draft-ietf-tcpm-rfc1948bis-02.txt   rfc6528.txt 
TCP Maintenance and Minor Extensions F. Gont Internet Engineering Task Force (IETF) F. Gont
(tcpm) SI6 Networks / UTN-FRH Request for Comments: 6528 SI6 Networks / UTN-FRH
Internet-Draft S. Bellovin Obsoletes: 1948 S. Bellovin
Obsoletes: 1948 (if approved) Columbia University Updates: 793 Columbia University
Updates: 793 (if approved) December 16, 2011 Category: Standards Track February 2012
Intended status: Standards Track ISSN: 2070-1721
Expires: June 18, 2012
Defending Against Sequence Number Attacks Defending against Sequence Number Attacks
draft-ietf-tcpm-rfc1948bis-02.txt
Abstract Abstract
This document specifies an algorithm for the generation of TCP This document specifies an algorithm for the generation of TCP
Initial Sequence Numbers (ISNs), such that the chances of an off-path Initial Sequence Numbers (ISNs), such that the chances of an off-path
attacker guessing the sequence numbers in use by a target connection attacker guessing the sequence numbers in use by a target connection
are reduced. This document revises (and formally obsoletes) RFC are reduced. This document revises (and formally obsoletes) RFC
1948, and takes the ISN generation algorithm originally proposed in 1948, and takes the ISN generation algorithm originally proposed in
that document to Standards Track, formally updating RFC 793. that document to Standards Track, formally updating RFC 793.
Status of this Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering This is an Internet Standards Track document.
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months This document is a product of the Internet Engineering Task Force
and may be updated, replaced, or obsoleted by other documents at any (IETF). It represents the consensus of the IETF community. It has
time. It is inappropriate to use Internet-Drafts as reference received public review and has been approved for publication by the
material or to cite them other than as "work in progress." Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
This Internet-Draft will expire on June 18, 2012. Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6528.
Copyright Notice Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Generation of Initial Sequence Numbers . . . . . . . . . . . . 3 2. Generation of Initial Sequence Numbers . . . . . . . . . . . . 3
3. Proposed Initial Sequence Number generation algorithm . . . . 4 3. Proposed Initial Sequence Number Generation Algorithm . . . . 4
4. Security Considerations . . . . . . . . . . . . . . . . . . . 6 4. Security Considerations . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7 6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7 6.1. Normative References . . . . . . . . . . . . . . . . . . . 6
7.1. Normative References . . . . . . . . . . . . . . . . . . . 7 6.2. Informative References . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . . 8 Appendix A. Address-Based Trust-Relationship Exploitation
Appendix A. Address-based trust relationship exploitation Attacks . . . . . . . . . . . . . . . . . . . . . . . 10
attacks . . . . . . . . . . . . . . . . . . . . . . . 10 A.1. Blind TCP Connection-Spoofing . . . . . . . . . . . . . . 10
A.1. Blind TCP connection-spoofing . . . . . . . . . . . . . . 10
Appendix B. Changes from RFC 1948 . . . . . . . . . . . . . . . . 12 Appendix B. Changes from RFC 1948 . . . . . . . . . . . . . . . . 12
Appendix C. Changes from previous versions of the document
(this section should be removed by the RFC Editor
before publication of this document as an RFC) . . . 12
C.1. Changes from draft-ietf-tcpm-rfc1948bis-00 . . . . . . . . 12
C.2. Changes from draft-gont-tcpm-rfc1948bis-00 . . . . . . . . 12
C.3. Changes from RFC 1948 . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction 1. Introduction
For a long time, the Internet has experienced a number of off-path For a long time, the Internet has experienced a number of off-path
attacks against TCP connections. These attacks have ranged from attacks against TCP connections. These attacks have ranged from
trust relationships exploitation to Denial of Service attacks trust-relationship exploitation to denial-of-service attacks
[CPNI-TCP]. Discussion of some of these attacks dates back to at [CPNI-TCP]. Discussion of some of these attacks dates back to at
least 1985, when Morris [Morris1985] described a form of attack based least 1985, when Morris [Morris1985] described a form of attack based
on guessing what sequence numbers TCP [RFC0793] will use for new on guessing what sequence numbers TCP [RFC0793] will use for new
connections between two known end-points. connections between two known end-points.
