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Versions: 00 01                                                         
IETF Mobile IP Working Group                                    T. Aura
INTERNET DRAFT                                                Microsoft
Expires May 2002                                               J. Arkko
                                                               Ericsson
                                                          November 2001
                     MIPv6 BU Attacks and Defenses
                  draft-aura-mipv6-bu-attacks-00.txt

Status of This Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026. 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.

Abstract

   This document overviews various attacks against mobile and fixed IP
   nodes by exploiting features of the Mobile IPv6 Route Optimization.
   Many of the attacks can be prevented by authenticating the Mobile
   IPv6 Binding Updates (BU) but some cannot, and some denial-of-
   service attacks specifically exploit features of authentication
   protocols. The purpose of this document is to list attacks that
   should be taken into consideration when designing protocols for BU
   authentication and to outline available protection mechanisms. We
   also discuss the choice between different levels of protection.













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                        Table of Contents

Status of This Memo...............................................1
Abstract..........................................................1
1. Introduction...................................................3
1. Attacks that Corrupt Routing Tables............................3
 1.1. Spoofing Binding Updates....................................4
 1.2. Attacks against Secrecy and Integrity.......................4
 1.3. Basic Denial of Service Attacks.............................5
 1.4. Replaying and Blocking Binding Updates......................5
 1.5. Bombing CoA with Unwanted Data..............................6
2. Authentication of Binding Updates..............................7
 2.1. Public Key Authentication...................................7
 2.2. Two Independent Routes......................................8
 2.3. Reducing the Number of Attackers and Targets................9
3. DoS Attacks against BU Authentication.........................11
 3.1. Inducing Unnecessary Binding Updates.......................11
 3.2. Consuming Authentication Resources.........................12
 3.3. Forcing Non-Optimized Routing..............................13
4. Preventing Resource Exhaustion................................13
 4.1. Delaying Commitment........................................14
 4.2. Controlling Damage.........................................15
 4.3. Favoring Regular Customers.................................15
5. The Right Level of Protection.................................16
 5.1. Prioritizing the Goals.....................................16
 5.2. Multiple Levels of Authentication..........................18
 5.3. Home Agent and Binding Acknowledgments.....................19
6. Security Considerations.......................................20
7. Conclusions...................................................20
Acknowledgments..................................................20
References.......................................................21
Authors' Addresses...............................................22



















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1.   Introduction

   This document describes attacks against Mobile IPv6 [JP00] Route
   Optimization and related protection mechanisms. The goal of the
   attacker can be to corrupt the correspondent host's routing table
   (the Binding Cache) and to cause packets to be routed to a wrong
   address. This can compromise secrecy and integrity of communication
   and cause denial-of-service (DoS) both at the address that does not
   receive the wanted packets and at the one that receives the
   unwanted packets. The attacker may also exploit features of a
   Binding Update (BU) protocol to exhaust the resources of either the
   mobile or the correspondent. The aim of this document is to
   describe the major attacks and to overview various protocol
   mechanisms and their limitations.

   In particular, we want to make known several attacks which should
   be considered when designing a protocol for authenticating BUs but
   which have not received sufficient attention at the Working Group
   (e.g. they are not mentioned in [MPH+01]). First, data flows can be
   redirected to flood a third party who is not taking part in the BU
   protocol (Section 2.5). Second, an attacker can consume the
   resources of any mobile or correspondent by inducing authentic but
   unnecessary Binding Updates (Section 4.1). Third, some proposed BU
   authentication protocols can be broken by attackers located on the
   route from the correspondent to the Home Agent (Section 3.2). Some
   of these attacks may be acceptable or too expensive to prevent, but
   their existence should be clearly acknowledged.

2.   Attacks that Corrupt Routing Tables

   Route optimization and Binding Updates create a new opportunity for
   attackers. By sending false BUs, they can create false entries in
   the correspondent host's Binding Cache and, thus, reroute IP
   packets to wrong destinations. If the data in the packets is not
   protected cryptographically, this can lead to compromise of secrecy
   and integrity. The attacker may also cause denial-of-service by
   keeping data from arriving at the right destination and by bombing
   a target host or network with unwanted data.

   We consider only active attackers. The rationale behind this is
   that in order to corrupt a routing table, the attacker must sooner
   or later send one or more messages. Thus, it makes little sense to
   consider attackers that only observe messages but do not send any.
   In fact, the active attacks are easier to for the average attacker
   than passive ones would be. In most active attacks, the attacker
   can initiate an unauthentic BU protocol execution at any time and
   it does not need to wait for a suitable messages to be sent by the
   targets hosts.



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2.1.     Spoofing Binding Updates

   If Binding Updates are not authenticated, an attacker can send
   spoofed BUs. All Internet hosts are vulnerable to this attack
   because they all must support the correspondent functionality.
   There is also no way of telling which addresses belong to mobile
   hosts that really could send BUs. Consider an IP host A sending IP
   packets to another IP host B. The attacker can redirect the packets
   to an arbitrary address C by sending to A a Binding Update where
   the HoA is B and the CoA is C. After receiving this BU, A will send
   all packets intended for B to the address C.

