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rfc:rfc3756

Network Working Group P. Nikander, Ed. Request for Comments: 3756 Ericsson Research Nomadic Lab Category: Informational J. Kempf

                                                       DoCoMo USA Labs
                                                           E. Nordmark
                                         Sun Microsystems Laboratories
                                                              May 2004
       IPv6 Neighbor Discovery (ND) Trust Models and Threats

Status of this Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

 The existing IETF standards specify that IPv6 Neighbor Discovery (ND)
 and Address Autoconfiguration mechanisms may be protected with IPsec
 Authentication Header (AH).  However, the current specifications
 limit the security solutions to manual keying due to practical
 problems faced with automatic key management.  This document
 specifies three different trust models and discusses the threats
 pertinent to IPv6 Neighbor Discovery.  The purpose of this discussion
 is to define the requirements for Securing IPv6 Neighbor Discovery.

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
     1.1. Remarks . . . . . . . . . . . . . . . . . . . . . . . . .  3
 2.  Previous Work. . . . . . . . . . . . . . . . . . . . . . . . .  4
 3.  Trust Models . . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.1. Corporate Intranet Model. . . . . . . . . . . . . . . . .  5
     3.2. Public Wireless Network with an Operator. . . . . . . . .  6
     3.3. Ad Hoc Network. . . . . . . . . . . . . . . . . . . . . .  7
 4.  Threats on a (Public) Multi-Access Link. . . . . . . . . . . .  8
     4.1. Non router/routing related threats. . . . . . . . . . . .  9
          4.1.1. Neighbor Solicitation/Advertisement Spoofing . . .  9
          4.1.2. Neighbor Unreachability Detection (NUD) failure. . 10
          4.1.3. Duplicate Address Detection DoS Attack . . . . . . 11
     4.2. Router/routing involving threats. . . . . . . . . . . . . 12
          4.2.1. Malicious Last Hop Router. . . . . . . . . . . . . 12

Nikander, et al. Informational [Page 1] RFC 3756 IPv6 ND Trust Models and Threats May 2004

          4.2.2. Default router is 'killed' . . . . . . . . . . . . 13
          4.2.3. Good Router Goes Bad . . . . . . . . . . . . . . . 14
          4.2.4. Spoofed Redirect Message . . . . . . . . . . . . . 14
          4.2.5. Bogus On-Link Prefix . . . . . . . . . . . . . . . 14
          4.2.6. Bogus Address Configuration Prefix . . . . . . . . 15
          4.2.7. Parameter Spoofing . . . . . . . . . . . . . . . . 16
     4.3. Replay attacks and remotely exploitable attacks . . . . . 17
          4.3.1. Replay attacks . . . . . . . . . . . . . . . . . . 17
          4.3.2. Neighbor Discovery DoS Attack. . . . . . . . . . . 18
     4.4. Summary of the attacks. . . . . . . . . . . . . . . . . . 19
 5.  Security Considerations. . . . . . . . . . . . . . . . . . . . 20
 6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
 7.  Informative References . . . . . . . . . . . . . . . . . . . . 21
 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22
 Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 23

1. Introduction

 The IPv6 Neighbor Discovery (ND) RFC 2461 [2] and Address
 Autoconfiguration RFC 2462 [3] mechanisms are used by nodes in an
 IPv6 network to learn the local topology, including the IP to MAC
 address mappings for the local nodes, the IP and MAC addresses of the
 routers present in the local network, and the routing prefixes served
 by the local routers.  The current specifications suggest that IPsec
 AH RFC 2402 [1] may be used to secure the mechanisms, but does not
 specify how.  It appears that using current AH mechanisms is
 problematic due to key management problems [8].
 To solve the problem, the Secure Neighbor Discovery (SEND) working
 group was chartered in Fall 2002.  The goal of the working group is
 to define protocol support for securing IPv6 Neighbor Discovery
 without requiring excessive manual keying.
 The purpose of this document is to define the types of networks in
 which the Secure IPv6 Neighbor Discovery mechanisms are expected to
 work, and the threats that the security protocol(s) must address.  To
 fulfill this purpose, this document first defines three different
 trust models, roughly corresponding to secured corporate intranets,
 public wireless access networks, and pure ad hoc networks.  After
 that, a number of threats are discussed in the light of these trust
 models.  The threat catalog is aimed to be exhaustive, but it is
 likely that some threats are still missing.  Thus, ideas for new
 threats to consider are solicited.

Nikander, et al. Informational [Page 2] RFC 3756 IPv6 ND Trust Models and Threats May 2004

1.1. Remarks

 Note that the SEND WG charter limits the scope of the working group
 to secure Neighbor Discovery functions.  Furthermore, the charter
 explicitly mentions zero configuration as a fundamental goal behind
 Neighbor Discovery.  Network access authentication and access control
 are outside the scope of this work.
 During the discussions while preparing this document, the following
 aspects that may help to evaluate the eventual solutions were
 mentioned.
    Zero configuration
    Interaction with access control solutions
    Scalability
    Efficiency
 However, the main evaluation criteria are formed by the trust models
 and threat lists.  In other words, the solutions are primarily
 evaluated by seeing how well they secure the networks against the
 identified threats, and only secondarily from the configuration,
 access control, scalability, and efficiently point of view.
 IMPORTANT.  This document occasionally discusses solution proposals,
 such as Cryptographically Generated Addresses (CGA) [7] and Address
 Based Keys (ABK) [6].  However, such discussion is solely for
 illustrative purposes.  Its purpose is to give the readers a more
 concrete idea of *some* possible solutions.  Such discussion does NOT
 indicate any preference on solutions on the behalf of the authors or
 the working group.
 It should be noted that the term "trust" is used in this document in
 a rather non-technical manner.  The most appropriate interpretation
 is to consider it as an expression of an organizational or collective
 belief, i.e., an expression of commonly shared beliefs about the
 future behavior of the other involved parties.  Conversely, the term
 "trust relationship" denotes a mutual a priori relationship between
 the involved organizations or parties where the parties believe that
 the other parties will behave correctly even in the future.  A trust
 relationship makes it possible to configure authentication and
 authorization information between the parties, while the lack of such
 a relationship makes it impossible to pre-configure such information.

