Network Working Group P. Nikander
Internet-Draft Ericsson Research Nomadic Lab
Expires: December 29, 2003 T. Aura
Microsoft Research
J. Arkko
Ericsson Research Nomadic Lab
G. Montenegro
E. Nordmark
Sun Microsystems
June 30, 2003
Mobile IP version 6 Route Optimization Security Design Background
draft-nikander-mobileip-v6-ro-sec-01
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at http://
www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on December 29, 2003.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document is a succint account of the rationale behind the Mobile
IPv6 (MIPv6) Route Optimization Security Design. The purpose of this
document is to present the thinking and to preserve the reasoning
behind the Mobile IPv6 Security Design in 2001-2002.
The document has two target audiences: (1) MIPv6 implementors (so
Nikander, et al. Expires December 29, 2003 [Page 1]
Internet-Draft Mobile IPv6 RO Security Design June 2003
that they can better understand the design choices in MIPv6 security
procedures); and (2) people dealing with mobility or multi-homing (so
that they can avoid a number of potential security pitfalls in their
design).
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Assumptions about the Existing IP Infrastructure . . . . . . 5
1.1.1 A note on source addresses and ingress filtering . . . . . . 6
1.2 The Mobility Problem and the Mobile IPv6 Solution . . . . . 6
1.3 Design Principles and Goals . . . . . . . . . . . . . . . . 8
1.3.1 End-to-end principle . . . . . . . . . . . . . . . . . . . . 8
1.3.2 Trust assumptions . . . . . . . . . . . . . . . . . . . . . 8
1.3.3 Protection level . . . . . . . . . . . . . . . . . . . . . . 9
1.4 About Mobile IPv6 Mobility and its Variations . . . . . . . 9
2. Dimensions of Danger . . . . . . . . . . . . . . . . . . . . 11
2.1 Target . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.2 Timing . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.3 Location . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3. Threats and limitations . . . . . . . . . . . . . . . . . . 13
3.1 Attacks against address 'owners' aka. address 'stealing' . . 13
3.1.1 Basic address stealing . . . . . . . . . . . . . . . . . . . 14
3.1.2 Stealing addresses of stationary nodes . . . . . . . . . . . 14
3.1.3 Future address stealing . . . . . . . . . . . . . . . . . . 15
3.1.4 Attacks against Secrecy and Integrity . . . . . . . . . . . 16
3.1.5 Basic Denial of Service Attacks . . . . . . . . . . . . . . 17
3.1.6 Replaying and Blocking Binding Updates . . . . . . . . . . . 17
3.2 Attacks against other nodes and networks (flooding) . . . . 17
3.2.1 Basic flooding . . . . . . . . . . . . . . . . . . . . . . . 18
3.2.2 Return-to-home flooding . . . . . . . . . . . . . . . . . . 19
3.3 Attacks against binding update protocols . . . . . . . . . . 19
3.3.1 Inducing Unnecessary Binding Updates . . . . . . . . . . . . 20
3.3.2 Forcing Non-Optimized Routing . . . . . . . . . . . . . . . 21
3.3.3 Reflection and Amplification . . . . . . . . . . . . . . . . 21
3.4 Classification of attacks . . . . . . . . . . . . . . . . . 23
3.5 Problems with infrastructure based authorization . . . . . . 23
4. The solution selected for Mobile IPv6 . . . . . . . . . . . 25
4.1 Return Routability . . . . . . . . . . . . . . . . . . . . . 25
4.1.1 Home Address check . . . . . . . . . . . . . . . . . . . . . 27
4.1.2 Care-of-Address check . . . . . . . . . . . . . . . . . . . 28
4.1.3 Forming the first Binding Update . . . . . . . . . . . . . . 28
4.2 Creating state safely . . . . . . . . . . . . . . . . . . . 28
4.2.1 Retransmissions and state machine . . . . . . . . . . . . . 30
4.3 Quick expiration of the Binding Cache Entries . . . . . . . 30
5. Security considerations . . . . . . . . . . . . . . . . . . 32
5.1 Time shifting attacks . . . . . . . . . . . . . . . . . . . 32
5.2 Interaction with IPsec . . . . . . . . . . . . . . . . . . . 32
Nikander, et al. Expires December 29, 2003 [Page 2]
Internet-Draft Mobile IPv6 RO Security Design June 2003
5.3 Pretending to be your neighbor . . . . . . . . . . . . . . . 33
5.4 Two mobile nodes talking to each other . . . . . . . . . . . 34
6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . 35
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 36
References (informative) . . . . . . . . . . . . . . . . . . 37
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 37
Intellectual Property and Copyright Statements . . . . . . . 39
Nikander, et al. Expires December 29, 2003 [Page 3]
Internet-Draft Mobile IPv6 RO Security Design June 2003
1. Introduction
Mobile IP is based on the idea of supporting mobility on top of
existing IP infrastructure, *without requiring* any modifications to
the routers, the applications or the stationary end hosts. However,
in Mobile IPv6 (as opposed to Mobile IPv4) the stationary end hosts
as well may provide support for mobility, i.e., *route optimization*.
In route optimization a *correspondent node* (CN), i.e., a peer for a
*mobile node*, learns a *binding* between the mobile node's
stationary *home address* and its current temporary
*care-of-address*. This binding is then used to modify the handling
of outgoing packets, leading to security risks. The purpose of this
document is the provide a relatively compact source of the background
assumptions, design choices, and other information needed to
understand the route optimization security design. This document
does not seek to compare the relative security of Mobile IPv6 and
other mobility protocols, or to list all the alternative security
mechanisms that were discussed during the Mobile IPv6 design process.
For a summary of the latter, we refer the reader to [1]. The goal of
this document is to explain the design choices and rationale behind
the current route optimization design. The authors participated in
the design team which produced the design, and hope, via this note,
to capture some of the lessons and reasoning behind that effort.
To fully understand the security implications of the design
constraints it is necessary to briefly explore the nature of the
existing IP infrastructure, the problems Mobile IP aims to solve, and
the design principles applied. In the light of this background, we
can then explore IP based mobility in more detail, and have a brief
look at the security problems. The background is given in the rest of
this section, starting from Section 1.1.
While the introduction in Section 1.1 may appear redundant to those
readers who are already familiar with Mobile IPv6, it may be valuable
to read it anyway. The approach taken in this document is very
different from the one in the Mobile IPv6 specification. That is, we
have explicitly aimed to expose the implicit assumptions and design
choices made in the base Mobile IPv6 design, while the Mobile IPv6
specification aims to state the result of the design. By
understanding the background it is much easier to understand the
source of some of the related security problems, and to understand
the limitations intrinsic to the provided solutions.
The rest of this document is organized as follows. After this
introductory section, we start by considering the dimensions of the
danger in Section 2. The security problems and countermeasures are
studied in detail in Section 3. Section 4 explains the overall
operation and design choices behind the current security design. In
Nikander, et al. Expires December 29, 2003 [Page 4]
Internet-Draft Mobile IPv6 RO Security Design June 2003
Section 5 we analyze the design and discuss the remaining threats.
