Network Working Group M. Dalal
Internet-Draft Editor
Expires: May 23, 2005 November 22, 2004
Transmission Control Protocol security considerations
draft-ietf-tcpm-tcpsecure-02.txt
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of section 3 of RFC 3667. By submitting this Internet-Draft, each
author represents that any applicable patent or other IPR claims of
which he or she is aware have been or will be disclosed, and any of
which he or she become aware will be disclosed, in accordance with
RFC 3668.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as
Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on May 23, 2005.
Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
TCP (RFC793 [1]) is widely deployed and one of the most often used
reliable end to end protocols for data communication. Yet when it
was defined over 20 years ago the internet, as we know it, was a
different place lacking many of the threats that are now common.
Recently several rather serious threats have been detailed that can
pose new methods for both denial of service and possibly data
injection by blind attackers. This document details those threats
and also proposes some small changes to the way TCP handles inbound
Dalal Expires May 23, 2005 [Page 1]
Internet-Draft TCP Security November 2004
segments that either eliminate the threats or at least minimize them
to a more acceptable level.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Blind reset attack using the RST bit . . . . . . . . . . . . . 5
2.1 Description of the attack . . . . . . . . . . . . . . . . 5
2.2 Solution . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Blind reset attack using the SYN bit . . . . . . . . . . . . . 7
3.1 Description of the attack . . . . . . . . . . . . . . . . 7
3.2 Solution . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Blind data injection attack . . . . . . . . . . . . . . . . . 9
4.1 Description of the attack . . . . . . . . . . . . . . . . 9
4.2 Solution . . . . . . . . . . . . . . . . . . . . . . . . . 9
5. Backward Compatibility and Other considerations . . . . . . . 10
6. Middlebox considerations . . . . . . . . . . . . . . . . . . . 12
6.1 Middlebox that cache RST's . . . . . . . . . . . . . . . . 12
6.2 Middleboxes that advance sequence numbers . . . . . . . . 12
7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 14
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
9.1 Normative References . . . . . . . . . . . . . . . . . . . . 16
9.2 Informative References . . . . . . . . . . . . . . . . . . . 16
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 16
Intellectual Property and Copyright Statements . . . . . . . . 17
Dalal Expires May 23, 2005 [Page 2]
Internet-Draft TCP Security November 2004
1. Introduction
TCP (RFC793 [1]) is widely deployed and one of the most often used
reliable end to end protocols for data communication. Yet when it
was defined over 20 years ago the internet, as we know it, was a
different place lacking many of the threats that are now common.
Recently several rather serious threats have been detailed that can
pose new methods for both denial of service and possibly data
injection by blind attackers. This document details those threats
and also proposes some small changes to the way TCP handles inbound
segments that either eliminate the threats or at least minimize them
to a more acceptable level.
Most of these proposals modify handling procedures for DATA, RST and
SYN's as defined in RFC793 [1] but do not cause interoperability
issues. The authors feel that many of the changes proposed in this
document would, if TCP were being standardized today, be required to
be in the base TCP document and the lack of these procedures is more
an artifact of the time when TCP was developed than any strict
requirement of the protocol.
For some uses of TCP, an alternative protection against the threats
that these changes address has already been implemented and deployed
in the TCP MD5 Signature Option (RFC2385 [2]). Because this option
is not negotiated and is implemented with a manually established
shared key or password, it has been used for protecting uses of TCP
in which the endpoints are managed, such as for BGP peers. RFC3562
[2] provides importance guidance for users of RFC2385 [2] for
decreasing their vulnerability to key-guessing.
Yet another commonly known mitigation technique is cryptography,
especially IPSec. For IPSec to work, both ends of the connection
need to agree on the properties to use for the connection and also
agree upon a pre-shared key. In the absence of PKI infrastr1ucture,
this may be inconvenient. IPSec with manual keys can be used to
avoid using ISAKMP. However, this adds considerable burden on the
administration of such solution. If ISAKMP were to be used, this
would typically require all firewalls in the path between the two TCP
endpoints to allow UDP traffic for atleast ISAKMP to function.
Further, IPSec and NAT have long been known to have interoperability
issues.
TCP implementations SHOULD also introduce ephemeral port
randomization. By randomizing ephemeral ports an attacker would have
a less easy time in guessing the four tuples needed to mount a
successful attack. Since ephemeral ports are 16 bit values and are a
subset of the entire available port numbers, it is a weaker defense
than an exact sequence number match as proposed here which is a
Dalal Expires May 23, 2005 [Page 3]
Internet-Draft TCP Security November 2004
32-bit value and changes dramatically within the life of a
connection. Nevertheless, both of them are complimentary solutions
that will make it difficult to launch attacks discussed below.
