Secure Neighborhood Discovery P. Nikander (editor)
Working Group Ericsson Research Nomadic Lab
Internet-Draft J. Kempf
Expires: July 24, 2003 DoCoMo USA Labs
E. Nordmark
Sun Microsystems Laboratories
January 23, 2003
IPv6 Neighbor Discovery trust models and threats
draft-ietf-send-psreq-01
Status of this Memo
This document is an Internet-Draft and is in full conformance with
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
The existing IETF standards specify that IPv6 Neighbor Discovery and
Address Autoconfiguration mechanisms MAY be protected with IPsec 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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Previous Work . . . . . . . . . . . . . . . . . . . . . . . 4
3. Trust models . . . . . . . . . . . . . . . . . . . . . . . . 5
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 . . . . . . . . . . . . . 8
4.1.1 Neighbor Solicitation/Advertisement Spoofing . . . . . . . . 8
4.1.2 Neighbor Unreachability Detection (NUD) failure . . . . . . 9
4.1.3 Duplicate Address Detection DoS Attack . . . . . . . . . . . 10
4.2 Router/routing involving threats . . . . . . . . . . . . . . 10
4.2.1 Malicious Last Hop Router . . . . . . . . . . . . . . . . . 10
4.2.2 Good Router Goes Bad . . . . . . . . . . . . . . . . . . . . 11
4.2.3 Spoofed Redirect Message . . . . . . . . . . . . . . . . . . 11
4.2.4 Bogus On-Link Prefix . . . . . . . . . . . . . . . . . . . . 12
4.2.5 Bogus Address Configuration Prefix . . . . . . . . . . . . . 13
4.2.6 Parameter Spoofing . . . . . . . . . . . . . . . . . . . . . 14
4.3 Remotely exploitable attacks . . . . . . . . . . . . . . . . 15
4.3.1 Neighbor Discovery DoS Attack . . . . . . . . . . . . . . . 15
4.4 Summary of the attacks . . . . . . . . . . . . . . . . . . . 15
5. Security Considerations . . . . . . . . . . . . . . . . . . 18
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19
References . . . . . . . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 21
A. Changes between versions . . . . . . . . . . . . . . . . . . 22
Intellectual Property and Copyright Statements . . . . . . . 23
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1. Introduction
The IPv6 Neighbor Discovery RFC2461 [3] and Address Autoconfiguration
RFC2462 [4] 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 RFC2402 [2] 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 [10].
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 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 is 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.
This document occasionally discusses solution proposals, such as CGA
[9] and ABK [8]. However, the discussion is solely for illustrative
purposes. It is meant to give the readers a more concrete idea of
some possible solutions. It 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 here in a rather
non-technical and loose 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.
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2. Previous Work
The RFCs that specify the IPv6 Neighbor Discovery and Address
Autoconfiguration protocols [3][4] 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 [2] header. However, that solution is not always possible due to
key management problems. 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 4 billion "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.
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3. Trust models
When considering various security solutions for the IPv6 Neighbor
Discovery (ND) [3], 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.
In the following, the RFC2461/RFC2662 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
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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 extend 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
a WLAN with 802.11i RNS/EAS may be considered secure enough,
requiring no further protection under this trust model.
On the other hand, 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 case some
protection is desirable to secure ND. Providing such protection
falls within the main initial focus of the SEND working group.
Furthermore, even under this model 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 [10]. Furthermore, using a shared key would not
protect against a compromised node.
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, air port, 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
not usually trust each other. Typically the router (or routers) fall
under one administrative domain, and the client nodes each fall under
its 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
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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.
Notice that there exists authentication protocols, e.g., variants of
EAP, 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 RNS/
EAP the broadcast and multicast keys are shared between all nodes.
Even if the underlying link layer is aware of all the nodes' link
layer addresses, and is able to check that no source addresses were
falsified, there may still be vulnerabilities.
There seems to be two ways to bring in security into this scenario.
One is to enforce strong security between the clients and the access
router, and make the access router aware of the IP 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 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 other nodes
are met for the first time, and there are no guarantees that the
other nodes would behave correctly at the IP layer. They must be
considered prone to send falsified ND and RD messages.
Since there are no a priori trust relationships, the nodes cannot
rely on traditional authentication. However, it is still possible to
use self-identifying mechanisms, such as Cryptographically Generated
Addresses (CGA) [9]. 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.
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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 two 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.
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.
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, and only after then those that do. Finally, we consider
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 created bindings between IP addresses and MAC addresses.
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 IP address
could additionally be the subnet router anycast address, allowing the
attacker to capture traffic to that address.
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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.
This mechanism can be used for a DoS attack by specifying an unused
link-layer address, however, the 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.
This threat involves Neighbor Solicitation and Neighbor
Advertisement.
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 just the operator is trusted,
the nodes may rely on the operator to certify the address bindings
for other local nodes. 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.
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 [3].
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 first waits for a small random delay, and then 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. 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 cryptographically 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
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become 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.
This threat involves Neighbor Solicitation/Advertisement.
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 [4], 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.
This threat was identified in RF2462 [4]. This is a DoS attack.
This threat involves Neighbor Solicitation/Advertisement.
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 may allocated more than a few (hundreds of)
addresses. In the ad hoc network case one has to structure the
addresses in such a way that self authorization is possible. CGA [9]
provides one possible way to achieve that.