Finally Section 6 concludes this document.
1.1 Assumptions about the Existing IP Infrastructure
One of the design goals in the Mobile IP design was to make mobility
possible without changing too much. This was especially important for
IPv4, with its large installed base, but the same design goals was
inherited by Mobile IPv6. Some alternative proposals, such as the
Host Identity Protocol (HIP) [9], take a different approach and
propose larger modifications to the Internet architecture (see
Section 1.4).
To understand Mobile IPv6, it is important to understand the MIPv6
design view to the base IPv6 protocol and infrastructure. The most
important base assumptions can be expressed as follows:
The routing prefixes available to a node are determined by its
current location, and therefore the node must change its IP
address as its moves.
The routing infrastructure is assumed to be secure and well
functioning, delivering packets to their intended destinations as
identified by the destination address.
While these may appear as trivial, let us explore them a little more
for a moment. Firstly, in the current IPv6 operational practise the
IP address prefixes are distributed in a *hierarchical* manner. This
limits the amount of routing table entries each single router needs
to handle. An important implication is that the *topology determines*
what globally routable IP addresses are available at a given
location. That is, the nodes cannot freely decide what globally
routable IP address to use, but they must rely on the routing
prefixes served by the local routers via Router Advertisements or by
a DHCP server. In other words, IP addresses are just what they name
says, *addresses, *or locators, i.e., names of locations.
Secondly, in the current Internet structure, the routers collectively
maintain a distributed database of the network topology, and forward
each packet towards the location determined by the destination
address carried in the packet. To maintain the topology information,
the routers *must* trust each other, at least to a certain extent.
The routers learn the topology information from the other routers,
and they have no option but to trust their neighbor routers about
distant topology. At the borders of administrative domains, *policy
rules* are used to limit the amount of perhaps faulty routing table
information received from the peer domains. While this is mostly used
to weed out administrative mistakes, it also helps with security. The
Nikander, et al. Expires December 29, 2003 [Page 5]
Internet-Draft Mobile IPv6 RO Security Design June 2003
aim is to maintain a reasonably accurate idea of the network topology
even if someone is feeding faulty information to the routing system.
In the current Mobile IPv6 design it is explicitly assumed that the
routers and the policy rules are configured in a reasonable way, and
that the resulting routing infrastructure is trustworthy enough. That
is, it is assumed that the routing system maintains an accurate idea
of the network topology and that it is therefore able to route
packets to their destination locations, if at all. If this assumption
is broken, the Internet is broken in the sense that packets go to
wrong locations. Under such a circumstance it does not matter however
hard the mechanism above try to make sure that packets are not
delivered to wrong addresses, e.g., due to Mobile IP security
problems.
1.1.1 A note on source addresses and ingress filtering
Some of the threats and attacks discussed in this document take
advantage of the ease of source address spoofing. That is, in the
current Internet it is possible to send packets with false source IP
address. *Ingress filtering* is assumed to eventually prevent this.
When ingress filtering is used, the source address of all packets are
screened by the Internet service provider, and if the source address
has a routing prefix that should not be used by the customer, the
packets are dropped.
It should be noted that ingress filtering is relatively easy to apply
at the edges of the network, but almost impossible in the core
network. Basically, ingress filtering is easy only when the network
topology and prefix assignment do follow the same hierarchical
structure. Secondly, ingress filtering helps if and only if a large
part of the Internet uses it. Thirdly, ingress filtering has its own
technical problems, e.g. w.r.t. site multi-homing, and these problems
are likely to limit its usefulness.
1.2 The Mobility Problem and the Mobile IPv6 Solution
The Mobile IP design aims to solve two problems at the same time.
Firstly, it allows transport layer sessions (TCP connections,
UDP-based transactions) to continue even if the underlying host(s)
move and change their IP addresses. Secondly, it allows a node to be
reached through a static IP address, a *home address* (HoA).
The latter design choice can also be stated in other words: Mobile
IPv6 aims to preserve the *identifier* nature of IP addresses. That
is, Mobile IPv6 takes the view that IP addresses can be used as
natural identifiers of nodes, as they have been used since the
beginning of the Internet. This must be contrasted to proposed and
Nikander, et al. Expires December 29, 2003 [Page 6]
Internet-Draft Mobile IPv6 RO Security Design June 2003
existing alternative designs where the identifier and locator natures
of the IP addresses have been separated (see Section 1.4)
The basic idea in Mobile IP is to allow a *home agent * (HA) to work
as a stationary proxy for a *mobile node* (MN). Whenever the mobile
node is away from its *home network*, the home agent intercepts
packets destined to the node, and forwards the packets by tunneling
them to the node's current address, the *care-of-address* (CoA). The
transport layer (e.g., TCP, UDP) uses the home address as a
stationary identifier for the mobile node. Figure 1 (Figure 1)
illustrates this basic arrangement.
+----+ +----+
| MN |=#=#=#=#=#=#=#=#=tunnel=#=#=#=#=#=#=#=#|#HA |
+----+ ____________ +-#--+
| CoA ___/ \_____ # Home Link
-+-------/ Internet * * *-*-*-*-#-#-#-#-----
| * * | * Home Address
\___ * * _____/ + * -+
\_____*______/ | MN |
* + - -+
+----+
| CN | Data path as * * * *
+----+ it appears to correspondent node
Real data path # # # #
Figure 1
The basic solution requires tunneling through the home agent, thereby
leading to longer paths and degraded performance. This tunneling is
sometimes called *triangular routing* since it was originally planned
that the packets from the mobile node to its peer could still
traverse directly, bypassing the home agent.
To alleviate the performance penalty, Mobile IPv6 includes a mode of
operation that allows the mobile node and its peer, a *correspondent
node* (CN), to exchange packets directly, bypassing the home agent
completely after the initial setup phase. This mode of operation is
called *route optimization* (RO). When route optimization is used,
the mobile node sends its current care-of-address to the
correspondent node using *binding update* (BU) messages. The
correspondent node stores the binding between the home address and
care-of address into its *Binding Cache*.
Whenever MIPv6 route optimization is used, the correspondent node
Nikander, et al. Expires December 29, 2003 [Page 7]
Internet-Draft Mobile IPv6 RO Security Design June 2003
effectively functions in two roles. Firstly, it is the source of the
packets it sends, as usual. Secondly, it acts as *the first router*
for the packets, effectively performing *source routing*. That is,
when the correspondent node is sending out packets, it consults its
MIPv6 route optimization data structures, and *reroutes* the packets
if necessary. A *Binding Cache Entry (BCE)* contains the home address
and the care-of-address of the mobile node, and records the fact that
packets destined to the home address should now be sent to the
destination address. Thus, it represents a local routing exception.
