ISN generator described in [RFC0793] makes it
trivial for an off-path attacker to predict the ISN that a TCP will
use for new connections, thus allowing a variety of attacks against
TCP connections [CPNI-TCP]. One of the possible attacks that takes
advantage of weak sequence numbers was first described in
[Morris1985], and its exploitation was widely publicized about 10
years later [Shimomura1995]. [CERT2001] and [USCERT2001] are
advisories about the security implications of weak ISN generators.
[Zalewski2001] and [Zalewski2002] contain a detailed analysis of ISN
generators, and a survey of the algorithms in use by popular TCP
Simple random selection of the TCP ISNs would mitigate those attacks
that require an attacker to guess valid sequence numbers. However,
it would also break the 4.4BSD "heuristics" to accept a new incoming
connection when there is a previous incarnation of that connection in
the TIME-WAIT state [Silbersack2005].
We can prevent sequence number guessing attacks by giving each
connection -- that is, each four-tuple of (localip, localport,
remoteip, remoteport) -- a separate sequence number space. Within
each space, the ISN is incremented according to [RFC0793]; however,
there is no obvious relationship between the numbering in different
An obvious way to prevent sequence number guessing attacks while not
breaking the 4.4BSD heuristics would be to perform a simple random
selection of TCP ISNs while maintaining state for dead connections
(e.g. changing the TCP state transition diagram so that both end-
points of all connections go to TIME-WAIT state). That would work
but would consume system memory to store the additional state.
Instead, we propose an improvement to the TCP ISN generation
algorithm that does not require TCP to keep state for all recently
3. Proposed Initial Sequence Number Generation Algorithm
TCP SHOULD generate its Initial Sequence Numbers with the expression:
ISN = M + F(localip, localport, remoteip, remoteport, secretkey)
where M is the 4 microsecond timer, and F() is a pseudorandom
function (PRF) of the connection-id. F() MUST NOT be computable from
the outside, or an attacker could still guess at sequence numbers
from the ISN used for some other connection. The PRF could be
implemented as a cryptographic hash of the concatenation of the
connection-id and some secret data; MD5 [RFC1321] would be a good
choice for the hash function.
The result of F() is no more secure than the secret key. If an
attacker is aware of which cryptographic hash function is being used
by the victim (which we should expect), and the attacker can obtain
enough material (i.e., ISNs selected by the victim), the attacker may
simply search the entire secret-key space to find matches. To
protect against this, the secret key should be of a reasonable
length. Key lengths of 128 bits should be adequate. The secret key
can either be a true random number [RFC4086] or some per-host secret.
A possible mechanism for protecting the secret key would be to change
it on occasion. For example, the secret key could be changed
whenever one of the following events occur:
o The system is being bootstrapped (e.g., the secret key could be a
combination of some secret and the boot time of the machine).
o Some predefined/random time has expired.
o The secret key has been used sufficiently often that it should be
regarded as insecure at that point.
Note that changing the secret would change the ISN space used for
reincarnated connections, and thus could cause the 4.4BSD heuristics
to fail; to maintain safety, either dead connection state could be
kept or a quiet time observed for two maximum segment lifetimes
before such a change.
It should be noted that while there have been concerns about the
security properties of MD5 [RFC6151], the algorithm specified in this
document simply aims at reducing the chances of an off-path attacker
guessing the ISN of a new connection, and thus in our threat model it
is not worth the effort for an attacker to try to learn the secret
key. Since MD5 is faster than other "stronger" alternatives, and is
used in virtually all existing implementations of this algorithm, we
consider that use of MD5 in the specified algorithm is acceptable.
However, implementations should consider the trade-offs involved in
using functions with stronger security properties, and employ them if
it is deemed appropriate.
