TCP Maintenance and Minor A. Ramaiah Extensions Working Group R. Stewart Internet-Draft M. Dalal Intended status: Standards Track Editor Expires: August 26, 2007 February 22, 2007 Improving TCP's Robustness to Blind In-Window Attacks draft-ietf-tcpm-tcpsecure-07.txt Status of this Memo 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 becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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 August 26, 2007. Copyright Notice Copyright (C) The IETF Trust (2007). Ramaiah, et al. Expires August 26, 2007 [Page 1] Internet-Draft TCP Security February 2007 Abstract TCP has historically been considered protected against spoofed 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 a longer term connections (e.g. H-323 or Border Gateway Protocol [RFC4271]) have left modern TCP implementation more vulnerable to these types of spoofed packet injection attacks. Many of these long term TCP applications tend to have predictable IP addresses and ports which makes it far easier for the 4-tuple to be guessed. Having guessed the 4-tuple correctly, an attacker can inject a RST, SYN or DATA segment into a TCP connection by carefully crafting the sequence number of the spoofed segment to be in the current receive window. This can cause the connection to either abort or possibly cause data corruption. This document specifies small modifications to the way TCP handles inbound segments that can reduce the chances of a successful attack. Ramaiah, et al. Expires August 26, 2007 [Page 2] Internet-Draft TCP Security February 2007 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Basic Attack Methodology . . . . . . . . . . . . . . . . . 4 1.2. Attack probabilities . . . . . . . . . . . . . . . . . . . 6 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 8 3. Blind reset attack using the RST bit . . . . . . . . . . . . . 9 3.1. Description of the attack . . . . . . . . . . . . . . . . 9 3.2. Mitigation . . . . . . . . . . . . . . . . . . . . . . . . 9 4. Blind reset attack using the SYN bit . . . . . . . . . . . . . 11 4.1. Description of the attack . . . . . . . . . . . . . . . . 11 4.2. Mitigation . . . . . . . . . . . . . . . . . . . . . . . . 11 5. Blind data injection attack . . . . . . . . . . . . . . . . . 13 5.1. Description of the attack . . . . . . . . . . . . . . . . 13 5.2. Mitigation . . . . . . . . . . . . . . . . . . . . . . . . 13 6. ACK throttling . . . . . . . . . . . . . . . . . . . . . . . . 15 7. Backward Compatibility and Other considerations . . . . . . . 16 8. Middlebox considerations . . . . . . . . . . . . . . . . . . . 17 8.1. Middlebox that resend RST's . . . . . . . . . . . . . . . 17 8.2. Middleboxes that advance sequence numbers . . . . . . . . 17 9. Security Considerations . . . . . . . . . . . . . . . . . . . 19 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 21 12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 22 13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23 13.1. Normative References . . . . . . . . . . . . . . . . . . . 23 13.2. Informative References . . . . . . . . . . . . . . . . . . 23 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25 Intellectual Property and Copyright Statements . . . . . . . . . . 26 Ramaiah, et al. Expires August 26, 2007 [Page 3] Internet-Draft TCP Security February 2007 1. Introduction TCP [RFC0793] is widely deployed and the most common reliable end to end transport protocol used for data communication in today's Internet. Yet when it was standardized over 20 years ago, the Internet, as we know it, was a different place, lacking many of the threats that are now common. TCP spoofing attacks are one such attack that are seen on the Internet today. In a TCP spoofing attack, an off-path attacker crafts TCP packets by forging the IP source and destination addresses as well as the source and destination ports (commonly referred to as a 4-tuple value). The targeted TCP endpoint will then associate such a packet with an existing TCP connection. Note that in and of itself guessing this 4-tuple value is not always easy for an attacker. But there are some applications (e.g. BGP [RFC4271]) that may have a tendency to use the same set(s) of ports on either endpoint making the odds of guessing correctly the 4-tuple value much easier. When an attacker is successful in guessing the 4-tuple value, one of three types of injection attacks may be waged against a long-lived connection. RST - Where an attacker injects a RST segment hoping to cause the connection to be torn down. SYN - Where an attacker injects a SYN hoping to cause the receiver to believe the peer has restarted and so tear down the connection state. DATA - Where an attacker tries to inject a DATA segment to corrupt the contents of the transmission. 1.1. Basic Attack Methodology Focusing upon the RESET 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 emphasised 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: Ramaiah, et al. Expires August 26, 2007 [Page 4] Internet-Draft TCP Security February 2007 1) The 4-tuple value containing the IP address and TCP port number of both ends of the connection. For one side (usually the server) guessing the port number is a trivial exercise. The client side may or may not be easy for an attacker to guess depending on a number of factors, most notably the operating system and application involved. 