Nikander, et al. Informational [Page 3] RFC 3756 IPv6 ND Trust Models and Threats May 2004

2. Previous Work

 The RFCs that specify the IPv6 Neighbor Discovery and Address
 Autoconfiguration protocols [2] [3] contain the required discussion
 of security in a Security Considerations section.  Some of the
 threats identified in this document were raised in the original RFCs.
 The recommended remedy was to secure the involved packets with an
 IPsec AH [1] header.  However, that recommendation oversimplifies the
 problem by leaving the AH key management for future work.  For
 example, a host attempting to gain access to a Public Access network
 may or may not have the required IPsec security associations set up
 with the network.  In a roaming (but not necessarily mobile)
 situation, where a user is currently accessing the network through a
 service provider different from the home provider, it is not likely
 that the host will have been preconfigured with the proper mutual
 trust relationship for the foreign provider's network, allowing it to
 directly authenticate the network and get itself authenticated.
 As of today, any IPsec security association between the host and the
 last hop routers or other hosts on the link would need to be
 completely manually preconfigured, since the Neighbor Discovery and
 Address Autoconfiguration protocols deal to some extent with how a
 host obtains initial access to a link.  Thus, if a security
 association is required for initial access and the host does not have
 that association, there is currently no standard way that the host
 can dynamically configure itself with that association, even if it
 has the necessary minimum prerequisite keying material.  This
 situation could induce administration hardships when events such as
 re-keying occur.
 In addition, Neighbor Discovery and Address Autoconfiguration use a
 few fixed multicast addresses plus a range of 16 million "solicited
 node" multicast addresses.  A naive application of pre-configured SAs
 would require pre-configuring an unmanageable number of SAs on each
 host and router just in case a given solicited node multicast address
 is used.  Preconfigured SAs are impractical for securing such a large
 potential address range.

3. Trust Models

 When considering various security solutions for the IPv6 Neighbor
 Discovery (ND) [2], it is important to keep in mind the underlying
 trust models.  The trust models defined in this section are used
 later in this document, when discussing specific threats.

Nikander, et al. Informational [Page 4] RFC 3756 IPv6 ND Trust Models and Threats May 2004

 In the following, the RFC 2461/RFC 2462 mechanisms are loosely
 divided into two categories: Neighbor Discovery (ND) and Router
 Discovery (RD).  The former denotes operations that do not primarily
 involve routers while the operations in the latter category do.
 Three different trust models are specified:
 1.  A model where all authenticated nodes trust each other to behave
     correctly at the IP layer and not to send any ND or RD messages
     that contain false information.  This model is thought to
     represent a situation where the nodes are under a single
     administration and form a closed or semi-closed group.  A
     corporate intranet is a good example.
 2.  A model where there is a router trusted by the other nodes in the
     network to be a legitimate router that faithfully routes packets
     between the local network and any connected external networks.
     Furthermore, the router is trusted to behave correctly at the IP
     layer and not to send any ND or RD messages that contain false
     information.
     This model is thought to represent a public network run by an
     operator.  The clients pay to the operator, have its credentials,
     and trust it to provide the IP forwarding service.  The clients
     do not trust each other to behave correctly; any other client
     node must be considered able to send falsified ND and RD
     messages.
 3.  A model where the nodes do not directly trust each other at the
     IP layer.  This model is considered suitable for e.g., ad hoc
     networks.
 Note that even though the nodes are assumed to trust each other in
 the first trust model (corporate intranet), it is still desirable to
 limit the extent of damage a node is able to inflict to the local
 network if it becomes compromised.

3.1. Corporate Intranet Model

 In a corporate intranet or other network where all nodes are under
 one administrative domain, the nodes may be considered to be reliable
 at the IP layer.  Thus, once a node has been accepted to be a member
 of the network, it is assumed to behave in a trustworthy manner.
 Under this model, if the network is physically secured or if the link
 layer is cryptographically secured to the extent needed, no other
 protection is needed for IPv6 ND, as long as none of the nodes become
 compromised.  For example, a wired LAN with 802.1x access control or

Nikander, et al. Informational [Page 5] RFC 3756 IPv6 ND Trust Models and Threats May 2004

 a WLAN with 802.11i Robust Security Network (RSN) with AES encryption
 may be considered secure enough, requiring no further protection
 under this trust model.  On the other hand, ND security would add
 protection depth even under this model (see below).  Furthermore, one
 should not overestimate the level of security any L2 mechanism is
 able to provide.
 If the network is not physically secured and the link layer does not
 have cryptographic protection, or if the cryptographic protection is
 not secure enough (e.g., just 802.1x and not 802.11i in a WLAN), the
 nodes in the network may be vulnerable to some or all of the threats
 outlined in Section 4.  In such a case some protection is desirable
 to secure ND.  Providing such protection falls within the main
 initial focus of the SEND working group.
 Furthermore, it is desirable to limit the amount of potential damage
 in the case a node becomes compromised.  For example, it might still
 be acceptable that a compromised node is able to launch a denial-of-
 service attack, but it is undesirable if it is able to hijack
 existing connections or establish man-in-the-middle attacks on new
 connections.
 As mentioned in Section 2, one possibility to secure ND would be to
 use IPsec AH with symmetric shared keys, known by all trusted nodes
 and by no outsiders.  However, none of the currently standardized
 automatic key distribution mechanisms work right out-of-the-box.  For
 further details, see [8].  Furthermore, using a shared key would not
 protect against a compromised node.
 More specifically, the currently used key agreement protocol, IKE,
 suffers from a chicken-and-egg problem [8]: one needs an IP address
 to run IKE, IKE is needed to establish IPsec SAs, and IPsec SAs are
 required to configure an IP address.  Furthermore, there does not
 seem to be any easy and efficient ways of securing ND with symmetric
 key cryptography.  The required number of security associations would
 be very large [9].
 As an example, one possible approach to overcome this limitation is
 to use public key cryptography, and to secure ND packets directly
 with public key signatures.