   The attacker may select the CoA to be either its own current
   address (or another address in its local network) or any other IP
   address. If the attacker selects a local CoA where it can receive
   packets, it will be able to send further packets to a
   correspondent, which the correspondent believes to be coming from
   the mobile. Ingress filtering at the attacker's local network does
   not prevent the spoofing of Binding Updates but forces the attacker
   either to choose a CoA from inside its own network or to use the
   Alternate CoA sub-option. This may make it easier for the attack
   targets to selectively filter the spurious BUs at a firewall.

   The correspondent stores the HoA-CoA pair in its Binding Cache. The
   attacker needs to send a new BU every few minutes to refresh the
   cache entry.

   The attacker needs to know or guess the IP addresses of both the
   sender and receiver. This means that it is difficult to redirect
   all packets to or from a host because the attacker would need to
   know the IP addresses of all the hosts with which it is
   communicating. Hosts with well-known addresses, such as servers and
   those using stateless auto-configuration, are most vulnerable.
   Hosts that are a part of the network infrastructure, such as DNS
   servers, are particularly interesting targets for attackers.

   Hosts with frequently changing random addresses and no DNS names
   are relatively safe. However, hosts that visit publicly accessible
   networks such as airport WLANs risk revealing their addresses. IPv6
   addressing privacy features mitigate these risks to an extent but
   it should be noted that addresses cannot be completely recycled
   while there are still open sessions that use those addresses.

2.2.     Attacks against Secrecy and Integrity

   By spoofing Binding Updates, an attacker can redirect packets
   between two IP hosts to itself. By sending a spoofed BU to A, it
   can capture the data intended to B.  It can pretend to be B and
   highjack B's connections with A, or establish new spoofed


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   connections. The attacker can also send spoofed BUs to both A and B
   and insert itself to the middle of all connections between them
   (man-in-the-middle attack). Consequently, the attacker is able to
   see and modify the packets sent between A and B. The attacks are
   possible if the target host or its correspondent supports Route
   Optimization and the attacker knows their IP addresses.

   Strong encryption and integrity protection, such as authenticated
   IPSec, can prevent all the attacks against data secrecy and
   integrity. When the data is cryptographically protected, spoofed
   BUs can result in denial of service but not in disclosure or
   corruption of sensitive data beyond revealing the existence of the
   traffic flows. Two fixed hosts could also protect communication
   between themselves by refusing to accept BUs from each other.
   Ingress filtering, on the other hand, does not help because the
   attacker is using its own address as the CoA and is not spoofing
   source IP addresses.

   The attacks described above are a serious threat to the Internet
   for two reasons. First, a lot of Internet traffic is unprotected.
   Second, an attacker anywhere on the network can mount the attacks
   against any Internet hosts, also non-mobile ones. If Binding
   Updates did not exist, the attacker would need to be on the route
   between the attack targets in order to listen to the traffic
   between them.

2.3.     Basic Denial of Service Attacks

   By sending spoofed BUs, the attacker can redirect all packets sent
   between two IP hosts to a random or nonexistent address. This way,
   it may be able to stop or disrupt communication between the hosts.
   The requirements are that the target host or its correspondent must
   support Route Optimization and the attacker must know their IP
   addresses.

   The attack is serious because any Internet host can be targeted,
   also fixed hosts. Hosts belonging to the infrastructure necessary
   for other hosts to communicate are also vulnerable. Again, two
   hosts can protect the communication between themselves by refusing
   BUs from each other or by establishing an authenticated IPSec
   tunnel for the BUs.

2.4.     Replaying and Blocking Binding Updates

   Any protocol for authenticating BUs will have to consider replay
   attacks. That is, an attacker may be able to replay recent
   authenticated BUs to the correspondent and, that way, direct
   packets to the mobile host's previous location. Like spoofed BUs,
   this can be used both for capturing packets and for DoS. The


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   attacker can capture the packets and impersonate the mobile node if
   it reserves the mobile's previous address after the mobile has
   moved away and then replays the previous BU to redirect packets
   back to the previous location.

   The replays are a concern if a timestamps are used for checking the
   freshness of BUs and the mobile is moving so frequently that it
   sends the next BU before the timestamp in the previous BU has
   expired. Sequence numbers in authenticated BUs usually prevent the
   attack. The authentication protocol needs to be is carefully
   designed to avoid more complex replay attacks.

   In a related attack, the attacker blocks binding updates from the
   mobile at its new location, e.g. by jamming the radio link or by
   mounting a flooding attack, and takes over its connections at the
   old location. The attacker will be able to capture the packets sent
   to the mobile and to impersonate the mobile until the
   correspondent's Binding Cache entry expires.

   Both of the above attacks require the attacker to be on the same
   local network with the mobile, where it can relatively easily
   observe packets and block them even if the mobile does not move to
   a new location. Therefore, we believe that these attacks are not as
   serious as ones that can be mounted from remote locations.

2.5.     Bombing CoA with Unwanted Data

   By sending spoofed BUs, the attacker can redirect traffic to an
   arbitrary IP address. This can be used to bomb an arbitrary
   Internet address with excessive amounts of data. The attacker can
   also target a network by redirecting data to one or more IP
   addresses within the network.

   In the simplest attack, the attacker knows that there is a heavy
   data stream from host A to B and redirects this to the target
   address C. A will soon stop sending the data because it is not
   receiving acknowledgments from B.