2) A sequence number that will be used in the RST. This sequence number will be a starting point for a series of guesses to attempt to present a RST segment to a connection endpoint that would be acceptable to it. Any random value may be used to guess the initial sequence number. 3) The window size that the two endpoints are using. This value does NOT have to be the exact window size since a smaller value used in lieu of the correct one will just cause the attacker to generate more segments before succeeding in his mischieve. Most modern operating systems have a default window size which usually is applied to most connections. Some applications however may change the window size to better suit the needs of the application. So often times the attacker, with a fair degree of certainty (knowing the application that is under attack), can come up with a very close approximation as to the actual window size in use on the connection. After assembling the above set of information the attacker begins sending spoofed TCP segments with the RST bit set and a guessed TCP sequence number. Each time a new RST segment is sent, the sequence number guess is incremented by the window size. The feasibility of this methodology (without mitigations) was first shown in [SITW]. This is because [RFC0793] specifies that any RST within the current window is acceptable. Also [I-D.ietf-tcpm-tcp-antispoof] talks about the probability of a successful attack with varying window sizes and bandwidth. A slight enhancement to the TCP's segment processing rules can be made which makes such an attack much more difficult to accomplish. If the receiver examines the incoming RST segment and validates that the sequence number exactly matches the sequence number that is next expected, then such an attack becomes much more difficult than outlined in [SITW] (i.e. the attacker would have to generate 1/2 the entire sequence space, on average). This document will discuss the exact details of what needs to be changed within TCP's segment processing rules to mitigate all three types of attacks (RST, SYN and DATA). Ramaiah, et al. Expires August 26, 2007 [Page 5] Internet-Draft TCP Security February 2007 1.2. Attack probabilities Every application has control of a number of factors that effect drastically the probability of a successful spoofing attack. These factors include such things as: Window Size - Normally settable by the application but often times defaulting to 32,768 or 65,535 depending upon the operating system (Medina05 [Medina05]). Server Port number - This value is normally a fixed value so that a client will know where to connect to the peer at. Thus this value normally provides no additional protection. Client Port number - This value may be a random ephemeral value, if so, this makes a spoofing attack more difficult. There are some clients, however, that for whatever reason either pick a fixed client port or have a very guessable one (due to the range of ephemeral ports available with their operating system or other application considerations) for such applications a spoofing attack becomes less difficult. For the purposes of the rest of this discussion we will assume that the attacker knows the 4-tuple values. This assumption will help us focus on the effects of the window size versus the number of TCP packets an attacker must generate. This assumption will rarely be true in the real Internet since at least the client port number will provide us with some amount of randomness (depending on the operating system). To successfully inject a spoofed packet (RST, SYN or DATA), in the past, the entire sequence space (i.e. 2^32) was often considered available to make such an attack unlikely. [SITW] demonstrated that this assumption was incorrect and that instead of [1/2 * 2^32] packets (assuming a random distribution) [1/2 * (2^32/window)] packets is required. Placing real numbers on 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. 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. Ramaiah, et al. Expires August 26, 2007 [Page 6] Internet-Draft TCP Security February 2007 As we can see from the above discussion this weakness lowers the bar quite considerably for likely attacks. But there is one additional dependency which is the duration of the TCP connection. A TCP connection that lasts only a few brief packets, as often is the case for web traffic, would not be subject to such an attack since the connection may not be established long enough for an attacker to generate enough traffic. However there is a set of applications such as BGP [RFC4271] which is judged to be potentially most affected by this vulnerability. BGP relies on a persistent TCP session between BGP peers. Resetting the connection can result in medium term unavailability due to the need to rebuild routing tables and route flapping; see [NISCC] for further details. It also needs to be emphasized that, for applications like BGP, use of the TCP MD5 option [RFC2385], can make the attacks described in this specification effectively impossible. It should be noted that there are existing alternative protections against the threats that this document addresses. For further details regarding the attacks and the existing techniques, please refer to draft [I-D.ietf-tcpm-tcp-antispoof] Ramaiah, et al. Expires August 26, 2007 [Page 7] Internet-Draft TCP Security February 2007 2. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. TCP terminology should be interpreted as described in [RFC0793]. Ramaiah, et al. Expires August 26, 2007 [Page 8] Internet-Draft TCP Security February 2007 3. Blind reset attack using the RST bit 3.1. Description of the attack As described in the introduction, it is possible for an attacker to generate a "RST" segment that would be acceptable to a TCP receiver by guessing "in-window" sequence numbers. In particular [RFC0793], p37, states the following: "In all states except SYN-SENT, all reset (RST) segments are validated by checking their SEQ-fields [sequence numbers]. A reset is valid if its sequence number is in the window. In the SYN-SENT state (a RST received in response to an initial SYN), the RST is acceptable if the ACK field acknowledges the SYN." 3.2. Mitigation [RFC0793] currently requires handling of a segment with the RST bit when in a synchronized state to be processed as follows: 1) If the RST bit is set and the sequence number is outside the current receive window (SEG.SEQ <= RCV.NXT || SEG.SEQ > RCV.NXT+ RCV.WND) , silently drop the segment. 2) If the RST bit is set and the sequence number is acceptable i.e.: (RCV.NXT <= SEG.SEQ < RCV.NXT+RCV.WND) then reset the connection. Instead, this document requires that implementations SHOULD implement the following steps in place of those specified in [RFC0793](as listed above). A) If the RST bit is set and the sequence number is outside the current receive window, silently drop the segment. B) If the RST bit is set and the sequence number exactly matches the next expected sequence number (RCV.NXT), then TCP MUST reset the connection. C) If the RST bit is set and the sequence number does not exactly match the next expected sequence value, yet is within the current receive window (RCV.NXT < SEG.SEQ < RCV.NXT+RCV.WND), TCP MUST send an acknowledgment (challenge ACK): Ramaiah, et al. Expires August 26, 2007 [Page 9] Internet-Draft TCP Security February 2007 After sending the challenge ACK, TCP MUST drop the unacceptable segment and stop processing the incoming packet further. The previous text, quoted from [RFC0793] pg 37 would thus become: In all states except SYN-SENT, all reset (RST) segments are validated by checking their SEQ-fields [sequence numbers]. A reset is valid if its sequence number exactly matches the next expected sequence number. If the RST arrives and its sequence number field does NOT match the next expected sequence number but is within the window, then the receiver should generate an ACK. In all other cases where the SEQ-field does not match and is outside the window, the receiver MUST silently discard the segment. In the SYN-SENT state (a RST received in response to an initial SYN), the RST is acceptable if the ACK field acknowledges the SYN. In all other cases the receiver MUST silently discard the segment. With the above slight change to the TCP state machine, it becomes much harder for an attacker to generate an acceptable reset segment. In cases where the remote peer did generate a RST but it fails to meet the above criteria (the RST sequence number was within the window but NOT the exact expected sequence number) when the challenge ACK is sent back, it will no longer have the transmission control block (TCB) related to this connection and hence as per [RFC0793], the remote peer will send a second RST back. The sequence number of the second RST is derived from the acknowledgment number of the incoming ACK. This second RST, if it reaches the sender, will cause the connection to be aborted since the sequence number would now be an exact match. Note that the above mitigation may cause a non-amplification ACK exchange. This concern is discussed in Section 9. Ramaiah, et al. Expires August 26, 2007 [Page 10] Internet-Draft TCP Security February 2007 4. Blind reset attack using the SYN bit 4.1. Description of the attack The analysis of the reset attack using the RST bit highlights another possible avenue for a blind attacker using a similar set of sequence number guessing. Instead of using the RST bit an attacker can use the SYN bit with the exact same semantics to tear down a connection. 4.2. Mitigation [RFC0793] currently requires handling of a segment with the SYN bit set in the synchronized state to be as follows: 1) If the SYN bit is set and the sequence number is outside the expected window, send an ACK back to the sender. 