3.2. Public Wireless Network with an Operator

 A scenario where an operator runs a public wireless (or wireline)
 network, e.g., a WLAN in a hotel, airport, or cafe, has a different
 trust model.  Here the nodes may be assumed to trust the operator to
 provide the IP forwarding service in a trustworthy manner, and not to
 disrupt or misdirect the clients' traffic.  However, the clients do

Nikander, et al. Informational [Page 6] RFC 3756 IPv6 ND Trust Models and Threats May 2004

 not usually trust each other.  Typically the router (or routers) fall
 under one administrative domain, and the client nodes each fall under
 their own administrative domain.
 It is assumed that under this scenario the operator authenticates all
 the client nodes, or at least requires authorization in the form of a
 payment.  At the same time, the clients must be able to authenticate
 the router and make sure that it belongs to the trusted operator.
 Depending on the link-layer authentication protocol and its
 deployment, the link layer may take care of the mutual
 authentication.  The link-layer authentication protocol may allow the
 client nodes and the access router to create a security association.
 Note that there exist authentication protocols, e.g., particular EAP
 methods, that do not create secure keying material and/or do not
 allow the client to authenticate the network.
 In this scenario, cryptographically securing the link layer does not
 necessarily block all the threats outlined in Section 4; see the
 individual threat descriptions.  Specifically, even in 802.11i RSN
 with AES encryption the broadcast and multicast keys are shared
 between all nodes.  Even if the underlying link layer was aware of
 all the nodes' link-layer addresses, and were able to check that no
 source addresses were falsified, there would still be
 vulnerabilities.
 One should also note that link-layer security and IP topology do not
 necessarily match.  For example, the wireless access point may not be
 visible at the IP layer at all.  In such a case cryptographic
 security at the link layer does not provide any security with regard
 to IP Neighbor Discovery.
 There seems to be at least two ways to bring in security into this
 scenario.  One possibility seems to be to enforce strong security
 between the clients and the access router, and make the access router
 aware of the IP and link-layer protocol details.  That is, the router
 would check ICMPv6 packet contents, and filter packets that contain
 information which does not match the network topology.  The other
 possibly acceptable way is to add cryptographic protection to the
 ICMPv6 packets carrying ND messages.

3.3. Ad Hoc Network

 In an ad hoc network, or any network without a trusted operator, none
 of the nodes trust each other.  In a generic case, the nodes meet
 each other for the first time, and there are no guarantees that the
 other nodes would behave correctly at the IP layer.  They must be
 considered suspicious to send falsified ND and RD messages.

Nikander, et al. Informational [Page 7] RFC 3756 IPv6 ND Trust Models and Threats May 2004

 Since there are no a priori trust relationships, the nodes cannot
 rely on traditional authentication.  That is, the traditional
 authentication protocols rely on some existing relationship between
 the parties.  The relationship may be direct or indirect.  The
 indirect case relies on one or more trusted third parties, thereby
 creating a chain of trust relationships between the parties.
 In the generic ad hoc network case, there are no trusted third
 parties, nor do the parties trust each other directly.  Thus, the
 traditional means of first authenticating and then authorizing the
 users (to use their addresses) do not work.
 It is still possible to use self-identifying mechanisms, such as
 Cryptographically Generated Addresses (CGA) [7].  These allow the
 nodes to ensure that they are talking to the same nodes (as before)
 at all times, and that each of the nodes indeed have generated their
 IP address themselves and not "stolen" someone else's address.  It
 may also be possible to learn the identities of any routers using
 various kinds of heuristics, such as testing the node's ability to
 convey cryptographically protected traffic towards a known and
 trusted node somewhere in the Internet.  Methods like these seem to
 mitigate (but not completely block) some of the attacks outlined in
 the next section.

4. Threats on a (Public) Multi-Access Link

 In this section we discuss threats against the current IPv6 Neighbor
 Discovery mechanisms, when used in multi-access links.  The threats
 are discussed in the light of the trust models defined in the
 previous section.
 There are three general types of threats:
 1.  Redirect attacks in which a malicious node redirects packets away
     from the last hop router or other legitimate receiver to another
     node on the link.
 2.  Denial-of-Service (DoS) attacks, in which a malicious node
     prevents communication between the node under attack and all
     other nodes, or a specific destination address.
 3.  Flooding Denial-of-Service (DoS) attacks, in which a malicious
     node redirects other hosts' traffic to a victim node, and thereby
     creates a flood of bogus traffic at the victim host.
 A redirect attack can be used for DoS purposes by having the node to
 which the packets were redirected drop the packets, either completely
 or by selectively forwarding some of them and not others.