The packets leaving the correspondent node are *source routed* to the
care-of-address. Each packet includes a routing header that contains
the home address of the mobile node. Thus, logically, the packet is
first routed to the care-of-address, and then *virtually* from the
care-of-address to the home address. In practise, of course, the
packet is consumed by the mobile node at the care-of-address, and the
header just allows the mobile node to select a socket associated with
the home address instead of one with the care-of-address. However,
the mechanism resembles source routing since there is routing state
involved at the correspondent node, and a routing header is used.
1.3 Design Principles and Goals
The MIPv6 design and security design aimed to follow the *end-to-end
principle*, to duly notice the differences in trust relationships
between the nodes, and to establish an explicit goal in the provided
level of protection.**
1.3.1 End-to-end principle
Perhaps the leading design principle for Internet protocols is the so
called end-to-end principle [3] [4]. According to this principle, it
is beneficial to avoid polluting the network with state, and to limit
new state creation to the involved end nodes.
In the case of Mobile IPv6, the end-to-end principle is applied by
restricting mobility related state primarily to the home agent.
Additionally, if route optimization is used, the correspondent nodes
also maintain a soft state about the mobile nodes' current
care-of-addresses, the Binding Cache. This can be contrasted to an
approach that would use individual host routes within the basic
routing system. Such an approach would crate state to a huge number
of routers around the network. In Mobile IPv6, only the home agent
and the communicating nodes need to create state.
1.3.2 Trust assumptions
In the Mobile IPv6 security design, different approaches were chosen
Nikander, et al. Expires December 29, 2003 [Page 8]
Internet-Draft Mobile IPv6 RO Security Design June 2003
for securing the communication between *the mobile node and its home
agent* and between *the mobile node and its correspondent nodes*. In
the home agent case it was assumed that the mobile node and the home
agent know each other through a prior arrangement, e.g., due to a
business relationships. In contrast, it was strictly assumed that the
mobile node and the correspondent node do not need to have any prior
arrangement, thereby allowing Mobile IPv6 to function in a *scalable*
manner, without requiring any configuration at the correspondent
nodes.
1.3.3 Protection level
As a security goal, Mobile IPv6 design aimed to be "as secure as the
(non-mobile) IPv4 Internet" was at the time of the design, in period
2001-2002. In particular, that means that there is little protection
against attackers that are able to attach themselves between a
correspondent node and a home agent. The rational is simple: in the
2001 Internet, if a node was able to attach itself to the
communication path between two arbitrary nodes, it was able to
disrupt, modify, and eavesdrop all the traffic between the two nodes,
unless IPsec protection was used. Even when IPsec *was* used, the
attacker was still able to selectively block communication by simply
dropping the packets. The attacker in control of a router between
the two nodes could also mount a flooding attack by redirecting the
data flows between the two nodes (or, more practically, an equivalent
flow of bogus data) to a third party.
1.4 About Mobile IPv6 Mobility and its Variations
Taking a more technical angle, IPv6 mobility can be defined as a
mechanism for *managing local exceptions to routing information* in
order to direct packets that are sent to one address (the home
address) to another address (the care-of-address). It is *managing*
in the sense that the local routing exceptions (source routes) are
created and deleted dynamically, based on the instructions sent by
the mobile node. It is *local* in the sense that the routing
exceptions are valid only at the home agent, and in the correspondent
nodes if route optimization is used. The created pieces of state are
*exceptions* in the sense that they override the normal topological
routing information carried collectively by the routers.
Using the terminology introduced by J. Noel Chiappa [8], we can say
that the home address functions in the dual role of being an
*end-point identifier (EID)* and a *permanent locator*. The
care-of-address is a pure, temporary *locator*, identifying the
current location of the mobile node. The correspondent nodes
effectively perform source routing, redirecting traffic destined to
the home address to the care-of-address. This is even reflected in
Nikander, et al. Expires December 29, 2003 [Page 9]
Internet-Draft Mobile IPv6 RO Security Design June 2003
the packet structure; the packets carry an explicit routing header.
The relationshiop between EID's and permanent locators has been
exploited by other proposals. Their technical merits and security
problems, however, are beyond the scope of this document.
Nikander, et al. Expires December 29, 2003 [Page 10]
Internet-Draft Mobile IPv6 RO Security Design June 2003
2. Dimensions of Danger
Based on the discussion above it should now be clear that the dangers
in Mobile IPv6 lie in creation (or deletion) of the local routing
exceptions. In Mobile IPv6 terms, the danger is in the possibility of
unauthorized creation of *Binding Cache Entries (BCE). *The affects
of an attack differ depending on the *target of the attack, the
timing of the attack, *and *the location of the attacker*.
2.1 Target
Basically, the target of an attack can be any node or network in the
Internet (stationary or mobile). The basic differences lie in the
goals of the attack: does the attacker aim to *divert* (steal) the
traffic destined to and/or sourced at the target node, or does it aim
to cause denial-of-service to the target node or network. The target
does not typically play much of a active role attack. As an example,
an attacker may launch a denial-of-service attack on a given node A
by contacting a large number of nodes, claiming to be A, and
subsequently diverting the traffic at these other nodes so that A is
harmed. A itself need not be involved at all before its
communications start to break. Furthermore, A is not necessarily a
mobile node; it may very well be stationary.
Mobile IPv6 uses the same class of IP addresses for both mobile nodes
(i.e., home and care-of addresses) and stationary nodes. That is,
mobile and stationary addresses are indistinguishable from each
other. Attackers can take advantage of this by taking any IP address
and using it in a context where normally only mobile (home or care-of
addresses) appear. This means that attacks that otherwise would only
concern mobiles are, in fact, a threat to all IPv6 nodes.
In fact, the role of being a mobile node appears to be most
protected, since in that role a node does not need to maintain state
about the whereabouts of some remote nodes. Conversely, the role of
being a correspondent node appears to be the weakest point since
there are very few assumptions upon which it can base its state
formation. That is, an attacker has much easier task to fool a
correspondent node to believe that an presumably mobile node is
somewhere where it is not than to fool a mobile node to believe
something similar. On the other hand, since it is possible to attack
a node indirectly by first targetting its peers, all nodes are
equally vulnerable in some sense. Furthermore, a (usually) mobile
node often also plays the role of being a correspondent node, since
it can exchange packets with other mobile nodes; see also Section
5.4.
Nikander, et al. Expires December 29, 2003 [Page 11]
Internet-Draft Mobile IPv6 RO Security Design June 2003
2.2 Timing
An important aspect in understanding Mobile IPv6 related dangers is
timing. In a stationary IPv4 network, an attacker must be between the
communication nodes at the same time as the nodes communicate. With
the Mobile IPv6 ability of creating binding cache entries, the
situation changes. A new danger is created. Without proper
protection, an attacker could attach itself between the home agent
and a correspondent node for a while, create a BCE at the
correspondent node, leave the position, and continuously update the
correspondent node about the mobile node's whereabouts. This would
make the correspondent node to send packets destined to the mobile
node to an incorrect address as long as the BCE remained valid, i.e.,
typically until the correspondent node is rebooted. The converse
would also be possible: an attacker could also launch an attack by
first creating a BCE and then letting it *expire* at a carefully
selected time. If a large number of active BCEs carrying large
amounts of traffic expired at the same time, the result might be an
overload towards the home agent or the home network. (See Section
3.2.2 for a more detailed explanation.)