   A more sophisticated attacker acts itself as B. It first subscribes
   to a data stream (e.g. a video stream from a news web site) and
   then redirects this to the target address C. The attacker may even
   be able to spoof the acknowledgements. For example, consider a
   constant-rate TCP stream. The attacker performs the TCP handshake
   itself and thus knows the initial sequence numbers. After
   redirecting the data to C, it suffices to send one spoofed
   acknowledgment per window. In a constant-rate stream, the attacker
   is able to predict the sequence numbers, the window size, and the
   right times for sending the acknowledgments. (The sender A is



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   likely to ignore any ICMP Destination Unreachable messages if it
   also receives acknowledgments.)

   This way, the attacker is able to redirect a steady stream of
   unwanted data to the target address without doing much work itself.
   It can carry on opening more streams and refresh the Binding Cache
   entries by sending a new BU every few minutes.

   The attack is serious because the target can be any host or
   network, not only mobile one. What makes it particularly serious
   compared to the other attacks is that the target itself cannot do
   anything to prevent the attack. For example, it does not help if
   the target network stops using Route Optimization. The damage is
   the worst if these techniques are used to amplify the effect of a
   distributed denial of service (DDoS) attack. Ingress filtering in
   the attacker's local network prevents the spoofing of source
   addresses but the attack is still possible by setting the Alternate
   CoA sub-option to the target address.

   The attacker needs to find a correspondent that is willing to send
   data streams to unauthenticated recipients. Many popular web sites
   provide such streams. If the target is a single host, the attacker
   needs to know or guess the target's IP address. On the other hand,
   if the target is an entire network, the attacker can congest the
   link toward that network by bombing random addresses within its
   routing prefix or group of prefixes. In some cases, a firewall on
   the border of the target network may be able filter out data that
   is sent to nonexistent addresses. Whether this may be possible
   depends on the way that the addresses within that network are
   managed. It seems likely that e.g. IPv6 addressing privacy features
   would preclude such filtering in the general case.

3.   Authentication of Binding Updates

   In order to prevent the corruption of correspondent routing tables,
   the Binding Updates must be authenticated. In this section, we
   discuss both strong and weak authentication methods.

3.1.     Public Key Authentication

   The assumption in Mobile IPv6, as in many other systems, seems to
   have been that generic security mechanism such as IPSec, IKE, and a
   public-key infrastructure (PKI) will eventually solve all
   authentication issues. The current lack of suitable BU
   authentication protocol can thus be seen as a direct consequence of
   the failure of global PKIs. Also, it turns out to be difficult to
   mix strong BU authentication with weaker schemes (see Section 6.2).
   There are nevertheless situations where it is possible, and
   advisable, to follow the original plan. In closed user groups and


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   high-security environments, it may be possible to set up a PKI and
   require BUs to be strongly authenticated.

   A recently discovered technique [OR01] provides an intermediate
   level of security between strong public-key authentication and weak
   methods such as routability tests. The idea is to form the last 64
   bits of the IP address (the interface identifier) by hashing the
   host's public signature key. Binding Updates can then be signed
   with this key. A secure one-way hash function makes it difficult
   for the attacker to come up with a key that matches a given address
   and to forge signed BUs.

   The main weakness of the scheme is that only 62-64 bits of the IP
   address can be used for the hash. Thus, an attacker may be able to
   mount a brute force attack and find a matching signature key by
   trial and error. It is relatively expensive to generate strong
   signature keys but the attacker does not need to care about the
   strength of the keys. There may be relatively fast ways of
   generating weak signature keys, which the correspondent is not able
   to tell apart from strong ones. In order to make such brute-force
   attacks less attractive, one should include the routing prefix of
   the network into the hash. This forces the attacker to perform the
   search separately for each prefix. Without this, it a global table
   of all hash values and their corresponding keys might become
   feasible in the future, even if that seems out of practical reach
   given current storage technology.

   Another shortcoming of the CAM-type addresses is that although they
   prevent the theft of another host's address, they do not stop the
   attacker from inventing new false addresses with an arbitrary
   routing prefix. The attacker can generate a public key and a
   matching IP address in any network and use it to mount bombing
   attacks (as described in Section 2.5).

   While the public-key protocols provide a reasonable protection
   against unauthentic BUs, they expose the correspondent to denial-
   of-service attacks (see Section 4.2).

3.2.     Two Independent Routes

   Some weak BU authentication schemes have been proposed (Bake
   [NP01], HA Cookies [TO01]) where the security essentially depends
   on sending two pieces of the authentication data between the
   correspondent and the mobile or its Home Agent through two
   independent routes and hoping that most attackers are unable to
   capture both of them. This may not help as much as has perhaps been
   hoped. In all these protocols, a single attacker on the route
   between the correspondent and Home Agent can spoof BUs by
   pretending to be both the mobile and its Home Agent. If the


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   attacker uses its own address as the false CoA, it can spoof
   packets from both the mobile and the Home Agent to the
   correspondent, and it can receive messages sent by the
   correspondent to both HoA and CoA.