Nikander, et al. Informational [Page 8] RFC 3756 IPv6 ND Trust Models and Threats May 2004

 The subsections below identify specific threats for IPv6 network
 access.  The threat descriptions are organized in three subsections.
 We first consider threats that do not involve routers or routing
 information.  We next consider threats that do involve routers or
 routing information.  Finally, we consider replay attacks and threats
 that are remotely exploitable.  All threats are discussed in the
 light of the trust models.

4.1. Non router/routing related threats

 In this section we discuss attacks against "pure" Neighbor Discovery
 functions, i.e., Neighbor Discovery (ND), Neighbor Unreachability
 Detection (NUD), and Duplicate Address Detection (DAD) in Address
 Autoconfiguration.

4.1.1. Neighbor Solicitation/Advertisement Spoofing

 Nodes on the link use Neighbor Solicitation and Advertisement
 messages to create bindings between IP addresses and MAC addresses.
 More specifically, there are two cases when a node creates neighbor
 cache entries upon receiving Solicitations:
 1.  A node receives a Neighbor Solicitation that contains a node's
     address.  The node can use that to populate its neighbor cache.
     This is basically a performance optimization, and a SHOULD in the
     base documents.
 2.  During Duplicate Address Detection (DAD), if a node receives a
     Neighbor Solicitation for the same address it is soliciting for,
     the situation is considered a collision, and the node must cease
     to solicit for the said address.
 In contrast to solicitation messages that create or modify state only
 in these specific occasions, state is usually modified whenever a
 node receives a solicited-for advertisement message.
 An attacking node can cause packets for legitimate nodes, both hosts
 and routers, to be sent to some other link-layer address.  This can
 be done by either sending a Neighbor Solicitation with a different
 source link-layer address option, or sending a Neighbor Advertisement
 with a different target link-layer address option.
 The attacks succeed because the Neighbor Cache entry with the new
 link-layer address overwrites the old.  If the spoofed link-layer
 address is a valid one, as long as the attacker responds to the
 unicast Neighbor Solicitation messages sent as part of the Neighbor
 Unreachability Detection, packets will continue to be redirected.
 This is a redirect/DoS attack.

Nikander, et al. Informational [Page 9] RFC 3756 IPv6 ND Trust Models and Threats May 2004

 This mechanism can be used for a DoS attack by specifying an unused
 link-layer address; however, this DoS attack is of limited duration
 since after 30-50 seconds (with default timer values) the Neighbor
 Unreachability Detection mechanism will discard the bad link-layer
 address and multicast anew to discover the link-layer address.  As a
 consequence, the attacker will need to keep responding with
 fabricated link-layer addresses if it wants to maintain the attack
 beyond the timeout.
 The threat discussed in this subsection involves Neighbor
 Solicitation and Neighbor Advertisement messages.
 This attack is not a concern if access to the link is restricted to
 trusted nodes; if a trusted node is compromised, the other nodes are
 exposed to this threat.  In the case where just the operator is
 trusted, the nodes may rely on the operator to certify the address
 bindings for other local nodes.  From the security point of view, the
 router may act as a trusted proxy for the other nodes.  This assumes
 that the router can be trusted to represent correctly the other nodes
 on the link.  In the ad hoc network case, and optionally in the other
 two cases, the nodes may use self certifying techniques (e.g., CGA)
 to authorize address bindings.
 Additionally, some implementations log an error and refuse to accept
 ND overwrites, instead requiring the old entry to time out first.

4.1.2. Neighbor Unreachability Detection (NUD) failure

 Nodes on the link monitor the reachability of local destinations and
 routers with the Neighbor Unreachability Detection procedure [2].
 Normally the nodes rely on upper-layer information to determine
 whether peer nodes are still reachable.  However, if there is a
 sufficiently long delay on upper-layer traffic, or if the node stops
 receiving replies from a peer node, the NUD procedure is invoked.
 The node sends a targeted NS to the peer node.  If the peer is still
 reachable, it will reply with a NA.  However, if the soliciting node
 receives no reply, it tries a few more times, eventually deleting the
 neighbor cache entry.  If needed, this triggers the standard address
 resolution protocol to learn the new MAC address.  No higher level
 traffic can proceed if this procedure flushes out neighbor cache
 entries after determining (perhaps incorrectly) that the peer is not
 reachable.
 A malicious node may keep sending fabricated NAs in response to NUD
 NS messages.  Unless the NA messages are somehow protected, the
 attacker may be able to extend the attack for a long time using this
 technique.  The actual consequences depend on why the node become

Nikander, et al. Informational [Page 10] RFC 3756 IPv6 ND Trust Models and Threats May 2004

 unreachable for the first place, and how the target node would behave
 if it knew that the node has become unreachable.  This is a DoS
 attack.
 The threat discussed in this subsection involves Neighbor
 Solicitation/Advertisement messages.
 This attack is not a concern if access to the link is restricted to
 trusted nodes; if a trusted node is compromised, the other nodes are
 exposed to this DoS threat.  Under the two other trust models, a
 solution requires that the node performing NUD is able to make a
 distinction between genuine and fabricated NA responses.