2.3 Location
In a static IPv4 Internet, an attacker can only receive packets
destined to a given address if it is able to attach itself to or
control a node on the topological path between the sender and the
recipient. On the other hand, an attacker can easily send spoofed
packets from almost anywhere. If Mobile IPv6 allowed sending
unprotected Binding Updates, an attacker could create a BCE on any
correspondent node from anywhere in the Internet, simply by sending a
fraudulent Binding Update to the correspondent node. Instead of being
required to be between the two target nodes, the attacker could act
from anywhere in the Internet.
In summary, by introducing the new source routing state (binding
cache) at the correspondent nodes, Mobile IPv6 introduces the dangers
of time and space shifting. Without proper protection, Mobile IPv6
would allow an attacker to act from anywhere in the Internet and well
before the time of the actual attack. In contrast, in the static IPv4
Internet the attacking nodes must be present at the time of the
attack and they must be positioned in a suitable way, or the attack
would not be possible in the first place.
Nikander, et al. Expires December 29, 2003 [Page 12]
Internet-Draft Mobile IPv6 RO Security Design June 2003
3. Threats and limitations
This section describes attacks against Mobile IPv6 Route Optimization
and related protection mechanisms. The goal of the attacker can be to
corrupt the correspondent node's binding cache and to cause packets
to be delivered to a wrong address. This can compromise secrecy and
integrity of communication and cause denial-of-service (DoS) both at
the communicating parties and at the address that receives the
unwanted packets. The attacker may also exploit features of the
Binding Update (BU) mechanism to exhaust the resources of the mobile
node, the home agent, or the correspondent nodes. The aim of this
section is to describe the major attacks and to overview various
protocol mechanisms and their limitations. The details of the
mechanisms are covered in Section 4.
It is essential to understand that some of the threats are more
serious than others, some can be mitigated but not removed, some
threats may represent acceptable risk, and some threats may be
considered too expensive to be prevented.
We consider only active attackers. The rationale behind this is that
in order to corrupt the binding cache, 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, some active attacks are easier, for the average attacker, to
launch than a passive one would be. That is, in many active attacks
the attacker can initiate binding update processing at any time,
while most passive attacks require the attacker to wait for suitable
messages to be sent by the targets nodes.
We first consider attacks against nodes that are supposed to have a
specified address (Section 3.1), continuing with flooding attacks
(Section 3.2) and attacks against the basic Binding Update protocol
(Section 3.3). After that we present a classification of the attacks
(Section 3.4). Finally, we considering the applicability of solutions
relying on some kind of a global security infrastructure (Section
3.5).
3.1 Attacks against address 'owners' aka. address 'stealing'
The most obvious danger in Mobile IPv6 is address "stealing", i.e.,
an attacker illegitimately claiming to be a given node at a given
address, and then trying to "steal" traffic destined to that address.
There are several variants of this attack. We first describe the
basic variant, followed by a description how the situation is
affected if the target is a stationary node, and continuing more
complicated issues related to timing (the so called "future"
attacks), confidentiality and integrity, and DoS aspects.
Nikander, et al. Expires December 29, 2003 [Page 13]
Internet-Draft Mobile IPv6 RO Security Design June 2003
3.1.1 Basic address stealing
If Binding Updates were not authenticated at all, an attacker could
fabricate and send spoofed binding updates from anywhere in the
Internet. All nodes that support the correspondent node functionality
would be vulnerable to this attack. As explained in Section 2.1,
there is no way of telling which addresses belong to mobile nodes
that really could send binding updates and which addresses belong to
stationary nodes (see below).
+---+ original +---+ new packet +---+
| B |<----------------| A |- - - - - - ->| C |
+---+ packet flow +---+ flow +---+
^
|
| False BU: B -> C
|
+----------+
| Attacker |
+----------+
Figure 2
Consider an IP node A sending IP packets to another IP node B. The
attacker could redirect the packets to an arbitrary address C by
sending a Binding Update to A. The home address (HoA) in the binding
update would be B and the care-of address (CoA) would be C. After
receiving this binding update, A would send all packets intended for
the node B to the address C. See Figure 2 (Figure 2).
The attacker might select the care-of address to be either its own
current address (or another address in its local network) or any
other IP address. If the attacker selected a local care-of address
allowing it to receive the packets, it would be able to send replies
to the correspondent node. Ingress filtering at the attacker's local
network does not prevent the spoofing of Binding Updates but forces
the attacker either to choose a care-of address from inside its own
network or to use the Alternate care-of address sub-option.
The *binding update authorization mechanism* used in the MIPv6
security design is primarily aimed to mitigate this threat, and to
limit the location of attackers to the path between a correspondent
node and the home agent.
3.1.2 Stealing addresses of stationary nodes
The attacker needs to know or guess the IP addresses of both the
Nikander, et al. Expires December 29, 2003 [Page 14]
Internet-Draft Mobile IPv6 RO Security Design June 2003
source of the packets to be diverted (A in the example above) and the
destination of the packets (B). This means that it is difficult to
redirect *all* packets to or from a specific node because the
attacker would need to know the IP addresses of all the nodes with
which it is communicating.
Nodes with well-known addresses, such as servers and those using
stateful configuration, are most vulnerable. Nodes that are a part of
the network infrastructure, such as DNS servers, are particularly
interesting targets for attackers, and particularly easy to identify.
Nodes that frequently change their address and use random addresses
are relatively safe. However, if they register their address into
DynDNS, they become more exposed. Similarly, nodes that visit
publicly accessible networks such as airport wireless LANs risk
revealing their addresses. IPv6 addressing privacy features [ND01]
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.
Thus, it is *not* the mobile nodes that are most vulnerable to
address stealing attacks, it is the well known static servers.
Furthermore, the servers often run old or heavily optimized operating
systems, and may not have any mobility related code at all. Thus, the
security design cannot be based on the idea that mobile nodes might
somehow be able to detect if someone has stolen their address, and
reset the state at the correspondent node. Instead, the security
design must make reasonable measures to *prevent the creation of
fraudulent binding cache entries in the first place*.
3.1.3 Future address stealing
If an attacker knows an address that a node is likely to select in
the future, it can launch a "future" address stealing attack. The
attacker creates a Binding Cache Entry, using the home address that
it anticipates the target node to use. If the Home Agent allows
dynamic home addresses, the attacker may be able to do this
legitimately. That is, if the attacker is a client of the Home Agent,
and able to acquire the home address temporarily, it may be able to
do so, and then return the home address back to the Home Agent once
the BCE is in place.
Now, if the BCE state had a long expiration time, the target node
would acquire the same home address while the BCE is still effective,
and the attacker would be able to launch a successful
man-in-the-middle or denial-of-service attack. The mechanism applied
in the MIPv6 security design is to *limit the lifetime of Binding
Cache Entries to a few minutes*.