In 1996, RFC 1948 [RFC1948] proposed an algorithm for the selection In 1996, RFC 1948 [RFC1948] proposed an algorithm for the selection
of TCP Initial Sequence Numbers (ISNs), such that the chances of an of TCP Initial Sequence Numbers (ISNs), such that the chances of an
off-path attacker guessing valid sequence numbers are reduced. With off-path attacker guessing valid sequence numbers are reduced. With
the aforementioned algorithm, such attacks would remain possible if the aforementioned algorithm, such attacks would remain possible if
and only if the attacker already has the ability to perform "man in and only if the attacker already has the ability to perform "man-in-
the middle" attacks. the-middle" attacks.
This document revises (and formally obsoletes) RFC 1948, and takes This document revises (and formally obsoletes) RFC 1948, and takes
the ISN generation algorithm originally proposed in that document to the ISN generation algorithm originally proposed in that document to
Standards Track. Standards Track.
Section 2 provides a brief discussion of the requirements for a good Section 2 provides a brief discussion of the requirements for a good
ISN generation algorithm. Section 3 specifies a good ISN selection ISN generation algorithm. Section 3 specifies a good ISN selection
algorithm. Finally, Appendix A provides a discussion of the trust- algorithm. Appendix A provides a discussion of the trust-
relationship exploitation attacks that originally motivated the relationship exploitation attacks that originally motivated the
publication of RFC 1948 [RFC1948]. 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", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
2. Generation of Initial Sequence Numbers 2. Generation of Initial Sequence Numbers
RFC 793 [RFC0793] suggests that the choice of the ISN of a connection 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 is not arbitrary, but aims to reduce the chances of a stale segment
from being accepted by a new incarnation of a previous connection. from being accepted by a new incarnation of a previous connection.
RFC 793 [RFC0793] suggests the use of a global 32-bit ISN generator RFC 793 [RFC0793] suggests the use of a global 32-bit ISN generator
that is incremented by 1 roughly every 4 microseconds. that is incremented by 1 roughly every 4 microseconds.
It is interesting to note that, as a matter of fact, protection It is interesting to note that, as a matter of fact, protection
against stale segments from a previous incarnation of the connection against stale segments from a previous incarnation of the connection
is enforced by preventing the creation of a new incarnation of a is enforced by preventing the creation of a new incarnation of a
previous connection before 2*MSL have passed since a segment previous connection before 2*MSL have passed since a segment
corresponding to the old incarnation was last seen. This is corresponding to the old incarnation was last seen (where "MSL" is
accomplished by the TIME-WAIT state, and TCP's "quiet time" concept the "Maximum Segment Lifetime" [RFC0793]). This is accomplished by
(see Appendix B of [RFC1323]). the TIME-WAIT state and TCP's "quiet time" concept (see Appendix B of
[RFC1323]).
Based on the assumption that ISNs are monotonically-increasing across Based on the assumption that ISNs are monotonically increasing across
connections, many stacks (e.g., 4.2BSD-derived) use the ISN of an connections, many stacks (e.g., 4.2BSD-derived) use the ISN of an
incoming SYN segment to perform "heuristics" that enable the creation incoming SYN segment to perform "heuristics" that enable the creation
of a new incarnation of a connection while the previous incarnation of a new incarnation of a connection while the previous incarnation
is still in the TIME-WAIT state (see pp. 945 of [Wright1994]). This is still in the TIME-WAIT state (see p. 945 of [Wright1994]). This
avoids an interoperability problem that may arise when a node avoids an interoperability problem that may arise when a node
establishes connections to a specific TCP end-point at a high rate establishes connections to a specific TCP end-point at a high rate
[Silbersack2005]. [Silbersack2005].
Unfortunately, the ISN generator described in [RFC0793] makes it Unfortunately, the ISN generator described in [RFC0793] makes it
trivial for an off-path attacker to predict the ISN that a TCP will 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 use for new connections, thus allowing a variety of attacks against
TCP connections [CPNI-TCP]. One of the possible attacks that takes TCP connections [CPNI-TCP]. One of the possible attacks that takes
advantage of weak sequence numbers was first described in advantage of weak sequence numbers was first described in
[Morris1985], and its exploitation was widely publicized about 10 [Morris1985], and its exploitation was widely publicized about 10
skipping to change at page 4, line 33 skipping to change at page 3, line 50
generators, and a survey of the algorithms in use by popular TCP generators, and a survey of the algorithms in use by popular TCP
implementations. implementations.
