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
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Internet-Drafts are draft documents valid for a maximum of six
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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.
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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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