Nikander, et al. Expires December 29, 2003 [Page 15]
Internet-Draft Mobile IPv6 RO Security Design June 2003
Note that this attack applies only to fairly specific conditions.
There are also some variations of this attack that are theoretically
possible under some other conditions. However, all of these attacks
are limited by the Binding Cache Entry lifetime, and therefore not a
real concern under the current design.
3.1.4 Attacks against Secrecy and Integrity
By spoofing Binding Updates, an attacker could redirect all packets
between two IP nodes to itself. By sending a spoofed binding update
to A, it could capture the data intended to B. That is, it could
pretend to be B and high-jack A's connections with B, or establish
new spoofed connections. The attacker could also send spoofed binding
updates to both A and B and insert itself to the middle of all
connections between them (man-in-the-middle attack). Consequently,
the attacker would be able to see and modify the packets sent between
A and B. See Figure 3 (Figure 3)
Original data path, before man-in-the-middle attack
+---+ +---+
| A | | B |
+---+ +---+
\___________________________________/
Modified data path, after the falsified binding updates
+---+ +---+
| A | | B |
+---+ +---+
\ /
\ /
\ +----------+ /
\---------| Attacker |-------/
+----------+
Figure 3
Strong end-to-end 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
binding updates could result in denial of service (see below) but not
in disclosure or corruption of sensitive data beyond revealing the
existence of the traffic flows. Two fixed nodes could also protect
communication between themselves by refusing to accept binding
updates from each other. Ingress filtering, on the other hand, does
not help because the attacker is using its own address as the care-of
Nikander, et al. Expires December 29, 2003 [Page 16]
Internet-Draft Mobile IPv6 RO Security Design June 2003
address and is not spoofing source IP addresses.
The protection adopted in MIPv6 Security Design is to weakly
authenticate the addresses by *return routability* (RR), which limits
the topological locations from which the attack is possible (see
Section 4.1).
3.1.5 Basic Denial of Service Attacks
By sending spoofed binding updates, the attacker could redirect all
packets sent between two IP nodes to a random or nonexistent
address(es). This way, it might be able to stop or disrupt
communication between the nodes. This attack is serious because any
Internet node could be targeted, also fixed nodes belonging to the
infrastructure (e.g. DNS servers) are vulnerable. Again, the selected
protection mechanism is *return routability *(RR).
3.1.6 Replaying and Blocking Binding Updates
Any protocol for authenticating binding update has to consider replay
attacks. That is, an attacker may be able to replay recent
authenticated binding updates to the correspondent and, that way,
direct packets to the mobile node's previous location. Like spoofed
binding updates, this could be used both for capturing packets and
for DoS. The attacker could capture the packets and impersonate the
mobile node if it reserved the mobile's previous address after the
mobile node has moved away and then replayed the previous binding
update to redirect packets back to the previous location.
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. *The
*limited lifetime* of the Binding Cache entry and the associated
nonces limit the time frame within which the replay attacks are
possible.
3.2 Attacks against other nodes and networks (flooding)
By sending spoofed binding updates, an attacker could redirect
Nikander, et al. Expires December 29, 2003 [Page 17]
Internet-Draft Mobile IPv6 RO Security Design June 2003
traffic to an arbitrary IP address. This could be used to bomb an
arbitrary Internet address with excessive amounts of packets. The
attacker could also target a network by redirecting data to one or
more IP addresses within the network. There are two main variations
of flooding: basic flooding and return-to-the-home flooding. We
consider them separate.
3.2.1 Basic flooding
In the simplest attack, the attacker knows that there is a heavy data
stream from node A to B and redirects this to the target address C.
However, A would soon stop sending the data because it is not
receiving acknowledgments from B.
(B is attacker)
+---+ original +---+ flooding packet +---+
| B |<================| A |==================>| C |
+---+ packet flow +---+ flow +---+
| ^
\ /
\__________________/
False binding update + false acknowledgements
Figure 4
A more sophisticated attacker would act itself as B; see Figure 4
(Figure 4). It would first subscribe to a data stream (e.g. a video
stream) and then redirects this stream to the target address C. The
attacker would even be able to spoof the acknowledgements. For
example, consider a TCP stream. The attacker would perform the TCP
handshake itself and thus know the initial sequence numbers. After
redirecting the data to C, the attacker would continue to send one
spoofed acknowledgments. It would even be able to accelerate the data
rate by simulating a fatter pipe [5].
This attack might be even easier with UDP/RTP. The attacker could
create spoofed RTCP acknowledgements. Either way, the attacker would
be able to redirect an increasing stream of unwanted data to the
target address without doing much work itself. It could carry on
opening more streams and refreshing the Binding Cache entries by
sending a new binding update every few minutes. Thus, the limitation
of BCE lifetime to a few minutes does *not* help here alone.
During the Mobile IPv6 design process, the effectiveness of this
attack was debated. It was mistakenly assumed that the target node
Nikander, et al. Expires December 29, 2003 [Page 18]
Internet-Draft Mobile IPv6 RO Security Design June 2003
would send a TCP Reset to the source of the unwanted data stream,
which would then stop sending. In reality, all practical TCP/IP
implementations fail to send the Reset. The target node drops the
unwanted packets at the IP layer because it does not have a Binding
Update List entry corresponding to the Routing Header on the incoming
packet. Thus, the flooding data is never processed at the TCP layer
of the target node and no Reset is sent. This means that the attack
using TCP streams is more effective than was originally believed.
This attack is serious because the target can be any node or network,
not only a 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 other distributed
denial of service (DDoS) attacks. Ingress filtering in the attacker's
local network prevents the spoofing of source addresses but the
attack would still be possible by setting the Alternate care-of
address sub-option to the target address.
Again, the *protection mechanism adopted for MIPv6 is return
routability. *This time it is necessary to check that there is indeed
a node at the new care-of-address, and that the node is indeed to one
that requested redirecting packets to that very address (see Section
4.1.2).
3.2.2 Return-to-home flooding
A variation of the bombing attack targets the home address or the
home network instead of the care-of-address or a visited network. The
attacker would claim to be a mobile with the home address equal to
the target address. While claiming to be away from home, the attacker
would start downloading a data stream. The attacker would then send a
binding update cancellation (i.e. a request to delete the binding
from the Binding Cache), or just allow the cache entry to expire.
Either would redirect the data stream to the home network. Just like
when bombing a care-of-address, the attacker can keep the stream
alive and even increase data rate by spoofing acknowledgments. When
successful, the bombing attack against the home network is just as
serious as the one against a care-of-address.
The basic protection mechanism adopted is *return routability.
*However, it is hard to fully protect against this attack; see
Section 4.1.1.
3.3 Attacks against binding update protocols
Security protocols that successfully protect the secrecy and
Nikander, et al. Expires December 29, 2003 [Page 19]
Internet-Draft Mobile IPv6 RO Security Design June 2003
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.