4.1.3. Duplicate Address Detection DoS Attack

 In networks where the entering hosts obtain their addresses using
 stateless address autoconfiguration [3], an attacking node could
 launch a DoS attack by responding to every duplicate address
 detection attempt made by an entering host.  If the attacker claims
 the address, then the host will never be able to obtain an address.
 The attacker can claim the address in two ways: it can either reply
 with an NS, simulating that it is performing DAD, too, or it can
 reply with an NA, simulating that it has already taken the address
 into use.  This threat was identified in RFC 2462 [3].  The issue may
 also be present when other types of address configuration is used,
 i.e., whenever DAD is invoked prior to actually configuring the
 suggested address.  This is a DoS attack.
 The threat discussed in this subsection involves Neighbor
 Solicitation/Advertisement messages.
 This attack is not a concern if access to the link is restricted to
 trusted nodes; if a trusted node is compromised, the other nodes
 become exposed to this DoS threat.  Under the two other trust models,
 a solution requires that the node performing DAD is able to verify
 whether the sender of the NA response is authorized to use the given
 IP address or not.  In the trusted operator case, the operator may
 act as an authorizer, keeping track of allocated addresses and making
 sure that no node has allocated more than a few (hundreds of)
 addresses.  On the other hand, it may be detrimental to adopt such a
 practice, since there may be situations where it is desirable for one
 node to have a large number of addresses, e.g., creating a separate
 address per TCP connection, or when running an ND proxy.  Thus, it
 may be inappropriate to suggest that ISPs could control how many
 addresses a legitimate host can have; the discussion above must be
 considered only as examples, as stated in the beginning of this
 document.

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 In the ad hoc network case one may want to structure the addresses in
 such a way that self authorization is possible.

4.2. Router/routing involving threats

 In this section we consider threats pertinent to router discovery or
 other router assisted/related mechanisms.

4.2.1. Malicious Last Hop Router

 This threat was identified in [5] but was classified as a general
 IPv6 threat and not specific to Mobile IPv6.  It is also identified
 in RFC 2461 [2].  This threat is a redirect/DoS attack.
 An attacking node on the same subnet as a host attempting to discover
 a legitimate last hop router could masquerade as an IPv6 last hop
 router by multicasting legitimate-looking IPv6 Router Advertisements
 or unicasting Router Advertisements in response to multicast Router
 Advertisement Solicitations from the entering host.  If the entering
 host selects the attacker as its default router, the attacker has the
 opportunity to siphon off traffic from the host, or mount a man-in-
 the-middle attack.  The attacker could ensure that the entering host
 selected itself as the default router by multicasting periodic Router
 Advertisements for the real last hop router having a lifetime of
 zero.  This may spoof the entering host into believing that the real
 access router is not willing to take any traffic.  Once accepted as a
 legitimate router, the attacker could send Redirect messages to
 hosts, then disappear, thus covering its tracks.
 This threat is partially mitigated in RFC 2462; in Section 5.5.3 of
 RFC 2462 it is required that if the advertised prefix lifetime is
 less than 2 hours and less than the stored lifetime, the stored
 lifetime is not reduced unless the packet was authenticated.
 However, the default router selection procedure, as defined in
 Section 6.3.6. of RFC 2461, does not contain such a rule.
 The threat discussed in this subsection involves Router Advertisement
 and Router Advertisement Solicitation messages.
 This attack is not a concern if access to the link is restricted to
 trusted nodes; if a trusted node is compromised, the other nodes are
 exposed to this threat.  However, the threat can be partially
 mitigated through a number of means, for example, by configuring the
 nodes to prefer existing routers over new ones.  Note that this
 approach does not necessarily prevent one from introducing new
 routers into the network, depending on the details of implementation.
 At minimum, it just makes the existing nodes to prefer the existing
 routers over the new ones.

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 In the case of a trusted operator, there must be a means for the
 nodes to make a distinction between trustworthy routers, run by the
 operator, and other nodes.  There are currently no widely accepted
 solutions for the ad hoc network case, and the issue remains as a
 research question.

4.2.2. Default router is 'killed'

 In this attack, an attacker 'kills' the default router(s), thereby
 making the nodes on the link to assume that all nodes are local.  In
 Section 5.2 of RFC 2461 [2] it is stated that "[if] the Default
 Router List is empty, the sender assumes that the destination is on-
 link."  Thus, if the attacker is able to make a node to believe that
 there are no default routers on the link, the node will try to send
 the packets directly, using Neighbor Discovery.  After that the
 attacker can use NS/NA spoofing even against off-link destinations.
 There are a few identified ways how an attacker can 'kill' the
 default router(s).  One is to launch a classic DoS attack against the
 router so that it does not appear responsive any more.  The other is
 to send a spoofed Router Advertisement with a zero Router Lifetime
 (see Section 6.3.4 of RFC 2461 [2]).  However, see also the
 discussion in Section 4.2.1, above.
 This attack is mainly a DoS attack, but it could also be used to
 redirect traffic to the next better router, which may be the
 attacker.
 The threat discussed in this subsection involves Router Advertisement
 messages.  One variant of this threat may be possible by overloading
 the router, without using any ND/RD messages.
 This attack is not a concern if access to the link is restricted to
 trusted nodes; if a trusted node is compromised, the other nodes are
 exposed to this threat.  In the case of a trusted operator, there
 must be a means for the nodes to make a distinction between
 trustworthy routers, run by the operator, and other nodes.  That
 protects against spoofed Router Advertisements, but it does not
 protect against router overloading.  There are currently no widely
 accepted solutions for the ad hoc network case, and the issue remains
 as a research question.
 Thanks to Alain Durand for identifying this threat.

Nikander, et al. Informational [Page 13] RFC 3756 IPv6 ND Trust Models and Threats May 2004

4.2.3. Good Router Goes Bad

 In this attack, a router that previously was trusted is compromised.
 The attacks available are the same as those discussed in Section
 4.2.1.  This is a redirect/DoS attack.
 There are currently no known solutions for any of the presented three
 trust models.  On the other hand, on a multi-router link one could
 imagine a solution involving revocation of router rights.  The
 situation remains as a research question.