   Without ingress filtering at the attacker's local network, the
   attacker is, in fact, able to select an arbitrary CoA. Ingress
   filtering at the attacker's network forces the attacker to use a
   CoA from its local network. (We assume that the Alternate CoA sub-
   option does cannot be used as a pert of these protocols.) Ingress
   filtering also prevent attacks against the HA Cookie protocol
   because the attacker could not spoof packets from Home Agent to the
   correspondent.

   A false sense of security is created if one assumes that the three
   sides of the triangle in MIPv6 triangle routing are independent
   routes. This is usually the case for an authentic mobile, but need
   not be true not for an attacker who claims to be the mobile. When
   the false mobile (i.e. the attacker) is on the route between the
   correspondent and the Home Agent, the triangle collapses.

   On the other hand, if one finds it a reasonable assumption that the
   attacker cannot listen to traffic between the correspondent and the
   Home Agent, then an equivalent level of security is achieved by a
   much simpler protocol: the correspondent sends a secret key in
   plaintext to HoA and Home Agent forwards it through a secure tunnel
   to the mobile. (See the next section.)

3.3.     Reducing the Number of Attackers and Targets

   Instead of trying to prevent all attacks, the best strategy is
   often to limit the number of potential attackers that can attack a
   particular target, and to reduce the number of targets a potential
   attacker can threaten. Such weak methods can be used either when
   the stronger ones are too expensive or to fix shortcomings of the
   stronger protocols. Limiting the number of attackers is also
   essential in defending against denial-of-service attacks on the
   Internet, because it is rarely possible to entirely prevent the
   attacks.

   Some proposed BU authentication protocols make the assumption that
   the communication between two specific hosts, e.g. on the route
   between the correspondent and the mobile's home agent, is safe from
   attackers, even though it is not cryptographically protected. As
   mentioned above, this assumption may be reasonable because the
   average Internet host cannot listen to or modify packets on the
   specific routers. Moreover, even an attacker in control of controls
   some routers can only interfere with communication between a
   limited number of hosts because most Internet traffic will not be


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   routed through the compromised routers. The assumption can be
   justified also by the fact that an attacker on the route between
   two fixed hosts (the mobile at home and the same correspondent) can
   mount equally damaging attacks against the communication between
   them.

   Another way to limit the number of attackers and their targets is
   to test the routability of both the HoA and CoA. That is, the
   correspondent sends packets to both addresses and accepts Binding
   updates only from mobiles that are able to receive them. Some
   malicious entities may be able to capture both packets but most
   Internet users do not have such capabilities. A typical attacker is
   able to attack only the correspondents in its own local network.
   The routability test is, in fact, a variation of the cookie
   exchange, which has been used as part of the TCP handshake [RAOA01]
   and in authentication protocols, including Photuris [KS99] and IKE
   [HC98]. The routability test is particularly usable in combination
   with the CAM-type addresses (see Section 3.1) because it can
   prevent the bombing of a CoA with unwanted data. A reply to the
   routability test indicates that someone who can receive packets
   sent to the CoA wants to receive data from the correspondent.

   Yet another idea that has been floating around is that if the
   mobile sends a session key insecurely to the correspondent at the
   beginning of a connection, the key can be used to authenticate
   subsequent BUs (so called leap-of-faith authentication).
   Unfortunately, this fails even if we assume that attacker is
   unlikely to capture the keys sent by authentic mobiles. First, the
   attacker can send its false key before the authentic mobile sends
   the authentic key. Second, there must be a recovery mechanism for
   situations where the mobile or the correspondent loses its state,
   and the attacker can exploit this mechanism. Third, the attack may
   impersonate a random IP addresses in order to mount a variety of
   DoS attack against the correspondent. The leap-of-faith
   authentication is suitable for situations where a human user, or
   some other factor outside the attacker's control, at random times
   initiates the protocol. The party making the leap must always be
   the one that initiates the protocol. In such situations, it may be
   reasonable to argue that an attacker is unlikely to be present at
   the time of the key exchange. In BU authentication, the protocol is
   usually initiated by the mobile but the leap in faith should be
   made by the correspondent. Also, the attacker can trigger the BU
   protocol at any time by sending a spoofed packet from the
   correspondent to the mobile's HoA.

   Ingress filtering is another way of limiting the number potential
   of attackers and their targets. As we have noted above, many of the
   attacks against Route Optimization involve spoofed source IP
   addresses and are, thus, prevented by ingress filtering. There are


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   two well-known weaknesses in this thinking, though. Firsts, ingress
   filtering must be applied on the attacker's local network; on the
   target network it makes no difference. Second, the Home Address
   Option and the Alternate Care-of Address sub-option can be used for
   similar source spoofing. While it is advisable to apply ingress
   filtering in as many networks as possible, one cannot rely on this
   to stop all attacks against Mobile IPv6.

4.   DoS Attacks against BU Authentication

   Security protocols that successfully protect the secrecy and
   integrity of data can sometimes make the participants more
   vulnerable to denial-of-service attacks. In fact, the stronger the
   authentication, the easier it may be for an attacker to use the
   protocol features to exhaust the mobile's or the correspondent's
   resources.