2) If the SYN bit is set and the sequence number is acceptable i.e.: (RCV.NXT <= SEG.SEQ <= RCV.NXT+RCV.WND) then send a RST segment to the sender. Instead, the handling of the SYN in the synchronized state SHOULD be performed as follows: A) If the SYN bit is set, irrespective of the sequence number, TCP MUST send an ACK (also referred to as challenge ACK) to the remote peer: After sending the acknowledgment, TCP MUST drop the unacceptable segment and stop processing further. By sending an ACK, the remote end sender is challenged to confirm the loss of the previous connection and the request to start a new connection. A legitimate peer, after restart, would not have a TCB in the synchronized state. Thus when the ACK arrives the peer should send a RST segment back with the sequence number derived from the ACK field that caused the RST. This RST will confirm that the remote TCP endpoint has indeed closed the previous connection. Upon receipt of a valid RST, the local TCP endpoint MUST terminate its connection. The local TCP endpoint should then rely on SYN retransmission from the remote end to re- establish the connection. A spoofed SYN, on the other hand, will then have generated an additional ACK which the peer will discard as a duplicate ACK and Ramaiah, et al. Expires August 26, 2007 [Page 11] Internet-Draft TCP Security February 2007 will not affect the established connection. Note that this mitigation does leave one corner case un-handled which will prevent the reset of a connection when it should be reset (i.e. it is a non-spoofed SYN wherein a peer really did restart). This problem occurs when the restarting host chooses the exact same IP address and port number that it was using prior to its restart. By chance the restarted host must also choose an initial sequence number of exactly (RCV.NXT - 1) of the remote TCP endpoint that is still in the established state. Such a case would cause the receiver to generate a "challenge" ack as described above. But since the ACK would be within the outgoing connections window the inbound ACK would be acceptable, and the sender of the SYN will do nothing with the response ACK. This sequence will continue as the SYN sender continually times out and retransmits the SYN until such time as the connection attempt fails. This corner case is a result of the [RFC0793] specification and is not introduced by these new requirments. Note that the above mitigation may cause a non-amplification ACK exchange. This concern is discussed in Section 9. Ramaiah, et al. Expires August 26, 2007 [Page 12] Internet-Draft TCP Security February 2007 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). This means that an attacker has to guess two ACK values with every guessed sequence number so that the chances of successfully injecting data into a connection are 1 in ((2^32 / RCV.WND) * 2). When an attacker successfully injects data into a connection the data will sit in the receiver's re-assembly queue until the peer sends enough data to bridge the gap between the RCV.NXT value and the injected data. At that point one of two things will occur : a) A packet war will ensue with the receiver indicating that it has received data up until RCV.NXT (which includes the attackers data) and the sender sending an ACK with an acknowledgment number less than RCV.NXT. b) The sender will send enough data to the peer which will move RCV.NXT even further along past the injected data. Depending upon the TCP implementation in question and the TCP traffic characteristics at that time, data corruption may result. In case (a) the connection will eventually be reset by one of the sides unless the sender produces more data that will transform the ACK war into case (b). The reset will usually occur via User Time Out (UTO) (see section 4.2.3.5 of [RFC1122]). Note that the protections illustrated in this section neither cause an ACK war nor prevent one from occurring if data is actually injected into a connection. The ACK war is a product of the attack itself and cannot be prevented (other than by preventing the data from being injected). 5.2. Mitigation All TCP stacks SHOULD implement the following mitigation. TCP stacks which implement the mitigation requires that an additional input check MUST be added to any incoming segment. The ACK value MUST be 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 Ramaiah, et al. Expires August 26, 2007 [Page 13] Internet-Draft TCP Security February 2007 satisfy the above condition MUST be discarded silently. A new state variable MAX.SND.WND is defined as the largest window that the local sender has ever received from its peer. This window may be scaled to a value larger than 65,535 bytes ([RFC1323]). This small check will reduce the vulnerability to an attacker guessing a valid sequence number, since he/she not only must guess the in-window sequence number, but also guess a proper ACK value within a scoped range. This mitigation reduces, but does not eliminate, the ability to generate false segments. It does however reduce the probability that invalid data will be injected. Note that the above mitigation may cause a non-amplification ACK exchange. This concern is discussed in Section 9. Ramaiah, et al. Expires August 26, 2007 [Page 14] Internet-Draft TCP Security February 2007 6. ACK throttling In order to alleviate multiple RSTs/SYNs from triggering multiple challenge ACKs, an ACK throttling mechanism is suggested as follows : 1) The system administrator can configure the number of challenge ACK's that can be sent out in a given interval. For example, in any 5 second window, no more than 10 challenge ACK's should be sent. 2) The values for both the time and number of ACK's 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 mechanism implemented in some OS's. Also note that no timer is needed to implement the above mechanism, instead a timestamp and a counter can be used. An implementation SHOULD include an ACK throttling mechanism to be conservative. Currently there is no known bad behavior that can be attributed to the lack of ACK throttling, but as a general principle, if ever invoked, something incorrect is occurring and such a mechanism will act as a failsafe that protects both the sender and the network. An administrator who is more concerned about protecting his bandwidth and CPU utilization may set smaller ACK throttling values whereas an administrator who is more interested in faster cleanup of stale connections (i.e. concerned about excess TCP state) may decide to set a higher value thus allowing more RST's to be processed in any given time period. The time limit SHOULD be tunable to help timeout brute force attacks faster than a potential legitimate flood of RSTs. Ramaiah, et al. Expires August 26, 2007 [Page 15] Internet-Draft TCP Security February 2007 7. Backward Compatibility and Other considerations All of the new required mitigation techniques in this document are totally compatible with existing ([RFC0793]) compliant TCP implementations as this document introduces no new assumptions or conditions. There is a corner scenario in the above mitigations which will require more than one round trip time to successfully abort the connection as per the figure below. This scenario is similar to the one in which the original RST was lost in the network. TCP A TCP B 1.a. ESTAB <-- <-- ESTAB b. (delayed) ... <-- ESTAB c. (in flight) ... <-- CLOSED 2. ESTAB --> --> CLOSED (ACK for 1.a) ... <-- CLOSED 3. CHALLENGE --> --> CLOSED (for 1.c) ... <-- RESPONSE 4.a. ESTAB <-- 1.b reaches A b. ESTAB --> c. (in flight) ... <-- CLOSED 5. RESPONSE arrives at A, but dropped since its outside of window. 6. ESTAB <-- 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. Ramaiah, et al. Expires August 26, 2007 [Page 16] Internet-Draft TCP Security February 2007 8. Middlebox considerations 8.1. Middlebox that resend RST's Consider a middlebox M-B tracking connections between two TCP endhosts E-A and E-C. If E-C sends a RST with a sequence number that is within the window but not an exact match to reset the connection and M-B does not have the fix recommended in this document, it may clear the connection and forward the RST to E-A saving an incorrect sequence number. If E-A does not have the fix the connection would get cleared as required. However if E-A does have the required fix, it will send a challenge ACK to E-C. M-B, being a middlebox, may intercept this ACK and resend the RST on behalf of E-C with the old sequence number. This RST, will again, not be acceptable and may trigger a challenge ACK. The above situation may result in a RST/ACK war. However, we believe that if such a case exists in the Internet, the middle box design does not comply to [RFC0793]. [RFC0793] dictates that the sequence number of a RST has to be derived from the acknowledgment number of the incoming ACK segment. It is outside the scope of this document to suggest mitigations to the ill-behaved middleboxes. Consider a similar scenario where the RST from M-B to E-A gets lost, E-A will continue to hold the connection and E-A might send an ACK an arbitrary time later after the connection state was destroyed at M-B. For this case, M-B will have to cache the RST for an arbitrary amount of time till until it is confirmed that the connection has been cleared at E-A. 8.2. Middleboxes that advance sequence numbers Some middleboxes may compute RST sequence numbers at the higher end of the acceptable window. The scenario is the same as the earlier case, but in this case instead of sending the cached RST, the middlebox (M-B) sends a RST that computes its sequence number as the sum of the ack 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. Ramaiah, et al. Expires August 26, 2007 [Page 17] Internet-Draft TCP Security February 2007 TCP A Middlebox 1. ESTABLISHED <-- <-- CLOSED 2. ESTABLISHED --> --> CLOSED 3. ESTABLISHED <-- <-- CLOSED 4. ESTABLISHED --> --> CLOSED 5. ESTABLISHED <-- <-- 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. Ramaiah, et al. Expires August 26, 2007 [Page 18] Internet-Draft TCP Security February 2007 9. Security Considerations These changes to the TCP state machine do NOT protect an implementation from on-path attacks. It also needs to be emphasized that while mitigations within this document make it harder for off- path attackers to inject segments, it does NOT make it impossible. The only way to fully protect a TCP connection from both on and off path attacks is by using either IPSEC-AH [RFC4302] or IPSEC-ESP [RFC4303]. implementers also should be aware that the attacks detailed in this specification are not