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 [6] but was classified as a general
IPv6 threat and not specific to Mobile IPv6. It is also identified
in RFC2461 [3]. 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
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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 RFC2462; in Section 5.5.3 of
RFC2462 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
RFC2461, does not contain such a rule.
This threat involves Router Advertisement and Router Advertisement
Solicitation.
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.
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 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.
4.2.3 Spoofed Redirect Message
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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.
This threat involves Redirect.
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.
4.2.4 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 the same prefix is advertised with a zero lifetime.
The attack could also be perpetrated selectively for packets destined
to a particular prefix by using 128 bit prefixes, i.e. full
addresses.
This attack can be extended into a redirect attack if the tacker
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. 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.
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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.
4.2.5 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 [4],
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 causes the
bogus address to be added to the host's AAAA record in the 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 IP address in the AAAA RRset, 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 the 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,
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then the target link will get all the replies for those initial
communication attempts.
This threat involves Router Advertisement. 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.
4.2.6 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 [3]. An attacking node could
send out a valid-seeming Router Advertisement that duplicates the
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.
This threat involves Router Advertisements.
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
as a research question.
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4.3 Remotely exploitable attacks
4.3.1 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.
This threat involves Neighbor Solicitation.
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. Rate limitating Neighbor Solicitations, restricting the
amount of state reserved for unresolved soliciations, 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.
It is an open question whether the SEND WG should address this threat
or not.
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
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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:
- 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
+-------+----------------------------+-----+-------+-------+---+---+---+
| 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 | Good router goes bad | RD | Redir | RA RS | R | R | R |
| 4.2.3 | Spoofed redirect | RD | Redir | Redir | + | + | R |
| 4.2.4 | Bogus on-link prefix | RD | DoS | RA | - | + | R | 1)
| 4.2.5 | Bogus address config prefix| RD | DoS | RA | - | + | R | 2)
| 4.2.6 | Parameter spoofing | RD | DoS | RA | - | + | R |
+-------+----------------------------+-----+-------+-------+---+---+---+
| 4.3.1 | Remote ND DoS | ND | DoS | NS | + | + | + |
+------------------------------------+-----+-------+-------+---+---+---+
Figure 1
1. Note that the extended attack defined in Section 4.2.4 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 4.2.4 for redirect is most probably
blocked, too.
2. The bogus DNS registration resulting from blindly registering the
new address via DynDNS 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 [11].
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Especially the table in Section 7.7 of [11] is very good.
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5. Security Considerations
This document discusses security threats to network access in IPv6.
As such, it is concerned entirely with security.
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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 E. Yegin, Pekka Savola and Bill Sommerfeld for their
constructive comments.
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References
[1] Blunk, L. and J. Vollbrecht, "PPP Extensible Authentication
Protocol (EAP)", RFC 2284, March 1998.
[2] Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402,
November 1998.
[3] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery
for IP Version 6 (IPv6)", RFC 2461, December 1998.
[4] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[5] Haskin, D. and E. Allen, "IP Version 6 over PPP", RFC 2472,
December 1998.
[6] "Threat Models introduced by Mobile IPv6 and Requirements for
Security in Mobile IPv6",
draft-ietf-mobileip-mipv6-scrty-reqts-02 (work in progress),
November 2001.
[7] Droms, R., "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", draft-ietf-dhc-dhcpv6-28 (work in progress),
November 2002.
[8] Kempf, J., "Securing IPv6 Neighbor Discovery Using Address
Based Keys (ABKs)", draft-kempf-secure-nd-01 (work in
progress), June 2002.
[9] Roe, M., "Authentication of Mobile IPv6 Binding Updates and
Acknowledgments", draft-roe-mobileip-updateauth-02 (work in
progress), March 2002.
[10] Arkko, J., "Effects of ICMPv6 on IKE and IPsec Policies",
draft-arkko-icmpv6-ike-effects-01 (work in progress), June
2002.
[11] Arkko, J., "Manual SA Configuration for IPv6 Link Local
Messages", draft-arkko-manual-icmpv6-sas-01 (work in progress),
June 2002.
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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 Laboratories
29, Chemin du Vieux Chene
Meylan 38240
France
Phone: +33 4 76 18 88 03
EMail: erik.nordmark@sun.com
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Appendix A. Changes between versions
(To be removed before publication.)
-00 to -01
Added text to Section 4.2.4 to explained the combined NS/NA
spoofing + Bogus On-Link Prefix attack (suggested by Pekka
Savola).
Added text to Section 4.2.5 to describe how it can be extended to
include a valid reverse DNS mapping (suggested by Pekka Savola).
Added text to Section 4.2.5 to consider its potential flooding
effects (suggested by Jari Arkko).
Added text through the document to trust model 1, considering what
happens if a previously trusted node becomes compromised
(suggested by Pekka Savola and Bill Sommerfeld).
Changed the summary table in Section 4.4 so that the redirect
threats should be considered even in the corporate intranet case.
Added qualifying text to all occasions where a node is said to
trust another node, and even to some cases where a node is said
not to trust another node.
Added footnotes 1) and 2) to Figure 1
Added more discussion to threat Section 4.3.1 noting that it is
similar to TCP SYN flooding, and that also hosts need to worry
about it.
Converted to xml2rfc.
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