4. Security Considerations
Good sequence numbers are not a replacement for cryptographic
authentication, such as that provided by IPsec [RFC4301] or the TCP
Authentication Option (TCP-AO) [RFC5925]. At best, they are a
If random numbers are used as the sole source of the secret, they
MUST be chosen in accordance with the recommendations given in
A security consideration that should be made about the algorithm
proposed in this document is that it might allow an attacker to count
the number of systems behind a Network Address Translator (NAT)
[RFC3022]. Depending on the ISN generators implemented by each of
the systems behind the NAT, an attacker might be able to count the
number of systems behind a NAT by establishing a number of TCP
connections (using the public address of the NAT) and identifying the
number of different sequence number "spaces". [Gont2009] discusses
how this and other information leakages at NATs could be mitigated.
An eavesdropper who can observe the initial messages for a connection
can determine its sequence number state, and may still be able to
launch sequence number guessing attacks by impersonating that
connection. However, such an eavesdropper can also hijack existing
connections [Joncheray1995], so the incremental threat is not that
high. Still, since the offset between a fake connection and a given
real connection will be more or less constant for the lifetime of the
secret, it is important to ensure that attackers can never capture
such packets. Typical attacks that could disclose them include both
eavesdropping and the variety of routing attacks discussed in
Off-path attacks against TCP connections require the attacker to
guess or know the four-tuple (localip, localport, remoteip,
remoteport) that identifies the target connection. TCP port number
randomization [RFC6056] reduces the chances of an attacker of
guessing such a four-tuple by obfuscating the selection of TCP
ephemeral ports, therefore contributing to the mitigation of such
attacks. [RFC6056] provides advice on the selection of TCP ephemeral
ports, such that the overall protection of TCP connections against
off-path attacks is improved.
[CPNI-TCP] contains a discussion of all the currently known attacks
that require an attacker to know or be able to guess the TCP sequence
numbers in use by the target connection.
Matt Blaze and Jim Ellis contributed some crucial ideas to RFC 1948,
on which this document is based. Frank Kastenholz contributed
constructive comments to that memo.
The authors of this document would like to thank (in chronological
order) Alfred Hoenes, Lloyd Wood, Lars Eggert, Joe Touch, William
Allen Simpson, Tim Shepard, Wesley Eddy, Anantha Ramaiah, and Ben
Campbell for providing valuable comments on draft versions of this
Fernando Gont wishes to thank Jorge Oscar Gont, Nelida Garcia, and
Guillermo Gont for their love and support, and Daniel Bellomo and
Christian O'Flaherty for their support in his Internet engineering
Fernando Gont's attendance to IETF meetings was supported by ISOC's
"Fellowship to the IETF" program.
6.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm",
RFC 1321, April 1992.
[RFC1948] Bellovin, S., "Defending Against Sequence Number
Attacks", RFC 1948, May 1996.
[RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP
Network Address Translator (Traditional NAT)",
RFC 3022, January 2001.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn,
"The Kerberos Network Authentication Service (V5)",
RFC 4120, July 2005.
[RFC4251] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Protocol Architecture", RFC 4251, January 2006.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4954] Siemborski, R. and A. Melnikov, "SMTP Service
Extension for Authentication", RFC 4954, July 2007.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol",
RFC 5321, October 2008.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, June 2010.
[RFC5936] Lewis, E. and A. Hoenes, "DNS Zone Transfer
Protocol (AXFR)", RFC 5936, June 2010.
[RFC6151] Turner, S. and L. Chen, "Updated Security
Considerations for the MD5 Message-Digest and the
HMAC-MD5 Algorithms", RFC 6151, March 2011.
[Shimomura1995] Shimomura, T., "Technical details of the attack
described by Markoff in NYT",
usenet.txt, Message posted in USENET's
comp.security.misc newsgroup, Message-ID:
[Silbersack2005] Silbersack, M., "Improving TCP/IP security through
randomization without sacrificing
interoperability", EuroBSDCon 2005 Conference.