3.3.1 Inducing Unnecessary Binding Updates
When a mobile node receives an IP packet from a new correspondent via
the home agent, it automatically initiates the binding update
protocol. An attacker can exploit this by sending the mobile node a
spoofed IP packet (e.g. ping or TCP SYN packet) that appears to come
from a new correspondent node. Since the packet arrives via the home
agent, the mobile node would automatically start the binding update
protocol with the correspondent node, thereby spending resources
unnecessarily.
In a real attack the attacker would induce the mobile node to
initiate binding update protocols with a large number of
correspondent nodes at the same time. If the correspondent addresses
are real addresses of existing IP nodes, then most instances of the
binding update protocol might even complete successfully. The entries
created in the Binding Cache are correct but useless. This way, the
attacker can induce the mobile to execute the binding update protocol
unnecessarily, which can drain the mobile's resources.
A correspondent node (i.e. any IP node) can also be attacked in a
similar way. The attacker sends spoofed IP packets to a large number
of mobiles with the target node's address as the source address.
These mobiles will initiate the binding update protocol with the
target node. Again, most of the binding update protocol executions
will complete successfully. By inducing a large number of unnecessary
binding updates, the attacker is able to consume the target node's
resources.
This attack is possible against any binding update 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 binding updates. Ingress filtering helps a
little, since it makes it harder to forge the source address of the
spoofed packets, but it does not completely eliminate this threat.
*A node 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 binding
updates*. When the limit is exceeded, the node can simply stop
attempting route optimization. Sometimes it is possible to process
Nikander, et al. Expires December 29, 2003 [Page 20]
Internet-Draft Mobile IPv6 RO Security Design June 2003
some binding updates even when a node is under the attack. A mobile
node may have a local security policy listing a limited number of
addresses to which binding updates will be sent even when the mobile
node is under DoS attack. A correspondent node (i.e. any IP node) may
similarly have a local security policy listing a limited set of
addresses from which binding updates will be accepted even when the
correspondent is under a binding update DoS attack.
The node may also recognize addresses with which they have had
meaningful communication in the past and sent binding updates to or
accept them from those addresses. Since it may be impossible for the
IP layer to know about the protocol state in higher protocol layers,
a good measure of the meaningfulness of the past communication is
probably per-address packet counts.
3.3.2 Forcing Non-Optimized Routing
As an variant of the previous attack, the attacker can prevent a
correspondent node from using route optimization by filling its
Binding Cache with unnecessary entries so that most entries for real
mobiles are dropped.
Any successful DoS attack against a mobile or a correspondent node
can also prevent the processing of binding updates. 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 home agent based 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 node 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, or 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.
3.3.3 Reflection and Amplification
Attackers sometimes try to hide the source of a packet flooding
attack by reflecting the traffic from other nodes [Sav02]. That is,
instead of sending the flood of packets directly to the target, the
attacker sends data to other nodes, tricking them to send the same
number, or more, packets to the target. Such reflection can hide the
Nikander, et al. Expires December 29, 2003 [Page 21]
Internet-Draft Mobile IPv6 RO Security Design June 2003
attacker's address even when ingress filtering prevents source
address spoofing. Reflection is particularly dangerous if the packets
can be reflected multiple times, if they can be sent into a looping
path, or if the nodes can be tricked into sending many more packets
than they receive from the attacker, because such features can be
used to amplify the traffic by a significant factor. When designing
protocols, one should avoid creating services that can be used for
reflection and amplification.
Triangle routing would easily create opportunities for reflection: a
correspondent node receives packets (e.g. TCP SYN) from the mobile
node and replies to the home address given by the mobile node in the
Home Address Option (HAO). The mobile might not really be a mobile
and the home address could actually be the target address. The target
would only see the packets sent by the correspondent and could not
see the attacker's address (even if ingress filtering prevents the
attacker from spoofing its source address).
+----------+ TCP SYN with HAO +-----------+
| Attacker |-------------------->| Reflector |
+----------+ +-----------+
|
| TCP SYN-ACK to HoA
V
+-----------+
| Flooding |
| target |
+-----------+
Figure 5
A badly designed binding update protocol could also be used for
reflection: the correspondent would respond to a data packet by
initiating the binding update authentication protocol, which usually
involves sending a packet to the home address. In that case, the
reflection attack can be discouraged by copying the mobile's address
into the messages sent by the mobile to the correspondent. (The
mobile's source address is usually the same as the care-of address
but an Alternative care-of address suboption can specify a different
care-of address.) Some of the early proposals for MIPv6 security used
this approach, and were prone to the reflection attacks.
In some of the proposals for binding update authentication protocols,
the correspondent node responded to an initial message from the
mobile with two packets (one to the home address, one to the care-of
address). It would have been possible to use this to amplify a
flooding attack by a factor of two. Furthermore, with public-key
Nikander, et al. Expires December 29, 2003 [Page 22]
Internet-Draft Mobile IPv6 RO Security Design June 2003
authentication, the packets sent by the correspondent might have been
significantly larger than the one that triggers them.
These types of reflection and amplification can be avoided by
ensuring that the correspondent only *responds to the same address
from which it received a packet, and only with a single packet of the
same size*. These principles have been applied to MIPv6 security
design.
3.4 Classification of attacks
Sect. Attack name Target Sev. Mitigation
---------------------------------------------------------------------
3.1.1 Basic address stealing MN Med. RR
3.1.2 Stealing addresses of stationary nodes Any High RR
3.1.3 Future address stealing MN Low RR, lifetime
3.1.4 Attacks against Secrecy and Integrity MN Low RR, IPsec
3.1.5 Basic Denial of Service Attacks Any Med. RR
3.1.6 Replaying and Blocking Binding Updates MN Low lifetime,
cookies
3.2.1 Basic flooding Any High RR
3.2.2 Return-to-home flooding Any High RR
3.3.1 Inducing Unnecessary Binding Updates MN, CN Med. heuristics
3.3.2 Forcing Non-Optimized Routing MN Low heuristics
3.3.3 Reflection and Amplification N/A Med. BU design
Figure 6
Table 1 (Figure 6) gives a summary of the discussed attacks. As it
stands today, the return-to-the-home flooding and the induction of
unnecessary binding updates look like the threats that we have the
least amount of protection, compared to their severity.
3.5 Problems with infrastructure based authorization
Early in the MIPv6 design process it was assumed that plain IPsec
could be used for securing Binding Updates. However, this turned out
to be impossible for two reasons. The first reason can be inferred
from the attack descriptions above: IPsec is not designed to protect
against the kinds of DoS attacks that would be possible with MIPv6;
especially, protecting against the flooding attacks would be very
difficult or even impossible with plain vanilla IPsec. The second
reason is scalability.