Simple random selection of the TCP ISNs would mitigate those attacks Simple random selection of the TCP ISNs would mitigate those attacks
that require an attacker to guess valid sequence numbers. However, that require an attacker to guess valid sequence numbers. However,
it would also break the 4.4BSD "heuristics" to accept a new incoming it would also break the 4.4BSD "heuristics" to accept a new incoming
connection when there is a previous incarnation of that connection in connection when there is a previous incarnation of that connection in
the TIME-WAIT state [Silbersack2005]. the TIME-WAIT state [Silbersack2005].
We can prevent sequence number guessing attacks by giving each We can prevent sequence number guessing attacks by giving each
connection -- that is, each 4-tuple of (localip, localport, remoteip, connection -- that is, each four-tuple of (localip, localport,
remoteport) -- a separate sequence number space. Within each space, remoteip, remoteport) -- a separate sequence number space. Within
the ISN is incremented according to [RFC0793]; however, there is no each space, the ISN is incremented according to [RFC0793]; however,
obvious relationship between the numbering in different spaces. there is no obvious relationship between the numbering in different
spaces.
An obvious way to prevent sequence number guessing attacks while not An obvious way to prevent sequence number guessing attacks while not
breaking the 4.4BSD heuristics would be to maintain state for dead breaking the 4.4BSD heuristics would be to perform a simple random
connections, and the easiest way to do that would be to change the selection of TCP ISNs while maintaining state for dead connections
TCP state transition diagram so that both end-points of all (e.g. changing the TCP state transition diagram so that both end-
connections go to TIME-WAIT state. That would work, but would points of all connections go to TIME-WAIT state). That would work
consume system memory to store the additional state. Instead, we but would consume system memory to store the additional state.
propose an improvement to the TCP ISN generation algorithm, that does Instead, we propose an improvement to the TCP ISN generation
not require TCP to keep state for all recently-terminated algorithm that does not require TCP to keep state for all recently
connections. terminated connections.
3. Proposed Initial Sequence Number generation algorithm 3. Proposed Initial Sequence Number Generation Algorithm
TCP SHOULD generate its Initial Sequence Numbers with the expression: TCP SHOULD generate its Initial Sequence Numbers with the expression:
ISN = M + F(localip, localport, remoteip, remoteport) ISN = M + F(localip, localport, remoteip, remoteport, secretkey)
where M is the 4 microsecond timer, and F() is a pseudorandom where M is the 4 microsecond timer, and F() is a pseudorandom
function (PRF) of the connection-id. F() MUST NOT be computable from function (PRF) of the connection-id. F() MUST NOT be computable from
the outside, or an attacker could still guess at sequence numbers the outside, or an attacker could still guess at sequence numbers
from the ISN used for some other connection. The PRF could be from the ISN used for some other connection. The PRF could be
implemented as a cryptographic hash of the concatenation of the implemented as a cryptographic hash of the concatenation of the
connection-id and some secret data; MD5 [RFC1321] would be a good connection-id and some secret data; MD5 [RFC1321] would be a good
choice for the hash function. choice for the hash function.
The result of F() is no more secure than the secret key. If an 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 attacker is aware of which cryptographic hash function is being used
by the victim (which we should expect), and the attacker can obtain by the victim (which we should expect), and the attacker can obtain
enough material (i.e., ISNs selected by the victim), the attacker may enough material (i.e., ISNs selected by the victim), the attacker may
simply search the entire secret-key space to find matches. To simply search the entire secret-key space to find matches. To
protect against this, the secret key should be of a reasonable protect against this, the secret key should be of a reasonable
length. Key lengths of 128 bits should be adequate. The secret key length. Key lengths of 128 bits should be adequate. The secret key
can either be a true random number [RFC4086], or some per-host can either be a true random number [RFC4086] or some per-host secret.
secret. A possible mechanism for protecting the secret key would be A possible mechanism for protecting the secret key would be to change
to change it on occasion. For example, the secret key could be it on occasion. For example, the secret key could be changed
changed whenever one of the following events occur: whenever one of the following events occur:
o The system is being bootstrapped (e.g., the secret key could be a 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). combination of some secret and the boot time of the machine).
o Some predefined/random time has expired. o Some predefined/random time has expired.
o The secret key has been used sufficiently often that it should be o The secret key has been used sufficiently often that it should be
regarded as insecure now. regarded as insecure at that point.