Appendix A. Address-Based Trust-Relationship Exploitation Attacks
This section discusses the trust-relationship exploitation attack
that originally motivated the publication of RFC 1948 [RFC1948]. It
should be noted that while RFC 1948 focused its discussion of
address-based trust-relationship exploitation attacks on Telnet
[RFC0854] and the various UNIX "r" commands, both Telnet and the
various "r" commands have since been largely replaced by secure
counterparts (such as SSH [RFC4251]) for the purpose of remote login
and remote command execution. Nevertheless, address-based trust
relationships are still employed nowadays in some scenarios. For
example, some SMTP [RFC5321] deployments still authenticate their
users by means of their IP addresses, even when more appropriate
authentication mechanisms are available [RFC4954]. Another example
is the authentication of DNS secondary servers [RFC1034] by means of
their IP addresses for allowing DNS zone transfers [RFC5936], or any
other access control mechanism based on IP addresses.
In 1985, Morris [Morris1985] described a form of attack based on
guessing what sequence numbers TCP [RFC0793] will use for new
connections. Briefly, the attacker gags a host trusted by the
target, impersonates the IP address of the trusted host when talking
to the target, and completes the three-way handshake based on its
guess at the next ISN to be used. An ordinary connection to the
target is used to gather sequence number state information. This
entire sequence, coupled with address-based authentication, allows
the attacker to execute commands on the target host.
Clearly, the proper solution for these attacks is cryptographic
authentication [RFC4301] [RFC4120] [RFC4251].
The following subsection provides technical details for the trust-
relationship exploitation attack described by Morris [Morris1985].
A.1. Blind TCP Connection-Spoofing
In order to understand the particular case of sequence number
guessing, one must look at the three-way handshake used in the TCP
open sequence [RFC0793]. Suppose client machine A wants to talk to
rsh server B. It sends the following message:
A->B: SYN, ISNa
That is, it sends a packet with the SYN ("synchronize sequence
number") bit set and an initial sequence number ISNa.
B replies with
B->A: SYN, ISNb, ACK(ISNa)
In addition to sending its own ISN, it acknowledges A's. Note that
the actual numeric value ISNa must appear in the message.
A concludes the handshake by sending
RFC 793 [RFC0793] specifies that the 32-bit counter be incremented by
1 in the low-order position about every 4 microseconds. Instead,
Berkeley-derived kernels traditionally incremented it by a constant
every second, and by another constant for each new connection. Thus,
if you opened a connection to a machine, you knew to a very high
degree of confidence what sequence number it would use for its next
connection. And therein lied the vulnerability.
The attacker X first opens a real connection to its target B -- say,
to the mail port or the TCP echo port. This gives ISNb. It then
impersonates A and sends
Ax->B: SYN, ISNx
where "Ax" denotes a packet sent by X pretending to be A.
B's response to X's original SYN (so to speak)
B->A: SYN, ISNb', ACK(ISNx)
goes to the legitimate A, about which more anon. X never sees that
message but can still send
using the predicted value for ISNb'. If the guess is right -- and
usually it will be, if the sequence numbers are weak -- B's rsh
server thinks it has a legitimate connection with A, when in fact X
is sending the packets. X can't see the output from this session,
but it can execute commands as more or less any user -- and in that
case, the game is over and X has won.
There is a minor difficulty here. If A sees B's message, it will
realize that B is acknowledging something it never sent, and will
send a RST packet in response to tear down the connection. However,
an attacker could send the TCP segments containing the commands to be
executed back-to-back with the segments required to establish the TCP
connection, and thus by the time the connection is reset, the
attacker has already won.
In the past, attackers exploited a common TCP implementation bug
to prevent the connection from being reset (see subsection "A
Common TCP Bug" in [RFC1948]). However, all TCP implementations
that used to implement this bug have been fixed for a long time.
Appendix B. Changes from RFC 1948
o This document is Standards Track (rather than Informational).
o Formal requirements [RFC2119] are specified.
o The discussion of address-based trust-relationship attacks has
been updated and moved to an appendix.
o The subsection entitled "A Common TCP Bug" (describing a common
bug in the BSD TCP implementation) has been removed.
SI6 Networks / UTN-FRH
Evaristo Carriego 2644
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Phone: +54 11 4650 8472
Steven M. Bellovin
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New York, NY 10027
Phone: +1 212 939 7149