4.1.     Inducing Unnecessary Binding Updates

   When a mobile host receives an IP packet from a new correspondent
   via the HoA, it automatically sends a Binding Update to that
   correspondent. The attacker can exploit this by sending the mobile
   spoofed IP packets (e.g. ping or TCP SYN packets) that appear to
   come from different correspondent addresses. The attacker will
   automatically start the BU protocol with these correspondents. If
   the correspondent addresses are real addresses of existing IP
   hosts, then most instances of the BU protocol will complete
   successfully. The entries created in the Binding Cache are correct
   but useless. This way, the attacker can with little work induce the
   mobile to execute the BU protocol unnecessarily, which can drain
   the mobile's resources.

   A correspondent (i.e. any IP host) can also be attacked in a
   similar way. The attacker sends spoofed IP packets to a large
   number of mobiles with the target host's address as the source
   address. These mobiles will all send Binding Updates to the target
   host. Again, most of the BU protocol executions will complete
   successfully. By inducing a large number of unnecessary BUs, the
   attacker is able to consume the target host's resources.

   This attack is possible against any BU authentication protocol. The
   more resources the Binding Update protocol consumes, the more
   serious the attack. Hence, strong cryptographic authentication
   protocol is more vulnerable to the attack than a weak one or
   unauthenticated BUs.

   A host should protect itself from the attack by setting a limit on
   the amount of resources, i.e. processing time, memory, and
   communications bandwidth, which it uses for processing BUs. When


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   the limit is exceeded, the host can simply stop Route Optimization.
   (See Section 5.2.) Sometimes it is possible to process some BUs
   even when a host is under the attack. A mobile host may have a
   local security policy listing a limited number of addresses to
   which BUs will be sent even when the mobile host is under DoS
   attack. A correspondent (i.e. any IP host) may similarly have a
   local security policy listing a limited set of addresses from which
   BUs will be accepted even when the correspondent is under a DoS
   attack. The host may also recognize addresses with which they have
   had meaningful communication in the past and sent BUs to or accept
   them from those addresses. (See Section Error! Reference source not
   found..)

   Ingress filtering at the attacker's local network mitigates the
   attacks. When flooding a mobile, the attacker can only choose
   correspondent addresses in its own local network. When flooding a
   correspondent, the attacker can only target correspondents in its
   own network. Unfortunately, the Home Address Option (HAO) can be
   used to circumvent the ingress filtering and to spoof packets from
   any correspondent. It has been questioned whether the HAO should be
   allowed in all packets, or only on those that are associated with
   an already successfully created Binding Cache Entry. The latter
   would prevent the use of HAOs in DoS attacks against BU
   authentication.

   In the following section, we will explain some additional DoS
   attacks and defense mechanisms. However, these attacks are
   generally no more serious than the ones described in this section.
   It usually does not pay to defend against the other types of
   attacks unless one can also prevent the attacks of this section.

4.2.     Consuming Authentication Resources

   Authentication protocols are often vulnerable to flooding attacks
   that exploit the protocol features to consume the target host's
   resources. Computing power is consumed by flooding the host with
   messages that cause it to perform expensive cryptographic
   operations. If a host creates state during protocol execution, it
   is also vulnerable to attacks where the attacker starts excessive
   numbers of protocol runs and never finishes them.

   In order to exhaust the computing power of modern processors, the
   attacker needs to get them to perform public-key cryptographic
   operations. To do this, they may, for example, spoof large numbers
   of signed messages where the signatures are replaced by random
   numbers. The target host must verify the signatures before
   rejecting the messages. Symmetric encryption, cryptographic hash
   functions, and non-cryptographic computation are rarely the
   performance bottleneck. However, if the cryptographic library is


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   optimized only for bulk data, it may behave inefficiently when the
   functions are invoked on millions of short messages and the keys
   are changed on every invocation.

   One way protocols can avoid the unnecessary public-key operations
   is to require a weak authentication, such as a cookie exchange,
   before doing the expensive computation. Attacks against stateful
   protocols can be prevented by making the protocol parties stateless
   until the honesty of the other participant has been proved or by
   designing the stare management with flooding attacks in mind. (See
   Section 5.1.)

4.3.     Forcing Non-Optimized Routing

   If the BUs are not authenticated, an attacker can prevent a
   correspondent from using Route Optimization by filling its Binding
   Cache with false entries so that most entries for real mobiles are
   dropped. With authenticated BUs, the attacker can mount the same
   attack by inducing unnecessary Binding Updates that create
   unnecessary cache entries (see Sec. 4.1).

   Any successful DoS attack against a mobile or a correspondent can
   also prevent the processing of BUs. We have repeatedly suggested
   that the target of a DoS attack may respond by stopping Route
   Optimization for all or some communication. Obviously, an attacker
   can exploit this fallback mechanism and force the target to use the
   less efficient triangle routing. The attacker only needs to mount a
   noticeable DoS attack against the mobile or correspondent, and the
   target will default to non-optimized routing.

   The target host can mitigate the effects of the attack by reserving
   more space for the Binding Cache, by reverting to non-optimized
   routing only when it cannot otherwise cope with the DoS attack, by
   trying aggressively to return to optimized routing, and by favoring
   mobiles with which it has an established relationship. This attack
   is not as serious as the ones described earlier, but applications
   that rely on Route Optimization could still be affected. For
   instance, conversational multimedia sessions can suffer drastically
   from the additional delays caused by triangle routing.