Note that changing the secret would change the ISN space used for Note that changing the secret would change the ISN space used for
reincarnated connections, and thus could lead to the 4.4BSD reincarnated connections, and thus could cause the 4.4BSD heuristics
heuristics to fail; to maintain safety, either dead connection state to fail; to maintain safety, either dead connection state could be
could be kept or a quiet time observed for two maximum segment kept or a quiet time observed for two maximum segment lifetimes
lifetimes before such a change. before such a change.
It should be noted that while there have been concerns about the It should be noted that while there have been concerns about the
security properties of MD5 [RFC6151], the algorithm specified in this security properties of MD5 [RFC6151], the algorithm specified in this
document simply aims at reducing the chances of an off-path attacker 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 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 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 key. Since MD5 is faster than other "stronger" alternatives, and is
used in virtually all existing implementations of this algorithm, we used in virtually all existing implementations of this algorithm, we
consider that use of MD5 in the specified algorithm is acceptable. consider that use of MD5 in the specified algorithm is acceptable.
However, implementations should consider the trade-offs involved in However, implementations should consider the trade-offs involved in
using functions with stronger security properties, and employ them if using functions with stronger security properties, and employ them if
it is deemed appropriate. it is deemed appropriate.
4. Security Considerations 4. Security Considerations
Good sequence numbers are not a replacement for cryptographic Good sequence numbers are not a replacement for cryptographic
authentication, such as that provided by IPsec [RFC4301] or TCP-AO authentication, such as that provided by IPsec [RFC4301] or the TCP
[RFC5925]. At best, they are a palliative measure. Authentication Option (TCP-AO) [RFC5925]. At best, they are a
palliative measure.
If random numbers are used as the sole source of the secret, they If random numbers are used as the sole source of the secret, they
MUST be chosen in accordance with the recommendations given in MUST be chosen in accordance with the recommendations given in
[RFC4086]. [RFC4086].
A security consideration that should be made about the algorithm A security consideration that should be made about the algorithm
proposed in this document is that it might allow an attacker to count proposed in this document is that it might allow an attacker to count
the number of systems behind a Network Address Translator (NAT) the number of systems behind a Network Address Translator (NAT)
[RFC3022]. Depending on the ISN generators implemented by each of [RFC3022]. Depending on the ISN generators implemented by each of
the systems behind the NAT, an attacker might be able to count the 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 number of systems behind a NAT by establishing a number of TCP
connections (using the public address of the NAT) and identifying the connections (using the public address of the NAT) and identifying the
number of different sequence number "spaces". number of different sequence number "spaces". [Gont2009] discusses
[I-D.gont-behave-nat-security] discusses how this and other how this and other information leakages at NATs could be mitigated.
information leakages at NATs could be mitigated.
An eavesdropper who can observe the initial messages for a connection An eavesdropper who can observe the initial messages for a connection
can determine its sequence number state, and may still be able to can determine its sequence number state, and may still be able to
launch sequence number guessing attacks by impersonating that launch sequence number guessing attacks by impersonating that
connection. However, such an eavesdropper can also hijack existing connection. However, such an eavesdropper can also hijack existing
connections [Joncheray1995], so the incremental threat is not that connections [Joncheray1995], so the incremental threat is not that
high. Still, since the offset between a fake connection and a given 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 real connection will be more or less constant for the lifetime of the
secret, it is important to ensure that attackers can never capture secret, it is important to ensure that attackers can never capture
such packets. Typical attacks that could disclose them include both such packets. Typical attacks that could disclose them include both
eavesdropping and the variety of routing attacks discussed in eavesdropping and the variety of routing attacks discussed in
[Bellovin1989]. [Bellovin1989].
Off-path attacks against TCP connections require the attacker to Off-path attacks against TCP connections require the attacker to
guess or know the four-tuple (localip, localport, remoteip, guess or know the four-tuple (localip, localport, remoteip,
remoteport) that identifies the target connection. TCP port number remoteport) that identifies the target connection. TCP port number
randomization [RFC6056] reduces the chances of an attacker of randomization [RFC6056] reduces the chances of an attacker of
guessing such four-tuple by obfuscating the selection of TCP guessing such a four-tuple by obfuscating the selection of TCP
ephemeral ports, therefore contributing to the mitigation of such ephemeral ports, therefore contributing to the mitigation of such
attacks. [RFC6056] provides advice on the selection of TCP ephemeral attacks. [RFC6056] provides advice on the selection of TCP ephemeral
ports, such that the overall protection of TCP connections against ports, such that the overall protection of TCP connections against
off-path attacks is improved. off-path attacks is improved.