4.2.4. Spoofed Redirect Message

 The Redirect message can be used to send packets for a given
 destination to any link-layer address on the link.  The attacker uses
 the link-local address of the current first-hop router in order to
 send a Redirect message to a legitimate host.  Since the host
 identifies the message by the link-local address as coming from its
 first hop router, it accepts the Redirect.  As long as the attacker
 responds to Neighbor Unreachability Detection probes to the link-
 layer address, the Redirect will remain in effect.  This is a
 redirect/DoS attack.
 The threat discussed in this subsection involves Redirect messages.
 This attack is not a concern if access to the link is restricted to
 trusted nodes; if a trusted node is compromised, the other nodes are
 exposed to this threat.  In the case of a trusted operator, there
 must be a means for the nodes to make a distinction between
 trustworthy routers, run by the operator, and other nodes.  There are
 currently no widely accepted solutions for the ad hoc network case,
 and the issue remains as a research question.

4.2.5. Bogus On-Link Prefix

 An attacking node can send a Router Advertisement message specifying
 that some prefix of arbitrary length is on-link.  If a sending host
 thinks the prefix is on-link, it will never send a packet for that
 prefix to the router.  Instead, the host will try to perform address
 resolution by sending Neighbor Solicitations, but the Neighbor
 Solicitations will not result in a response, denying service to the
 attacked host.  This is a DoS attack.
 The attacker can use an arbitrary lifetime on the bogus prefix
 advertisement.  If the lifetime is infinity, the sending host will be
 denied service until it loses the state in its prefix list e.g., by
 rebooting, or after the same prefix is advertised with a zero

Nikander, et al. Informational [Page 14] RFC 3756 IPv6 ND Trust Models and Threats May 2004

 lifetime.  The attack could also be perpetrated selectively for
 packets destined to a particular prefix by using 128 bit prefixes,
 i.e., full addresses.
 Additionally, the attack may cause a denial-of-service by flooding
 the routing table of the node.  The node would not be able to
 differentiate between legitimate on-link prefixes and bogus ones when
 making decisions as to which ones are kept and which are dropped.
 Inherently, any finite system must have some point at which new
 received prefixes must be dropped rather than accepted.
 This attack can be extended into a redirect attack if the attacker
 replies to the Neighbor Solicitations with spoofed Neighbor
 Advertisements, thereby luring the nodes on the link to send the
 traffic to it or to some other node.
 This threat involves Router Advertisement message.  The extended
 attack combines the attack defined in Section 4.1.1 and in this
 section, and involves Neighbor Solicitation, Neighbor Advertisement,
 and Router Advertisement messages.
 This attack is not a concern if access to the link is restricted to
 trusted nodes; if a trusted node is compromised, the other nodes are
 exposed to this threat.  In the case of a trusted operator, there
 must be a means for the nodes to make a distinction between
 trustworthy routers, run by the operator, and other nodes.  There are
 currently no known solutions for the ad hoc network case, and the
 issue remains as a research question.
 As an example, one possible approach to limiting the damage of this
 attack is to require advertised on-link prefixes be /64s (otherwise
 it's easy to advertise something short like 0/0 and this attack is
 very easy).

4.2.6. Bogus Address Configuration Prefix

 An attacking node can send a Router Advertisement message specifying
 an invalid subnet prefix to be used by a host for address
 autoconfiguration.  A host executing the address autoconfiguration
 algorithm uses the advertised prefix to construct an address [3],
 even though that address is not valid for the subnet.  As a result,
 return packets never reach the host because the host's source address
 is invalid.  This is a DoS attack.
 This attack has the potential to propagate beyond the immediate
 attacked host if the attacked host performs a dynamic update to the
 DNS based on the bogus constructed address.  DNS update [4] causes
 the bogus address to be added to the host's address record in the

Nikander, et al. Informational [Page 15] RFC 3756 IPv6 ND Trust Models and Threats May 2004

 DNS.  Should this occur, applications performing name resolution
 through the DNS obtain the bogus address and an attempt to contact
 the host fails.  However, well-written applications will fall back
 and try the other addresses registered in DNS, which may be correct.
 A distributed attacker can make the attack more severe by creating a
 falsified reverse DNS entry that matches with the dynamic DNS entry
 created by the target.  Consider an attacker who has legitimate
 access to a prefix <ATTACK_PRFX>, and a target who has an interface
 ID <TARGET_IID>.  The attacker creates a reverse DNS entry for
 <ATTACK_PRFX>:<TARGET_IID>, pointing to the real domain name of the
 target, e.g., "secure.target.com".  Next the attacker advertises the
 <ATTACK_PRFX> prefix at the target's link.  The target will create an
 address <ATTACK_PRFX>:<TARGET_IID>, and update its DNS entry so that
 "secure.target.com" points to <ATTACK_PRFX>:<TARGET_IID>.
 At this point "secure.target.com" points to
 <ATTACK_PRFX>:<TARGET_IID>, and <ATTACK_PRFX>:<TARGET_IID> points to
 "secure.target.com".  This threat is mitigated by the fact that the
 attacker can be traced since the owner of the <ATTACK_PRFX> is
 available at the registries.
 There is also a related possibility of advertising a target prefix as
 an autoconfiguration prefix on a busy link, and then have all nodes
 on this link try to communicate to the external world with this
 address.  If the local router doesn't have ingress filtering on, then
 the target link may get a large number of replies for those initial
 communication attempts.
 The basic threat discussed in this subsection involves Router
 Advertisement messages.  The extended attack scenarios involve the
 DNS, too.
 This attack is not a concern if access to the link is restricted to
 trusted nodes; if a trusted node is compromised the other nodes are
 exposed to this threat.  In the case of a trusted operator, there
 must be a means for the nodes to make a distinction between
 trustworthy routers, run by the operator, and other nodes.  There are
 currently no known solutions for the ad hoc network case, and the
 issue remains as a research question.