5.   Preventing Resource Exhaustion

   In this section, we describe defenses against denial-of-service
   attacks that exploit features of the BU protocol to exhaust the
   target host's resources. It is usually impossible to completely
   prevent DoS attacks and the right approach may be to increase the
   cost and difficulty of the attacks and to mitigate their effects.




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5.1.     Delaying Commitment

   A standard protection against DoS attacks is to delay committing
   one's resources to the protocol until the other party has provided
   some assurance about its honesty. This assurance could, for
   example, be authentication or expensive computation of resources by
   the other party.

   Expensive public-key operations can be delayed by starting with a
   weak but inexpensive authentication mechanism and by proceeding
   with the public-key operations only after the weak authentication
   succeeds [Mea99]. This either limits the number of attackers who
   can get to the public-key stage or increases the cost of attack by
   forcing the attacker to break the weak mechanism. For example, a BU
   authentication protocol could start with a cookie exchange or,
   preferably, with a routability test for both HoA and CoA (see
   Section 3.3), and continue with a public-key authentication if the
   routability test succeeds.

   Memory space for storing protocol state is another resource that
   attackers can exhaust. The conventional way to defeat attacks that
   consume state storage is to reserve enough memory for storing the
   state data and to manage the storage efficiently. Another method is
   to remain stateless until the authentication is complete [AN97a].
   In particular, hosts with little memory and implementations aiming
   for simplicity are likely to find the stateless approach easier. We
   therefore recommend the that BU authentication protocol should
   allow stateless implementation of the correspondent. On the other
   hand, there is no compelling reason to require statelessness for
   hosts that can manage the state data in other ways.

   There are some difficulties in making the BU authentication
   protocol stateless, which should be understood. The main difficulty
   is that usually only the responder can be stateless, and it is not
   clear which party initiates the Binding Update process and which
   one responds. While the mobile normally initiates the
   authentication, this may be triggered by a packet belonging to
   another protocol that arrived from the correspondent. Moreover, if
   a packet from the correspondent triggers the BU, the correspondent
   may not know that this was the case. A further complication is that
   once an upper layer protocol has created a protocol state, it is of
   little value to refrain from doing so in the lower layers, but the
   IP layer cannot know when this is happening. For simplicity, we
   recommend making the correspondent stateless and advice against
   trying to guess which party initiated the protocol and whether the
   statelessness is necessary or not. We choose protection of the
   correspondent over the mobile for two reasons. First, any Internet
   host can be a correspondent. Second, it is less practical to make
   the mobile stateless because the mobile usually does not


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   authenticate the correspondent and, thus, there is no clear point
   after which it is safe to create a state. We acknowledge that the
   mobile is likely to have more stringent memory constraints than the
   correspondent and it may be left vulnerable to the state storage
   exhaustion.

   Cryptographic puzzles [JB99][ANL00] are another proposed protection
   against resource-exhaustion attacks. A server requires its clients
   to solve a cryptographic puzzle (e.g. brute-force search for some
   input bits of a one-way function) before committing its own
   resources to the protocol. The server can adjust the difficulty of
   the puzzles according to its load. Solving the puzzle creates a
   small cost for each protocol invocation, which makes flooding
   attacks expensive but has little effect on honest hosts.
   Unfortunately, there are several drawbacks to this strategy in the
   Mobile IP protocol. First, the IP layer does not know which one of
   the nodes is the server (i.e. the respondent), the mobile or the
   correspondent. Second, mobile nodes can have extremely limited
   processor and battery capacity, which makes the cost of solving
   puzzles too high for them.  The puzzle protocols work well only
   when all clients have approximately equal computational capacity.

5.2.     Controlling Damage

   As we suggested in Section 4.1, a host can protect itself from
   flooding attacks by limiting the amount of resources it uses for BU
   authentication. It can set limits on the processor time, memory and
   communications capacity used for this purpose. When this limit is
   exceeded, the host should stop all further participation in BU
   authentication protocols. It may stop processing BUs and let all
   Binding Cache entries expire. Alternatively, it may continue to
   update entries already in the Binding Cache if there is a light-
   weight mechanism (e.g. a session key) for authenticating them. The
   effect of this is to stop Route Optimization for all communication,
   or only for new connections. The host may return to normal
   operation when it believes the attack is over.

   Since authentication cannot prevent all the attacks, every host
   SHOULD implement the limit on resource usage. A host that does not
   implement the limit will be vulnerable to a flooding attack but it
   will not cause any damage to other hosts.

5.3.     Favoring Regular Customers

   A correspondent can give priority to mobiles with which it has a
   long-term relationship or recent meaningful communication in the
   upper protocols layers. The mobile may similarly favor selected
   correspondent addresses. The best way to secure Binding Updates
   when the hosts have a long-term relationship is to send to use an


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   authenticated IPSec tunnel. The following techniques should only be
   used when it is not possible to configure an IPSec Security
   Association.

   A host's local policy may have a list of addresses or address
   ranges, to and from which BUs are processed even when the host is
   under stress from attacks. These should not include addresses for
   which it is feasible to establish long-term IPSec Security
   Associations. The number of addresses in the list should not be
   large or public because otherwise the attacker might be able to
   mount the attack by using only these addresses. Taking into account
   authentication and other displays of commitment in the upper
   protocol layers can be difficult to implement because it violates
   the stateless nature of the IP protocol layer and creates
   dependences between protocol layers. In some common situations, it
   may be worth while to violate the layering principle. For example,
   a server could accept BUs from its clients after it has
   successfully executed the TCP handshake.