[CPNI-TCP] contains a discussion of all the currently-known attacks [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 that require an attacker to know or be able to guess the TCP sequence
numbers in use by the target connection. numbers in use by the target connection.
5. IANA Considerations 5. Acknowledgements
This document has no actions for IANA.
6. Acknowledgements
Matt Blaze and Jim Ellis contributed some crucial ideas to RFC 1948, Matt Blaze and Jim Ellis contributed some crucial ideas to RFC 1948,
on which this document is based. Frank Kastenholz contributed on which this document is based. Frank Kastenholz contributed
constructive comments to that memo. constructive comments to that memo.
The authors of this document would like to thank (in chronological The authors of this document would like to thank (in chronological
order) Alfred Hoenes, Lloyd Wood, Lars Eggert, Joe Touch, William order) Alfred Hoenes, Lloyd Wood, Lars Eggert, Joe Touch, William
Allen Simpson, Tim Shepard, Wesley Eddy, Anantha Ramaiah, and Ben Allen Simpson, Tim Shepard, Wesley Eddy, Anantha Ramaiah, and Ben
Campbell, for providing valuable comments on earlier versions of this Campbell for providing valuable comments on draft versions of this
document. document.
Fernando Gont would like to thank the United Kingdom's Centre for the Fernando Gont wishes to thank Jorge Oscar Gont, Nelida Garcia, and
Protection of National Infrastructure (UK CPNI) for their continued Guillermo Gont for their love and support, and Daniel Bellomo and
support. Christian O'Flaherty for their support in his Internet engineering
activities.
7. References Fernando Gont's attendance to IETF meetings was supported by ISOC's
"Fellowship to the IETF" program.
7.1. Normative References 6. References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, 6.1. Normative References
RFC 793, September 1981.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, [RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
April 1992. RFC 793, September 1981.
[RFC1323] Jacobson, V., Braden, B., and D. Borman, "TCP Extensions [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm",
for High Performance", RFC 1323, May 1992. RFC 1321, April 1992.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC1323] Jacobson, V., Braden, B., and D. Borman, "TCP
Requirement Levels", BCP 14, RFC 2119, March 1997. Extensions for High Performance", RFC 1323,
May 1992.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirements for Security", BCP 106, RFC 4086, June 2005. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC6056] Larsen, M. and F. Gont, "Recommendations for Transport- [RFC4086] Eastlake, D., Schiller, J., and S. Crocker,
Protocol Port Randomization", BCP 156, RFC 6056, "Randomness Requirements for Security", BCP 106,
January 2011. RFC 4086, June 2005.
7.2. Informative References [RFC6056] Larsen, M. and F. Gont, "Recommendations for
Transport-Protocol Port Randomization", BCP 156,
RFC 6056, January 2011.
[Bellovin1989] 6.2. Informative References
Morris, R., "Security Problems in the TCP/IP Protocol
Suite", Computer Communications Review, vol. 19, no. 2,
pp. 32-48, 1989.
[CERT2001] [Bellovin1989] Morris, R., "Security Problems in the TCP/IP
CERT, "CERT Advisory CA-2001-09: Statistical Weaknesses in Protocol Suite", Computer Communications Review,
TCP/IP Initial Sequence Numbers", vol. 19, no. 2, pp. 32-48, 1989.
http://www.cert.org/advisories/CA-2001-09.html, 2001.
[CPNI-TCP] [CERT2001] CERT, "CERT Advisory CA-2001-09: Statistical
CPNI, "Security Assessment of the Transmission Control Weaknesses in TCP/IP Initial Sequence Numbers",
Protocol (TCP)", http://www.cpni.gov.uk/Docs/ http://www.cert.org/advisories/CA-2001-09.html,
tn-03-09-security-assessment-TCP.pdf, 2009. 2001.
[I-D.gont-behave-nat-security] [CPNI-TCP] CPNI, "Security Assessment of the Transmission
Gont, F. and P. Srisuresh, "Security implications of Control Protocol (TCP)", http://www.gont.com.ar/
Network Address Translators (NATs)", papers/tn-03-09-security-assessment-TCP.pdf, 2009.
draft-gont-behave-nat-security-03 (work in progress),
October 2009.