the only attacks available to an off-path attacker and that the counter measures described herein are not a comprehensive defense against such attacks. In particular, administrators should be aware that forged ICMP messages provide off-path attackers the opportunity to disrupt connections or degrade service. Such attacks may be subject to even less scrutiny than the TCP attacks addressed here, especially in stacks not tuned for hostile environments. It is important to note that some ICMP messages, validated or not, are key to the proper function of TCP. Those ICMP messages used to properly set the path maximum transmission unit are the most obvious example. There are a variety of ways to choose which, if any, ICMP messages to trust in the presence of off-path attackers and choosing between them depends on the assumptions and guarantees developers and administrators can make about their network. This specification does not attempt to do more than note this and related issues. In any case, this RFC details only part of a complete strategy to prevent off-path attackers from disrupting services that use TCP. Administrators and implementers should consider the other attack vectors and determine appropriate mitigations in securing their systems. Another consideration that should be noted is, 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. Ramaiah, et al. Expires August 26, 2007 [Page 19] Internet-Draft TCP Security February 2007 10. IANA Considerations This document contains no IANA considerations. Ramaiah, et al. Expires August 26, 2007 [Page 20] Internet-Draft TCP Security February 2007 11. Contributors Mitesh Dalal and Amol Khare of Cisco Systems came up with the solution for the RST/SYN attacks. Anantha Ramaiah and Randall Stewart of Cisco Systems discovered the data injection vulnerability and together with Patrick Mahan and Peter Lei of Cisco Systems found solutions for the same. Paul Goyette, Mark Baushke, Frank Kastenholz, Art Stine and David Wang of Juniper Networks provided the insight that apart from RSTs, SYNs could also result in formidable attacks. Shrirang Bage of Cisco Systems, Qing Li and Preety Puri of Wind River Systems and Xiaodan Tang of QNX Software along with the folks above helped in ratifying and testing the interoperability of the suggested solutions. Ack throttling was introduced to this document by combining the suggestions from the tcpm working group. Ramaiah, et al. Expires August 26, 2007 [Page 21] Internet-Draft TCP Security February 2007 12. Acknowledgments Special thanks to Mark Allman, Ted Faber, Steve Bellovin, Vern Paxson, Allison Mankin, Sharad Ahlawat, Damir Rajnovic, John Wong, Joe Touch, and other members of the tcpm WG members for suggestions and comments. Ramaiah, et al. Expires August 26, 2007 [Page 22] Internet-Draft TCP Security February 2007 13. References 13.1. Normative References [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC4302] Kent, S., "IP Authentication Header", RFC 4302, December 2005. [RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, December 2005. 13.2. Informative References [I-D.ietf-tcpm-tcp-antispoof] Touch, J., "Defending TCP Against Spoofing Attacks", draft-ietf-tcpm-tcp-antispoof-05 (work in progress), October 2006. [Medina05] Medina, A., Allman, M., and S. Floyd, "Measuring the Evolution of Transport Protocols in the Internet. ACM Computer Communication Review, 35(2), April 2005. http://www.icir.org/mallman/papers/tcp-evo-ccr05.ps (figure 6)". [NISCC] NISCC, "NISCC Vulnerability Advisory 236929 - Vulnerability Issues in TCP". [RFC1122] Braden, R., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, October 1989. [RFC1323] Jacobson, V., Braden, B., and D. Borman, "TCP Extensions for High Performance", RFC 1323, May 1992. [RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP MD5 Signature Option", RFC 2385, August 1998. [RFC3562] Leech, M., "Key Management Considerations for the TCP MD5 Signature Option", RFC 3562, July 2003. [RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway Protocol 4 (BGP-4)", RFC 4271, January 2006. Ramaiah, et al. Expires August 26, 2007 [Page 23] Internet-Draft TCP Security February 2007 [SITW] Watson, P., "Slipping in the Window: TCP Reset attacks, Presentation at 2004 CanSecWest http://www.cansecwest.com/archives.html". Ramaiah, et al. Expires August 26, 2007 [Page 24] Internet-Draft TCP Security February 2007 Authors' Addresses Anantha Ramaiah Editor 170 Tasman Drive San Jose, CA 95134 USA Phone: +1 (408) 525-6486 Email: ananth@cisco.com Randall R. Stewart Editor 4875 Forest Drive Suite 200 Columbia, SC 29206 USA Phone: +1 (803) 345-0369 Email: rrs@cisco.com Mitesh Dalal Editor 170 Tasman Drive San Jose, CA 95134 USA Phone: +1 (408) 853-5257 Email: mdalal@cisco.com Ramaiah, et al. Expires August 26, 2007 [Page 25] Internet-Draft TCP Security February 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). 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. 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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. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Ramaiah, et al. Expires August 26, 2007 [Page 26]