4.2.7. Parameter Spoofing

 IPv6 Router Advertisements contain a few parameters used by hosts
 when they send packets and to tell hosts whether or not they should
 perform stateful address configuration [2].  An attacking node could
 send out a valid-seeming Router Advertisement that duplicates the

Nikander, et al. Informational [Page 16] RFC 3756 IPv6 ND Trust Models and Threats May 2004

 Router Advertisement from the legitimate default router, except the
 included parameters are designed to disrupt legitimate traffic.  This
 is a DoS attack.
 Specific attacks include:
 1.  The attacker includes a Current Hop Limit of one or another small
     number which the attacker knows will cause legitimate packets to
     be dropped before they reach their destination.
 2.  The attacker implements a bogus DHCPv6 server or relay and the
     'M' and/or 'O' flag is set, indicating that stateful address
     configuration and/or stateful configuration of other parameters
     should be done.  The attacker is then in a position to answer the
     stateful configuration queries of a legitimate host with its own
     bogus replies.
 The threat discussed in this subsection involves Router Advertisement
 messages.
 Note that securing DHCP alone does not resolve this problem.  There
 are two reasons for this.  First, the attacker may prevent the node
 from using DHCP in the first place.  Second, depending on the node's
 local configuration, the attacker may spoof the node to use a less
 trusted DHCP server.  (The latter is a variant of the so called
 "bidding down" or "down grading" attacks.)
 As an example, one possible approach to mitigate this threat is to
 ignore very small hop limits.  The nodes could implement a
 configurable minimum hop limit, and ignore attempts to set it below
 said limit.
 This attack is not a concern if access to the link is restricted to
 trusted nodes; if a trusted node is compromised the other nodes are
 exposed to this treat.  In the case of a trusted operator, there must
 be a means for the nodes to make a distinction between trustworthy
 routers, run by the operator, and other nodes.  There are currently
 no known solutions for the ad hoc network case, and the issue remains
 a research question.

4.3. Replay attacks and remotely exploitable attacks

4.3.1. Replay attacks

 All Neighbor Discovery and Router Discovery messages are prone to
 replay attacks.  That is, even if they were cryptographically
 protected so that their contents cannot be forged, an attacker would

Nikander, et al. Informational [Page 17] RFC 3756 IPv6 ND Trust Models and Threats May 2004

 be able to capture valid messages and replay them later.  Thus,
 independent on what mechanism is selected to secure the messages,
 that mechanism must be protected against replay attacks.
 Fortunately it is fairly easy to defeat most replay attacks.  In
 request-reply exchanges, such as Solicitation-Advertisement, the
 request may contain a nonce that must appear also in the reply.
 Thus, old replies are not valid since they do not contain the right
 nonce.  Correspondingly, stand-alone messages, such as unsolicited
 Advertisements or Redirect messages, may be protected with timestamps
 or counters.  In practise, roughly synchronized clocks and timestamps
 seem to work well, since the recipients may keep track of the
 difference between the clocks of different nodes, and make sure that
 all new messages are newer than the last seen message.

4.3.2. Neighbor Discovery DoS Attack

 In this attack, the attacking node begins fabricating addresses with
 the subnet prefix and continuously sending packets to them.  The last
 hop router is obligated to resolve these addresses by sending
 neighbor solicitation packets.  A legitimate host attempting to enter
 the network may not be able to obtain Neighbor Discovery service from
 the last hop router as it will be already busy with sending other
 solicitations.  This DoS attack is different from the others in that
 the attacker may be off-link.  The resource being attacked in this
 case is the conceptual neighbor cache, which will be filled with
 attempts to resolve IPv6 addresses having a valid prefix but invalid
 suffix.  This is a DoS attack.
 The threat discussed in this subsection involves Neighbor
 Solicitation messages.
 This attack does not directly involve the trust models presented.
 However, if access to the link is restricted to registered nodes, and
 the access router keeps track of nodes that have registered for
 access on the link, the attack may be trivially plugged.  However, no
 such mechanisms are currently standardized.
 In a way, this problem is fairly similar to the TCP SYN flooding
 problem.  For example, rate limiting Neighbor Solicitations,
 restricting the amount of state reserved for unresolved
 solicitations, and clever cache management may be applied.
 It should be noted that both hosts and routers need to worry about
 this problem.  The router case was discussed above.  Hosts are also
 vulnerable since the neighbor discovery process can potentially be
 abused by an application that is tricked into sending packets to
 arbitrary on-link destinations.

Nikander, et al. Informational [Page 18] RFC 3756 IPv6 ND Trust Models and Threats May 2004

4.4. Summary of the attacks

 Columns:
    N/R Neighbor Discovery (ND) or Router Discovery (RD) attack
    R/D Redirect/DoS (Redir) or just DoS attack
    Msgs Messages involved in the attack: NA, NS, RA, RS, Redir
    1  Present in trust model 1 (corporate intranet)
    2  Present in trust model 2 (public operator run network)
    3  Present in trust model 3 (ad hoc network)
 Symbols in trust model columns:
  1. The threat is not present or not a concern.
    +  The threat is present and at least one solution is known.
    R  The threat is present but solving it is a research problem.
 Note that the plus sign '+' in the table does not mean that there is
 a ready-to-be-applied, standardized solution.  If solutions existed,
 this document would be unnecessary.  Instead, it denotes that in the
 authors' opinion the problem has been solved in principle, and there
 exists a publication that describes some approach to solve the
 problem, or a solution may be produced by straightforward application
 of known research and/or engineering results.
 In the other hand, and 'R' indicates that the authors' are not aware
 of any publication describing a solution to the problem, and cannot
 at the time of writing think about any simple and easy extension of
 known research and/or engineering results to solve the problem.