Alternative proposals, including the use of cookies (or, use of the
timestamp option as a cookie) require both peers to implement the
changes before any additional protection can be realized.
Dalal Expires May 23, 2005 [Page 4]
Internet-Draft TCP Security November 2004
2. Blind reset attack using the RST bit
2.1 Description of the attack
It has been traditionally thought that for a blind attacker to reset
a TCP connection the attacker would have to guess a single sequence
number in the TCP sequence space. This would in effect require an
attacker to generate (2^^32) segments in order to reset a connection.
Recent papers have shown this to not necessarily be the case. An
attacker need only guess a number that lies between the last sequence
number acknowledged and the last sequence number acknowledged added
to the receiver window (RCV.WND)[4]. Modern operating systems
normally default the RCV.WND to about 32,768 bytes. This means that
a blind attacker need only guess 65,535 RST segments (2^^32/RCV.WND)
in order to reset a connection. At DSL speeds this means that most
connections (assuming the attacker can accurately guess both ports)
can be reset in under 200 seconds (usually far less). With the rise
of broadband availability and increasing available bandwidth, many
Operating Systems have raised their default RCV.WND to as much as
64k, thus making these attacks even easier.
2.2 Solution
RFC793 [1] 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
expected window, 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, the following changes should be made to provide some
protection against such an attack.
A) If the RST bit is set and the sequence number is outside the
expected window, silently drop the segment.
B) If the RST bit is set and the sequence number exactly matches the
next expected sequence number, reset the connection.
C) If the RST bit is set and the sequence number does not exactly
match the next expected sequence value, yet is within the
acceptable window (RCV.NXT < SEG.SEQ <= RCV.NXT+RCV.WND) send an
acknowledgment:
<SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
After sending the acknowledgment, drop the unacceptable segment
and return.
This solution forms a challenge/response with any RST where the
value does not exactly match the expected value and yet the RST is
within the window. In cases of a legitimate reset without the
exact sequence number, the consequences of this new
challenge/response will be that the peer requires an extra round
Dalal Expires May 23, 2005 [Page 5]
Internet-Draft TCP Security November 2004
trip time before the connection can be reset.
In order to alleviate multiple RSTs/SYNs from triggering multiple
challenge ACKs, a ACK throttling mechanism SHOULD be implemented.
Suggested values are to send no more than 10 challenge ACKs in a 5
second window. These values MUST be tunable to accomodate different
requirements. ACK throttling if implemented successfully can lead to
several advantages. Besides preventing the RST/ACK war outlined in
the section below, it can also alleviate spurious fast retransmits at
the remote end caused by flood of duplicate ACKs and also save
spurious processing required to send an ACK on the victim side.
Dalal Expires May 23, 2005 [Page 6]
Internet-Draft TCP Security November 2004
3. Blind reset attack using the SYN bit
3.1 Description of the attack
Analysis of the reset attack, which uses the RST flag bit, highlights
another possible avenue for a blind attacker. Instead of using the
RST bit an attacker can use the SYN bit as well to tear down a
connection. Using the same guessing technique, repeated SYN's can be
generated with sequence numbers incrementing by an amount not larger
than the window size apart and thus eventually cause the connection
to be terminated.
3.2 Solution
RFC793 [1] currently requires handling of a segment with the SYN bit
set in the synchronized state to be as follows:
1) If the SYN bit is set and the sequence number is outside the
expected window, send an ACK back to the sender.
2) If the SYN bit is set and the sequence number is acceptable i.e.:
(RCV.NXT <= SEG.SEQ <= RCV.NXT+RCV.WND) then send a RST segment to
the sender.
Instead, changing the handling of the SYN to the following will
provide complete protection from this attack:
1) If the SYN bit is set, irrespective of the sequence number, send
an ACK to the remote peer:
<SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
After sending the acknowledgment, drop the unacceptable segment
and return.
This solution agains forms a challenge response with the peer as
in the previous section.
By always sending an ACK to the sender, a challenge/response is setup
with the peer. 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 with the sequence number derived from the
ACK field that caused the RST.
Note, there is one corner case for the SYN attack problem that will
prevent the successful reset of the connection. This is a result of
the RFC793 [1] specification and is nothing to do with the proposed
solution. In this problem, if a restarting host generates a SYN with
an initial sequence number that is exactly equal to RCV.NXT - 1 of
the remote TCP endpoint that is still in the established state and if
the SYN arrives at the peer that is still holding the stale
connection, an ACK will be generated. This ACK will have an ack
value of RCV.NXT and will be acceptable to the restarting host which
will accept the ACK and do nothing. The SYN will then be
Dalal Expires May 23, 2005 [Page 7]
Internet-Draft TCP Security November 2004
retransmitted and the behavior will repeat. This could lead to an
initialization failure. Subsequent connection attempts will
hopefully succeed by choosing a new ISN that is not equal to RCV.NXT
- 1. A similar problem will be seen should the SYN contain data.