[Joncheray1995] [Gont2009] Gont, F. and P. Srisuresh, "Security implications
Joncheray, L., "A Simple Active Attack Against TCP", Proc. of Network Address Translators (NATs)", Work
Fifth Usenix UNIX Security Symposium, 1995. in Progress, October 2009.
[Morris1985] [Joncheray1995] Joncheray, L., "A Simple Active Attack Against
Morris, R., "A Weakness in the 4.2BSD UNIX TCP/IP TCP", Proc. Fifth Usenix UNIX Security Symposium,
Software", CSTR 117, AT&T Bell Laboratories, Murray Hill, 1995.
NJ, 1985.
[RFC0854] Postel, J. and J. Reynolds, "Telnet Protocol [Morris1985] Morris, R., "A Weakness in the 4.2BSD UNIX TCP/IP
Specification", STD 8, RFC 854, May 1983. Software", CSTR 117, AT&T Bell Laboratories, Murray
Hill, NJ, 1985.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities", [RFC0854] Postel, J. and J. Reynolds, "Telnet Protocol
STD 13, RFC 1034, November 1987. Specification", STD 8, RFC 854, May 1983.
[RFC1948] Bellovin, S., "Defending Against Sequence Number Attacks", [RFC1034] Mockapetris, P., "Domain names - concepts and
RFC 1948, May 1996. facilities", STD 13, RFC 1034, November 1987.
[RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network [RFC1948] Bellovin, S., "Defending Against Sequence Number
Address Translator (Traditional NAT)", RFC 3022, Attacks", RFC 1948, May 1996.
January 2001.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The [RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP
Kerberos Network Authentication Service (V5)", RFC 4120, Network Address Translator (Traditional NAT)",
July 2005. RFC 3022, January 2001.
[RFC4251] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) [RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn,
Protocol Architecture", RFC 4251, January 2006. "The Kerberos Network Authentication Service (V5)",
RFC 4120, July 2005.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the [RFC4251] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Internet Protocol", RFC 4301, December 2005. Protocol Architecture", RFC 4251, January 2006.
[RFC4954] Siemborski, R. and A. Melnikov, "SMTP Service Extension [RFC4301] Kent, S. and K. Seo, "Security Architecture for the
for Authentication", RFC 4954, July 2007. Internet Protocol", RFC 4301, December 2005.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, [RFC4954] Siemborski, R. and A. Melnikov, "SMTP Service
October 2008. Extension for Authentication", RFC 4954, July 2007.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP [RFC5321] Klensin, J., "Simple Mail Transfer Protocol",
Authentication Option", RFC 5925, June 2010. RFC 5321, October 2008.
[RFC5936] Lewis, E. and A. Hoenes, "DNS Zone Transfer Protocol [RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
(AXFR)", RFC 5936, June 2010. Authentication Option", RFC 5925, June 2010.
[RFC6151] Turner, S. and L. Chen, "Updated Security Considerations [RFC5936] Lewis, E. and A. Hoenes, "DNS Zone Transfer
for the MD5 Message-Digest and the HMAC-MD5 Algorithms", Protocol (AXFR)", RFC 5936, June 2010.
RFC 6151, March 2011.
[Shimomura1995] [RFC6151] Turner, S. and L. Chen, "Updated Security
Shimomura, T., "Technical details of the attack described Considerations for the MD5 Message-Digest and the
by Markoff in NYT", HMAC-MD5 Algorithms", RFC 6151, March 2011.
http://www.gont.com.ar/docs/post-shimomura-usenet.txt,
Message posted in USENET's comp.security.misc newsgroup,
Message-ID: <3g5gkl$5j1@ariel.sdsc.edu&gt, 1995.
[Silbersack2005] [Shimomura1995] Shimomura, T., "Technical details of the attack
Silbersack, M., "Improving TCP/IP security through described by Markoff in NYT",
randomization without sacrificing interoperability.", http://www.gont.com.ar/docs/post-shimomura-
EuroBSDCon 2005 Conference . usenet.txt, Message posted in USENET's
comp.security.misc newsgroup, Message-ID:
<3g5gkl$5j1@ariel.sdsc.edu>, 1995.