Nikander, et al. Informational [Page 19] RFC 3756 IPv6 ND Trust Models and Threats May 2004

 +-------+----------------------+-----+-------+-------+---+---+---+
 | Sec   | Attack name          | N/R | R/D   | Msgs  | 1 | 2 | 3 |
 +-------+----------------------+-----+-------+-------+---+---+---+
 | 4.1.1 | NS/NA spoofing       | ND  | Redir | NA NS | + | + | + |
 | 4.1.2 | NUD failure          | ND  | DoS   | NA NS | - | + | + |
 | 4.1.3 | DAD DoS              | ND  | DoS   | NA NS | - | + | + |
 +-------+----------------------+-----+-------+-------+---+---+---+
 | 4.2.1 | Malicious router     | RD  | Redir | RA RS | + | + | R |
 | 4.2.2 | Default router killed| RD  | Redir | RA    |+/R|+/R| R | 1)
 | 4.2.3 | Good router goes bad | RD  | Redir | RA RS | R | R | R |
 | 4.2.4 | Spoofed redirect     | RD  | Redir | Redir | + | + | R |
 | 4.2.5 | Bogus on-link prefix | RD  | DoS   | RA    | - | + | R | 2)
 | 4.2.6 | Bogus address config | RD  | DoS   | RA    | - | + | R | 3)
 | 4.2.7 | Parameter spoofing   | RD  | DoS   | RA    | - | + | R |
 +-------+----------------------+-----+-------+-------+---+---+---+
 | 4.3.1 | Replay attacks       | All | Redir | All   | + | + | + |
 | 4.3.2 | Remote ND DoS        | ND  | DoS   | NS    | + | + | + |
 +------------------------------+-----+-------+-------+---+---+---+
                              Figure 1
 1.  It is possible to protect the Router Advertisements, thereby
     closing one variant of this attack.  However, closing the other
     variant (overloading the router) does not seem to be plausible
     within the scope of this working group.
 2.  Note that the extended attack defined in Section 4.2.5 combines
     sending a bogus on-link prefix and performing NS/NA spoofing as
     per Section 4.1.1.  Thus, if the NA/NS exchange is secured, the
     ability to use Section 4.2.5 for redirect is most probably
     blocked, too.
 3.  The bogus DNS registration resulting from blindly registering the
     new address via DNS update [4] is not considered an ND security
     issue here.  However, it should be noted as a possible
     vulnerability in implementations.
 For a slightly different approach, see also Section 7 in [9].
 Especially the table in Section 7.7 of [9] is very good.

5. Security Considerations

 This document discusses security threats to network access in IPv6.
 As such, it is concerned entirely with security.

Nikander, et al. Informational [Page 20] RFC 3756 IPv6 ND Trust Models and Threats May 2004

6. Acknowledgements

 Thanks to Alper Yegin of DoCoMo Communications Laboratories USA for
 identifying the Neighbor Discovery DoS attack.  We would also like to
 thank Tuomas Aura and Michael Roe of Microsoft Research Cambridge as
 well as Jari Arkko and Vesa-Matti Mantyla of Ericsson Research
 Nomadiclab for discussing some of the threats with us.
 Thanks to Alper Yegin, Pekka Savola, Bill Sommerfeld, Vijay
 Devaparalli, Dave Thaler, and Alain Durand for their constructive
 comments.
 Thanks to Craig Metz for his numerous very good comments, and
 especially for more material of implementations that refuse to accept
 ND overrides, for the bogus on-link prefix threat, and for reminding
 us about replay attacks.

7. Informative References

 [1]   Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402,
       November 1998.
 [2]   Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery
       for IP Version 6 (IPv6)", RFC 2461, December 1998.
 [3]   Thomson, S. and T. Narten, "IPv6 Stateless Address
       Autoconfiguration", RFC 2462, December 1998.
 [4]   Wellington, B., "Secure Domain Name System (DNS) Dynamic
       Update", RFC 3007, November 2000.
 [5]   Mankin, A., "Threat Models introduced by Mobile IPv6  and
       Requirements for Security in Mobile IPv6", Work in Progress.
 [6]   Kempf, J., Gentry, C. and A. Silverberg, "Securing IPv6
       Neighbor Discovery Using Address Based Keys (ABKs)", Work in
       Progress, June 2002.
 [7]   Roe, M., "Authentication of Mobile IPv6 Binding Updates and
       Acknowledgments", Work in Progress, March 2002.
 [8]   Arkko, J., "Effects of ICMPv6 on IKE", Work in Progress, March
       2003.
 [9]   Arkko, J., "Manual Configuration of Security Associations for
       IPv6 Neighbor Discovery", Work in Progress, March 2003.

Nikander, et al. Informational [Page 21] RFC 3756 IPv6 ND Trust Models and Threats May 2004

Authors' Addresses

 Pekka Nikander (editor)
 Ericsson Research Nomadic Lab
 JORVAS  FIN-02420
 FINLAND
 Phone: +358 9 299 1
 EMail: pekka.nikander@nomadiclab.com
 James Kempf
 DoCoMo USA Labs
 181 Metro Drive, Suite 300
 San Jose, CA  95110
 USA
 Phone: +1 408 451 4711
 EMail: kempf@docomolabs-usa.com
 Erik Nordmark
 Sun Microsystems
 17 Network Circle
 Menlo Park, CA 94043
 USA
 Phone: +1 650 786 2921
 EMail: erik.nordmark@sun.com

Nikander, et al. Informational [Page 22] RFC 3756 IPv6 ND Trust Models and Threats May 2004

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Nikander, et al. Informational [Page 23]

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