   It may also help to keep updating the existing entries in the
   Binding Cache so that existing optimized routes can be maintained
   during the attack, although it is not certain that the existing
   cache entries belong to the most important mobiles or even to
   authentic ones. Some indication of this may be inferred from the
   packet counts associated with the traffic flowing through the
   entry.

6.   The Right Level of Protection

   We will conclude this document by discussing the criteria that
   should be used for selecting and comparing BU authentication
   protocols and issues that arise when there are several alternative
   protocols.

6.1.     Prioritizing the Goals

   The strength of the protection against DoS attacks that do not
   corrupt routing tables is independent of the strength of the BU
   authentication. As we have observed, strong authentication
   mechanisms may actually enable DoS attacks that would not otherwise
   be possible. Nevertheless, the following principles apply to both
   kinds of attacks.

   The attacks described in this document can be divided into three
   classes: The most serious attack is the bombing of third parties
   with unwanted data (Section 2.5) because they cannot do anything to
   stop the flood of data. The mobiles and correspondents MUST
   implement and use some kind of protection against this attack.



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   Hosts that do not want to implement such protection, MUST ignore
   all Binding Updates and, thus, cannot use Route Optimization.

   The attacks against correspondent hosts form the second most
   serious class of attacks. This includes attacks on data secrecy and
   integrity as well as denial-of-service attacks. Every IP host is a
   correspondent and if the Mobile IP protocol makes them vulnerable,
   then every IP host is vulnerable. It is important that a reliable
   protection mechanism is available for the correspondents and every
   IP host SHOULD implement and use it. If the protection for the
   correspondents requires support from mobiles, all mobile hosts MUST
   implement and use it.

   The third class includes all attacks against the mobiles. As with
   correspondents, it is important to have a protection mechanism for
   the mobiles and they SHOULD implement and use it. If the protection
   for the mobiles requires support from correspondents, all
   correspondents MUST implement and use such support.  Although
   attacks against mobile hosts cannot bring down the Internet as we
   know it, the number and significance of mobiles will increase. If
   Mobile IPv6 is to become the primary mobility protocol in the
   Internet, it is essential to protect its reliability against
   malicious parties. (Actually, there is a choice between defending
   only the Internet from Mobile IPv6 and defending also the
   reliability of Mobile IPv6 itself. We have chosen the former
   viewpoint. If one goes for the latter alternative, then it is
   unnecessary to prevent attack against the mobile hosts. This does
   not necessarily result in significant simplifications in the BU
   authentication protocols because most protections are needed in any
   case to protect the non-mobile hosts, i.e. correspondents and third
   parties.)

   The guiding principle above is that a protection mechanism MUST be
   implemented and used if the security of other hosts depends on it.
   If only the host's own security is at stake, it SHOULD implement
   and use the protection. However, some hosts, e.g. minimal
   implementations, MAY elect not to protect themselves against all
   the attacks as long as that does not compromise the protection for
   others. A host MAY always protect itself and others by ignoring all
   Binding Updates, which means giving up Route Optimization. A
   slightly smarter host MAY implement only the mandatory mechanisms
   that help protect others and stop using Route Optimization when it
   detects a DoS attack against itself.

   The cost of the defenses must not be excessive. In particular, the
   minimum requirements for correspondent nodes have to be low because
   all IP nodes must satisfy them. One possibility is to require
   everyone either to use a strong protocol or to refrain from using
   Route Optimization. The Route Optimization may, however, be crucial


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   for the performance of many low-end mobile hosts that cannot afford
   to implement strong authentication but receive audio or video
   streams.

   An ideal solution to BU authentication would allow a simple and
   cheap solution for the low-importance hosts and a more
   comprehensive protection for those who can afford it. A three level
   protection would appear to be particularly suitable to cover the
   needs of different situations:
   1. A method based on return routability and symmetric cryptography
     (similar to Bake).
   2. A method based on relationships of addresses and public keys
     (such as CAM or SUCV).
   3. Strong authentication through shared secrets and/or PKIs (e.g.
     IPSec/IKE).
   Unfortunately, as we will explain below, it is difficult to
   accommodate multiple levels of protection without compromising
   everyone's security.

6.2.     Multiple Levels of Authentication

   As explained above, there is the temptation to allow multiple
   authentication mechanism of different strength and cost.
   Unfortunately, this does not necessarily result in any better
   security than if the weakest method were used alone. The same is
   true to protocols that provide different levels of DoS protection.

   The decision about accepting or not accepting a BU is made by the
   correspondent. Therefore, the decision process at the correspondent
   is crucial for any BU authentication mechanism. If weak and a
   strong authentication are alternatives, the correspondent will have
   to make the decision when to allow the weak BU authentication and
   when to require the strong method. This section discusses the ways
   in which the correspondent can make the decision. The same
   principles apply to the combination of any stronger and weaker BU
   authentication mechanism regardless of their absolute strength and
   technical details.

   The correspondent MAY have a local policy that lists the HoA
   addresses or address ranges for which weak protection is allowed.
   For example, a correspondent could be configured to allow weak BU
   authentication for some low-end mobile devices that benefit from
   Route Optimization but cannot afford to run the stronger
   authentication protocol.