Dalal Expires May 23, 2005 [Page 8]
Internet-Draft TCP Security November 2004
4. Blind data injection attack
4.1 Description of the attack
A third type of attack is also highlighted by both the RST and SYN
attacks. It is quite possible to inject data into a TCP connection
by simply guessing a sequence value that is within the window. The
ACK value of any data segment is considered valid as long as it does
not acknowledge data ahead of the next segment to send. In other
words an ACK value is acceptable if it is (SND.UNA-(2^^31-1)) <=
SEG.ACK <= SND.NXT). This means that an attacker simply need guess
two ACK values with every guessed sequence number so that the chances
of successfully injecting data into a connection are 1 in ((2^^32 /
RCV.WND) * 2).
When an attacker successfully injects data into a connection the data
will sit in the receiver's re-assembly queue until the peer sends
enough data to bridge the gap between the RCV.NXT value and the
injected data. At that point one of two things will occur either:
a) An ACK war will ensue with the receiver indicating that it has
received data up until RCV.NXT (which includes the attackers data)
and the sender sending a corrective ACK with a value less than
RCV.NXT (the real sequence number of the last byte sent).
b) The sender will send enough data to the peer which will move
RCV.NXT even further along past the injected data.
In either case the injected data will be made readable to the upper
layer and in case <a> the connection will eventually be reset by one
of the sides. 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 natural
consequence of any data injection that is sucessful.
4.2 Solution
An additional input check should be added to any incoming segment.
The ACK value should be acceptable only if it is in the range of
((SND.UNA - MAX.SND.WND) <= SEG.ACK <= SND.NXT). MAX.SND.WND is
defined as the largest window that the local receiver has ever
advertised to it's peer. This window is the scaled value i.e. the
value may be larger than 65,535 bytes. This small check will greatly
reduce the vulnerability of an attacker guessing a valid sequence
number since not only must he/she guess the sequence number in
window, but must also guess a proper ACK value within a scoped range.
This solution reduces but does not eliminate the ability to generate
false segments. It does however reduce the probability that invalid
data will be injected to a more acceptable level. For those
applications that wish to close this attack completely RFC2385 [2]
should be deployed between the two endpoints.
Dalal Expires May 23, 2005 [Page 9]
Internet-Draft TCP Security November 2004
5. Backward Compatibility and Other considerations
1) The proposed solution is backward compatible as it uses the
semantics laid down in RFC793 [1] to defend against it's own
weakness. Referring to the figure below, if we assume that the
RST (1.c) was in flight when the ACK (2) left TCP A, TCP B has no
way of knowing what triggered the ACK. For all it cares, the ACK
might have been a result of the data or the RST that it had sent
earlier. Hence in either case, TCP B must reply to this ACK with
an appropriate RST that is in keeping with RFC793 [1].
2) Concerns have been raised that the challenge response mechanism
will lead to a reflector kind of attack. In this attack, it is
believed that an attacker with higher bandwidth can potentially
spoof SYN or RST packets within the window and cause ACK flooding
to a remote peer that may have a lower bandwidth. These concerns
are misplaced because it is trivial to cause a victim to generate
an ACK. A spoofer can simply send packets with sequence numbers
that are outside the acceptable window of the attacker or send an
ACK that acknowledges something that is not yet sent. Further, an
attacker can also simply generate data packets that fall within
the window to cause an ACK to be sent. RFC793 [1] also mandates
that an ACK be sent if the incoming SYN to an established
connection falls outside the acceptable window. All these
scenarios can be used to launch a flood attack. However, the
potential harm of such attacks are low and can be easily detected
due to the volume of packets generated. The latter is a strong
deterrent to such attacks.
3) There is a corner scenario in the above proposed solutions which
will require more than one round trip time to successfully abort
the connection as per the figure below. This scenario is similar
to the one in which the original RST was lost in the network.
TCP A TCP B
1.a. ESTABLISHED <-- <SEQ=300><ACK=101><CTL=ACK><DATA> <-- ESTABLISHED
b. (delayed) ... <SEQ=400><ACK=101><CTL=ACK><DATA> <-- ESTABLISHED
c. (in flight) ... <SEQ=500><ACK=101><CTL=RST> <-- CLOSED
2. ESTABLISHED --> <SEQ=101><ACK=400><CTL=ACK> --> CLOSED
(ACK for 1.a)
... <SEQ=400><ACK=0><CTL=RST> <-- CLOSED
3. CHALLENGE --> <SEQ=101><ACK=400><CTL=ACK> --> CLOSED
(for 1.c)