Relying on IPsec requires key management, and key management requires
infrastructure to distribute the keys. Furthermore, in MIPv6 it is
important to show whom an IP address belongs to, i.e., who has the
*authority* to control where packets destined to the given address
Nikander, et al. Expires December 29, 2003 [Page 23]
Internet-Draft Mobile IPv6 RO Security Design June 2003
may be redirected to. Only the "owner" of an address may send Binding
Updates to redirect packets to a care-of-address. [6]
On way of providing a global key infrastructure for mobile IP would
be DNSSEC. If there was secure reverse DNS that provided a public key
for each IP address, that could be used for verifying that a binding
update is indeed signed by an authorized party. However, in order to
be secure, each link in such a system must be secure. That is, there
must be a chain of keys and signatures all the way down from the root
to the given IP address. Furthermore, it is not enough that each key
is signed by the key above, it is also necessary that each signature
carries the meaning of authorizing the lower key to manage the
address block below it.
For example, consider the reverse DNS entry e.f.f.3.ip6.arpa . It
could be associated with a key, say K_3ffe. In order to be valid,
that key should be signed by an upper level key, let's say K3ff,
etc., up to the top level. Similarly, any subrange of addresses below
3ff0::/16 would need to be signed by K3ffe. Additionally, when the
human managing the K_3ffe key signs subkeys, he or she should make
sure that the singed subkey really belongs to a party that is
authorized to assign address blocks in the said address range. In
other words, the keys and signatures should form a tree reflecting
the actual address allocations.
Even though it would be theoretically possible to build a secure
reverse DNS infrastructure along the lines show above, the practical
problems would be insurmountable. That is, while the delegation and
key signing might work close to the root of the tree, it would
probably break down somewhere between the root and the individual
nodes. Furthermore, checking all the signatures up the tree would
place a considerable burden to the correspondent nodes, making route
optimization computationally very expensive. As the last nail on the
coffin, checking just that the mobile node is authorized to send
binding updates containing a given Home Address would not be enough,
since a malicious mobile node would still be able to launch flooding
attacks. On the other hand, relying on such an infrastructure to
assign and verify "ownership" of care-of-addresses would be even
harder than verifying home address "ownership".
Nikander, et al. Expires December 29, 2003 [Page 24]
Internet-Draft Mobile IPv6 RO Security Design June 2003
4. The solution selected for Mobile IPv6
The current Mobile IPv6 route optimization security has been
carefully designed to prevent or mitigate the threats that were
discussed in Section 3. The goal has been to produce a design whose
security is close to that of a static IPv4 based Internet, and whose
cost in terms of packets, delay and processing is not excessive. The
result is not what one would expect; the result is definitely not a
traditional cryptographic protocol. Instead, the result relies
heavily on the assumption of an uncorrupted routing infrastructure,
and builds upon the idea of checking that an alleged mobile node is
indeed reachable both through its home address and its
care-of-address. Furthermore, the lifetime of the state created at
the corresponded nodes is deliberately restricted to a few minutes,
in order to limit the potential ability of time shifting.
In this section we describe the solution in reasonable detail (for
the fine details see the specification), starting from Return
Routability (Section 4.1), continuing with a discussion about state
creation at the correspondent node (Section 4.2), and completing the
description with a discussion about the lifetime of Binding Cache
Entries (Section 4.3).
4.1 Return Routability
*Return Routability (RR) *is the name of the basic mechanism deployed
by Mobile IPv6 route optimization security design. Basically, it
means that a node verifies that there is a node that is able to
respond to packets sent to a given address. The check yields false
positives if the routing infrastructure is compromised or if there is
an attacker between the verifier and the address to be verified. With
these exceptions, it is assumed that a successful reply indicates
that there is indeed a node at the given address, and that the node
is willing to reply to the probes sent to it.
The basic return routability mechanism consist of two checks, a Home
Address check (see Section 4.1.1) and a care-of-address check (see
Section 4.1.2). The packet flow is depicted in Figure 7 (Figure 7).
First the mobile node sends two packets to the correspondent node: a
Home Test Init (HoTI) packet is sent through the home agent, and a
Care-of Test Init (CoTI) directly. The correspondent node replies to
both of these independently by sending a Home Test (HoT) in response
to the Home Test Init and a Care-of Test (CoT) in response to the
Care-of Test Init. Finally, once the mobile node has received both
the Home Test and Care-of Test packets, it sends a Binding Update to
the correspondent node.
Nikander, et al. Expires December 29, 2003 [Page 25]
Internet-Draft Mobile IPv6 RO Security Design June 2003
+------+ 1a) HoTI +------+
| |---------------------->| |
| MN | 2a) HoT | HA |
| |<----------------------| |
+------+ +------+
1b) CoTI | ^ | / ^
| |2b| CoT / /
| | | / /
| | | 3) BU / /
V | V / /
+------+ 1a) HoTI / /
| |<----------------/ /
| CN | 2a) HoT /
| |------------------/
+------+
Figure 7
It might appear that the actual design was somewhat convoluted. That
is, the real return routability checks are the message pairs < Home
Test, Binding Update > and < Care-of Test, Binding Update >. The Home
Test Init and Care-of Test Init packets are only needed to *trigger*
the test packets, and the Binding Update acts as a combined
routability response to both of the tests.
There are two main reasons behind this design:
avoidance of reflection and amplification (see Section 3.3.3), and
avoidance of state exhaustion DoS attacks (see Section 4.2).
The reason for sending two Init packets instead of one is the
avoidance of amplication.The correspondent node is replying to
packets that come out of the blue. It does not know anything about
the mobile node, and therefore it just suddenly receives an IP packet
from some arbitrarily looking IP address. In a way, this is similar
to a server receiving a TCP SYN from a previously unknown client. If
the correspondent node would send two packets in response to an
initial trigger, that would create a DoS amplification effect, as
discussed in Section 3.3.3.
Reflection avoidance is directly related. If the correspondent node
would reply to another address but the source address of the packet,
that would create a reflection effect. Thus, since the correspondent
node does not know better, the only safe way is to reply to the
received packet with just one packet, and to send the reply to the
source address of the received packet. Hence, two initial triggers
Nikander, et al. Expires December 29, 2003 [Page 26]
Internet-Draft Mobile IPv6 RO Security Design June 2003
are needed instead of just one.
Let us now consider the two return routability tests separately.
4.1.1 Home Address check
The Home Address check consists of a Home Test (HoT) packet and a
subsequent Binding Update (BU). It is triggered by the arrival of a
Home Test Init (HoTI). A correspondent node replies to a Home Test
Init by sending a Home Test to the source address of the Home Test
Init. The source address is assumed to be the home address of a
mobile node, and therefore the Home Test is assumed to be tunneled by
the Home Agent to the mobile node. The Home Test contains a
cryptographically generated token, *home keygen token, *which is
formed by calculating a hash function over the concatenation of a
secret key Kcn known only by the correspondent node, the source
address of the Home Test Init packet, and a nonce.
home keygen token = hash(Kcn | home address | nonce | 0)
An index to the nonce is also included in the Home Test packet,
allowing the correspondent node to easier find the appropriate nonce.
The token allows the correspondent node to make sure that the
subsequently received binding update is created by a node that has
seen the Home Test packet; see Section 4.2.