[USCERT2001] [Silbersack2005] Silbersack, M., "Improving TCP/IP security through
US-CERT, "US-CERT Vulnerability Note VU#498440: Multiple randomization without sacrificing
TCP/IP implementations may use statistically predictable interoperability", EuroBSDCon 2005 Conference.
initial sequence numbers",
http://www.kb.cert.org/vuls/id/498440, 2001.
[Wright1994] [USCERT2001] US-CERT, "US-CERT Vulnerability Note VU#498440:
Wright, G. and W. Stevens, "TCP/IP Illustrated, Volume 2: Multiple TCP/IP implementations may use
The Implementation", Addison-Wesley, 1994. statistically predictable initial sequence
numbers", http://www.kb.cert.org/vuls/id/498440,
2001.
[Zalewski2001] [Wright1994] Wright, G. and W. Stevens, "TCP/IP Illustrated,
Zalewski, M., "Strange Attractors and TCP/IP Sequence Volume 2: The Implementation", Addison-Wesley,
Number Analysis", 1994.
http://lcamtuf.coredump.cx/oldtcp/tcpseq.html, 2001.
[Zalewski2002] [Zalewski2001] Zalewski, M., "Strange Attractors and TCP/IP
Zalewski, M., "Strange Attractors and TCP/IP Sequence Sequence Number Analysis",
Number Analysis - One Year Later", http://lcamtuf.coredump.cx/oldtcp/tcpseq.html,
http://lcamtuf.coredump.cx/newtcp/, 2002. 2001.
Appendix A. Address-based trust relationship exploitation attacks [Zalewski2002] Zalewski, M., "Strange Attractors and TCP/IP
Sequence Number Analysis - One Year Later",
http://lcamtuf.coredump.cx/newtcp/, 2002.
Appendix A. Address-Based Trust-Relationship Exploitation Attacks
This section discusses the trust-relationship exploitation attack This section discusses the trust-relationship exploitation attack
that originally motivated the publication of RFC 1948 [RFC1948]. It that originally motivated the publication of RFC 1948 [RFC1948]. It
should be noted that while RFC 1948 focused its discussion of should be noted that while RFC 1948 focused its discussion of
address-based trust relationship exploitation attacks on Telnet address-based trust-relationship exploitation attacks on Telnet
[RFC0854] and the various UNIX "r" commands, both Telnet and the [RFC0854] and the various UNIX "r" commands, both Telnet and the
various "r" commands have since been largely replaced by secure various "r" commands have since been largely replaced by secure
counter-parts (such as SSH [RFC4251]) for the purpose of remote login counterparts (such as SSH [RFC4251]) for the purpose of remote login
and remote command execution. Nevertheless, address-based trust and remote command execution. Nevertheless, address-based trust
relationships are still employed nowadays in some scenarios. For relationships are still employed nowadays in some scenarios. For
example, some SMTP [RFC5321] deployments still authenticate their example, some SMTP [RFC5321] deployments still authenticate their
users by means of their IP addresses, even when more appropriate users by means of their IP addresses, even when more appropriate
authentication mechanisms are available [RFC4954]. Another example authentication mechanisms are available [RFC4954]. Another example
is the authentication of DNS secondary servers [RFC1034] by means of is the authentication of DNS secondary servers [RFC1034] by means of
their IP addresses for allowing DNS zone transfers [RFC5936], or any their IP addresses for allowing DNS zone transfers [RFC5936], or any
other access control mechanism based on IP addresses. other access control mechanism based on IP addresses.
In 1985, Morris [Morris1985] described a form of attack based on In 1985, Morris [Morris1985] described a form of attack based on
guessing what sequence numbers TCP [RFC0793] will use for new guessing what sequence numbers TCP [RFC0793] will use for new
connections. Briefly, the attacker gags a host trusted by the connections. Briefly, the attacker gags a host trusted by the
target, impersonates the IP address of the trusted host when talking target, impersonates the IP address of the trusted host when talking
to the target, and completes the 3-way handshake based on its guess to the target, and completes the three-way handshake based on its
at the next ISN to be used. An ordinary connection to the target is guess at the next ISN to be used. An ordinary connection to the
used to gather sequence number state information. This entire target is used to gather sequence number state information. This
sequence, coupled with address-based authentication, allows the entire sequence, coupled with address-based authentication, allows
attacker to execute commands on the target host. the attacker to execute commands on the target host.
Clearly, the proper solution for these attacks is cryptographic Clearly, the proper solution for these attacks is cryptographic
authentication [RFC4301] [RFC4120] [RFC4251]. authentication [RFC4301] [RFC4120] [RFC4251].