... <SEQ=400><ACK=0><CTL=RST> <-- RESPONSE
4.a. ESTABLISHED <-- <SEQ=400><ACK=101><CTL=ACK><DATA> 1.b reaches A
b. ESTABLISHED --> <SEQ=101><ACK=500><CTL=ACK>
c. (in flight) ... <SEQ=500><ACK=0><CTL=RST> <-- CLOSED
5. RESPONSE arrives at A, but is dropped because of being out of window.
Dalal Expires May 23, 2005 [Page 10]
Internet-Draft TCP Security November 2004
6. ESTABLISHED <-- <SEQ=500><ACK=0><CTL=RST> 4.c reaches A
7. CLOSED CLOSE
4) For the solution to be totally effective against the
vulnerabilities discussed in this document, both ends of the TCP
connection need to have the fix. Although, having it at one end
might prevent that end from being exposed to the attack, the
connection is still vulnerable at the other end.
Dalal Expires May 23, 2005 [Page 11]
Internet-Draft TCP Security November 2004
6. Middlebox considerations
The following scenarios were brought to notice by the tcpm working
group members as middlebox issues which may cause the proposed
solution to behave in an unexpected manner.
6.1 Middlebox that cache RST's
Consider a middlebox B tracking connection between two TCP endhosts A
and C. If C sends a RST with a sequence number that is within the
window but not an exact match to reset the connection and if B does
not have the fix proposed here, it will clear the connection and ask
A to do the same. If A does not have the fix it will clear the
connection and everything will be fine. However if A does have the
proposed fix above, it will send a challenge ACK. B being a
middlebox will intercept this ACK and resend the RST cached earlier
that was responsible for bringing down the connection. However, the
RST will again be not acceptable and will trigger a challenge ACK
again. This will cause a RST/ACK war to happen. However, we are not
aware of middleboxes that actually do this and believe the design
itself is flawed in that given the scenario that the RST from B to A
got lost on the way, A will continue to hold the connection and A
might send an ACK an arbitrary time after the connection was brought
down at B. In this case, B will have to cache the RST for an
arbitrary amount of time till it's confirmed that the connection has
been cleared at A.
6.2 Middleboxes that advance sequence numbers
Some Middleboxes may compute RST sequence numbers at the higher end
of the acceptable window. The setup is the same as the earlier case,
but in this case instead of sending the cached RST, the middlebox
sends a RST that computes it's sequence number as a sum of the ack
field in the ACK and the window advertised by the ACK that was sent
by 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
Dalal Expires May 23, 2005 [Page 12]
Internet-Draft TCP Security November 2004
5. ESTABLISHED <-- <SEQ=800><ACK=100><CTL=RST> <-- CLOSED
Although the authors are not aware of a working implementation that
does the above, it could be mitigated by implementing the RST
throttling mechanism described earlier.
Dalal Expires May 23, 2005 [Page 13]
Internet-Draft TCP Security November 2004
7. Contributors
Mitesh Dalal and Amol Khare of Cisco Systems came up with the
solution for the RST/SYN attacks. Anantha Ramaiah and Randall
Stewart of Cisco Systems discovered the data injection vulnerability
and together with Patrick Mahan and Peter Lei of Cisco Systems found
solutions for the same. Paul Goyette, Mark Baushke, Frank
Kastenholz, Art Stine and David Wang of Juniper Networks provided the
insight that apart from RSTs, SYNs could also result in formidable
attacks. Shrirang Bage of Cisco Systems, Qing Li and Preety Puri of
Wind River Systems and Xiaodan Tang of QNX Software along with the
folks above helped in ratifying and testing the interoperability of
the suggested solutions.
Dalal Expires May 23, 2005 [Page 14]
Internet-Draft TCP Security November 2004
8. Acknowledgments
Special thanks to Sharad Ahlawat, Mark Allman, Steve Bellovin, Vern
Paxson, Allison Mankin, Damir Rajnovic, John Wong and the tcpm WG
members for suggestions and comments.
Dalal Expires May 23, 2005 [Page 15]
Internet-Draft TCP Security November 2004
9. References
9.1 Normative References
[1] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
September 1981.
[2] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC 2385, August 1998.
9.2 Informative References
[3] Leech, M., "Key Management Considerations for the TCP MD5
Signature Option", RFC 3562, July 2003.
[4] Watson, P., ""Slipping in the Window: TCP Reset attacks"".
Author's Address
Mitesh Dalal
Editor
170 Tasman Drive
San Jose, CA 95134
USA
Phone: +1-408-853-5257
EMail: mdalal@cisco.com
Dalal Expires May 23, 2005 [Page 16]
Internet-Draft TCP Security November 2004
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Disclaimer of Validity
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
Dalal Expires May 23, 2005 [Page 17]