   It is not possible to negotiate the level of protection online
   between the mobile and the correspondent. If an attacker can
   impersonate the mobile in the weaker mechanism, it can always
   negotiate to choose the weak mechanism. A strong authentication


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   would be required to prove that the authentic mobile agrees to use
   a weak one. Therefore, the list of mobiles for which the
   correspondent allows weak BU authentication MUST either be
   statically configured into the correspondent's local policy, or
   dynamically determined by strong authentication or equivalent
   commitment shown in upper protocol layers.

   On the other hand, weak BU authentication in one correspondent does
   not compromise strong authentication in another correspondent. Even
   if the attacker defeats the weak mechanism, it cannot redirect
   packets to or from correspondents that use a strong protocol.
   Therefore, a degree of separation exists between the decisions of
   different nodes, and the decisions of one correspondent do not
   affect the strength of BU authentication at other correspondents.
   However, this separation does not apply to most DoS attacks and
   protection for the other party and for third parties must always be
   implemented, as explained in the previous section.

   The correspondents that do not allow weak BU authentication are
   unable use Route Optimization with low-end mobiles that do not
   implement the stronger mechanism.
   From the above it follows that if a public web site or other server
   does not register its clients, it must choose either strong or weak
   BU authentication for everyone. If it allows a weak mechanism, it
   is unnecessary to implement a strong one. On the other hand,
   different servers may choose the strength of BU authentication
   mechanisms independently. However, the economics of the Internet
   may force practically every public server to select the weakest
   allowed mechanism, which means that Mobile IPv6 may always remain
   vulnerable to attacks.

   In conclusion, there are technical and business reasons that will
   force most IP nodes to use the weakest level of authentication that
   is mandatory to implement and use in all IP hosts. We therefore
   recommend that the weakest allowed authentication should be fairly
   strong. There are nevertheless situations, such as closed groups
   and networks and high-security environments, where it is possible
   to deploy weaker or stronger mechanism than the minimum required by
   the standard, as long a single level of security is used
   consistently and outsiders on the Internet are not exposed to DoS
   attacks.

6.3.     Home Agent and Binding Acknowledgments

   We assume that the mobile has chosen a reliable Home Agent and that
   Binding Updates and Acknowledgments between the mobile and its Home
   Agent are strongly authenticated with techniques outside the scope
   of this document. For example, there could be a manually keyed
   IPSec tunnel to a router on the mobile's Home Network. There are


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   situations where no secure tunnel exist, for example if the mobile
   automatically contracts the services of temporary Home Agents along
   its path, but we feel that the security in such situations is based
   on much weaker assumptions and should be considered separately.

   It seems necessary to send the Binding Acknowledgments from other
   sources than the Home agent without authentication. In most BU
   authentication protocols, only the mobile is authenticated, and
   without authenticating the correspondent, there is no way of
   authenticating the acknowledgments. The consequence is that an
   attacker can send false acknowledgments and cause the mobile to
   send the next BU sooner or later than the correspondent expects. At
   worst, the effect is that the Binding Cache entry expires and Route
   Optimization is not used. Thus, an attacker anywhere on the network
   can prevent the use of Route Optimization by sending false Binding
   Acknowledgments. (Of course, if an authenticated IPSec tunnel
   between the mobile and the correspondent exists, it can be used
   also for the acknowledgments and the problem is solved.)

7.   Security Considerations

   This document discusses security of Mobile IPv6 Route Optimization
   but does not present any specific protocols or detailed solutions.
   A separate specification of the actual BU authentication protocol
   is needed.

   There are other security concerns with Mobile IPv6 that are not
   addresses in this document. The Home Agent Discovery needs to be
   secured so that the mobile can establish a secure tunnel to a
   reliable Home Agent. Also, some features of the Mobile IPv6
   protocol may reduce the effectiveness of ingress filtering. The
   Home Address Option, the Alternative CoA sub-option, and possibly
   IPv6 in IPv6 tunnels, may be used to spoof the origin of packets
   without spoofing the source IP address.

8.   Conclusions

   We have described attacks against Mobile IPv6 Route Optimization
   and mechanisms for protecting the protocol participants and third
   parties. Some of the attacks may be new in the sense that they have
   not been considered in existing BU authentication requirements and
   protocol drafts. It is our hope that this document will help in
   designing BU authentication protocols and in the process of
   choosing the protocol or protocols that will be part of the Mobile
   IPv6 standard. We are also working on a family of protocols that
   takes into account the points of this document [RAOA01].

Acknowledgments



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   Many of the ideas originate from Mike Roe, Greg O'Shea, and Pekka
   Nikander and various Internet Drafts.

References

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             montenegro-sucv-01.txt, IETF, July 2001. Work in
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Authors' Addresses

   Tuomas Aura
   7 J J Thompson Avenue
   Cambridge, CB3 0FB
   United Kingdom

   Phone: +44 1223 479708
   Email: tuomaura@microsoft.com

   Jari Arkko
   Oy LM Ericsson Ab
   02420 Jorvas
   Finland



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   Phone: +358 40 5079256
   Email: Jari.Arkko@ericsson.com

















































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