The following subsection provides technical details for the trust The following subsection provides technical details for the trust-
relationship exploitation attack described by Morris [Morris1985]. relationship exploitation attack described by Morris [Morris1985].
A.1. Blind TCP connection-spoofing A.1. Blind TCP Connection-Spoofing
In order to understand the particular case of sequence number In order to understand the particular case of sequence number
guessing, one must look at the 3-way handshake used in the TCP open guessing, one must look at the three-way handshake used in the TCP
sequence [RFC0793]. Suppose client machine A wants to talk to rsh open sequence [RFC0793]. Suppose client machine A wants to talk to
server B. It sends the following message: rsh server B. It sends the following message:
A->B: SYN, ISNa A->B: SYN, ISNa
That is, it sends a packet with the SYN ("synchronize sequence That is, it sends a packet with the SYN ("synchronize sequence
number") bit set and an initial sequence number ISNa. number") bit set and an initial sequence number ISNa.
B replies with B replies with
B->A: SYN, ISNb, ACK(ISNa) B->A: SYN, ISNb, ACK(ISNa)
skipping to change at page 12, line 17 skipping to change at page 12, line 15
connection, and thus by the time the connection is reset, the connection, and thus by the time the connection is reset, the
attacker has already won. attacker has already won.
In the past, attackers exploited a common TCP implementation bug In the past, attackers exploited a common TCP implementation bug
to prevent the connection from being reset (see subsection "A to prevent the connection from being reset (see subsection "A
Common TCP Bug" in [RFC1948]). However, all TCP implementations Common TCP Bug" in [RFC1948]). However, all TCP implementations
that used to implement this bug have been fixed for a long time. that used to implement this bug have been fixed for a long time.
Appendix B. Changes from RFC 1948 Appendix B. Changes from RFC 1948
o This document aims at Standards Track (rather than Informational). o This document is Standards Track (rather than Informational).
o Formal requirements ([RFC2119]) are specified. o Formal requirements [RFC2119] are specified.
o The discussion of address-based trust relationship attacks has o The discussion of address-based trust-relationship attacks has
been updated and moved to an Appendix. been updated and moved to an appendix.
o The subsection entitled "A Common TCP Bug" (describing a common o The subsection entitled "A Common TCP Bug" (describing a common
bug in the BSD TCP implementation) has been removed. bug in the BSD TCP implementation) has been removed.
Appendix C. Changes from previous versions of the document (this
section should be removed by the RFC Editor before
publication of this document as an RFC)
C.1. Changes from draft-ietf-tcpm-rfc1948bis-00
o Addresses WGLC feedback (posted on-list) by Wesley Eddy, and some
comments submitted by Anantha Ramaiah.
C.2. Changes from draft-gont-tcpm-rfc1948bis-00
o The recommended hash algorithm has been changed back to MD5
[RFC1321], with a note that the security implications of MD5 have
been carefully considered.
o The subsection entitled "An old BSD bug" (describing a common bug
in the BSD TCP implementation) has been removed.
o Minor editorial changes.
C.3. Changes from RFC 1948
o New document aims at Standards Track (rather than Informational).
o The discussion of address-based trust relationship attacks was
updated and moved to an Appendix.
o The recommended hash algorithm has been changed to SHA-256, in
response to the security concerns for MD5 [RFC1321].
o Formal requirements ([RFC2119]) are specified.
Authors' Addresses Authors' Addresses
Fernando Gont Fernando Gont
UTN-FRH / SI6 Networks SI6 Networks / UTN-FRH
Evaristo Carriego 2644 Evaristo Carriego 2644
Haedo, Provincia de Buenos Aires 1706 Haedo, Provincia de Buenos Aires 1706
Argentina Argentina
Phone: +54 11 4650 8472 Phone: +54 11 4650 8472
Email: fernando@gont.com.ar EMail: fgont@si6networks.com
URI: http://www.si6networks.com URI: http://www.si6networks.com
Steven M. Bellovin Steven M. Bellovin
Columbia University Columbia University
1214 Amsterdam Avenue 1214 Amsterdam Avenue
MC 0401 MC 0401
New York, NY 10027 New York, NY 10027
US US
Phone: +1 212 939 7149 Phone: +1 212 939 7149
Email: bellovin@acm.org EMail: bellovin@acm.org
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