In most cases the Home Test packet is forwarded over two different
segments of the Internet. It first traverses from the correspondent
node to the Home Agent. On this trip, it is not protected and any
eavesdropper on the path can learn its contents. The Home Agent then
forwards the packet to the mobile node. This path is taken inside the
IPsec ESP protected tunnel, making it impossible for the outsiders to
learn the contents of the packet.
At first it may sound unnecessary to protect the packet between the
home agent and the mobile node since it travelled unprotected between
the correspondent node and the mobile node. If all links in the
Internet were equally insecure, the situation would indeed be so,
that would be unnecessary. However, in most practical settings the
network is likely to be more secure near the Home Agent than near the
Mobile Node. For example, if the home agent hosts a virtual home link
and the mobile nodes are never actually at home, an eavesdropper
should be close to the correspondent node or on the path between the
correspondent node and the home agent, since it could not eavesdrop
at the home agent. If the correspondent node is a big server, all the
links on the path between it and the Home Agent are likely to be
fairly secure. On the other hand, the Mobile Node is probably using
Nikander, et al. Expires December 29, 2003 [Page 27]
Internet-Draft Mobile IPv6 RO Security Design June 2003
wireless access technology, making it sometimes trivial to eavesdrop
its access link. Thus, it is fairly easy to eavesdrop packets that
arrive at the mobile node. Consequently, protecting the HA-MN path is
likely to provide real security benefits even when the CN-HA path
remains unprotected.
4.1.2 Care-of-Address check
From the correspondent node's point of view, the Care-of check is
very similar to the Home check. The only difference is that now the
source address of the received Care-of Test Init packet is assumed to
be the care-of-address of the mobile node. Furthermore, the token is
created in a slightly different manner in order to make it impossible
to use home tokens for care-of tokens or vice versa.
care-of keygen token = hash(Kcn | care-of address | nonce | 1)
The Care-of Test traverses only one leg, directly from the
correspondent node to the mobile node. It remains unprotected all
along the way, making it vulnerable to eavesdroppers near the
correspondent node, on the path from the correspondent node to the
mobile node, or near the mobile node.
4.1.3 Forming the first Binding Update
When the mobile node has received both the Home Test and Care-of Test
messages, it creates a binding key Kbm by taking a hash function over
the concatenation of the tokens received.
This key is used to protect the first and the subsequent binding
updates, as long as the key remains valid.
Note that the key Kbm is available to anyone that is able to receive
both the Care-of Test and Home Test messages. However, they are
normally router through different routes through the network, and the
Home Test is transmitted over an encrypted tunnel from the home agent
to the mobile node; see also Section 5.4.
4.2 Creating state safely
The correspondent node may remain *stateless* until it receives the
first Binding Update. That is, it does not need to record receiving
and replying to the Home Test Init and Care-of Test Init messages.
The Home Test Init/Home Test and Care-of Test Init/Care-of Test
exchanges take place in parallel but independently from each other.
Thus, the correspondent can respond to each message immediately and
it does not need to remember doing that. This helps in potential
Denial-of-Service situations: no memory needs to be reserved when
Nikander, et al. Expires December 29, 2003 [Page 28]
Internet-Draft Mobile IPv6 RO Security Design June 2003
processing Home Test Init and Care-of Test Init messages.
Furthermore, Home Test Init and Care-of Test Init processing is
designed to be lightweight, and it can be rate limited if necessary.
When receiving a first binding update, the correspondent node goes
through a rather complicated procedure. The purpose of this procedure
is to ensure that there is indeed a mobile node that has recently
received a Home Test and a Care-of Test that were sent to the claimed
home and care-of-addresses, respectively, and to make sure that the
correspondent node does not unnecessarily spend CPU or other
resources while performing this check.
Since the correspondent node does not have any state when the binding
update arrives, the binding update itself must contain enough
information so that relevant state can be created. The binding update
contains the following pieces of information for that:
The source address must be equal to the source address used in the
Care-of Test Init message.
This must be the same address that was used as the source address
for the Home Test Init message and as the destination address for
the Home Test message.
These are copied over from the Home Test and Care-of Test
messages, and together with the other information they allow the
correspondent node to re-create the tokens sent in the Home Test
and Care-of Test messages and used for creating Kbm. Without them
the correspondent node might need to try the 2-3 latest nonces,
leading to unnecessary resource consumption.
The binding update is authenticated by computing a MAC function
over the care-of-address, the correspondent node's address and the
binding update message itself. The MAC is keyed with the key Kbm.
Given the addresses, the nonce indices and thereby the nonces, and
the key Kcn, the correspondent node can re-create the home and
care-of tokens at the cost of a few memory lookups and computation of
one MAC and one hash function.
Once the correspondent node has re-created the tokens, it hashes the
tokens together, giving the key Kbm. If the Binding Update is
authentic, Kbm is cached together with the binding. This key is then
used to verify the MAC that protects integrity and origin of the
actual Binding Update. Note that the same Kbm may be used for a
while, until either the mobile node moves (and needs to get a new
care-of-address token), the care-of token expires, or the home token
expires.
Nikander, et al. Expires December 29, 2003 [Page 29]
Internet-Draft Mobile IPv6 RO Security Design June 2003
4.2.1 Retransmissions and state machine
Note that since the correspondent node may remain stateless until it
receives a valid binding update, the mobile node is solely
responsible for retransmissions. That is, the mobile node should keep
sending the Home Test Init / Care-of Test Init messages until it
receives a Home Test / Care-of Test, respectively. Similarly, it may
need to send the binding update a few times in the case it is lost
while in transit.
4.3 Quick expiration of the Binding Cache Entries
A Binding Cache Entry, along the key Kbm, represents the return
routability state of the network *at the time* when the Home Test and
Care-of Test messages were sent out. Now, it is possible that a
specific attacker is able to eavesdrop a Home Test message at some
point of time but not later. If the Home Test had an infinite or a
long lifetime, that would allow the attacker to perform a *time
shifting* attack (see Section 2.2). That is, in the current IPv4
architecture an attacker at the path between the correspondent node
and the home agent is able to perform attacks only as long as the
attacker is able to eavesdrop (and possibly disrupt) communications
on that particular path. A long living Home Test, and consequently
the ability to send valid binding updates for a long time, would
allow the attacker to continue its attack even after the attacker is
not any more able to eavesdrop the path.
To limit the seriousness of this and other similar time shifting
threats, the validity of the tokens is limited to a few minutes. This
effectively limits the validity of the key Kbm and the lifetime of
the resulting binding updates and binding cache entries.
While short life times are necessary given the other aspects of the
security design and the goals, they are clearly detrimental for
efficiency and robustness. That is, a Home Test Init / Home Test
message pair must be exchanged through the home agent every few
minutes. These messages are unnecessary from a pure functional point
of view, thereby representing overhead. What is worse, though, is
that they make the home agent a single point of failure. That is, if
the Home Test Init / Home Test messages were not needed, the existing
connections from a mobile node to other nodes could continue even
when the home agent fails, but the current design forces the bindings
to expire after a few minutes.
This concludes our brief walkthrough of the selected security design.
The cornerstones of the design were the employment of the return
routability idea in the Home Test, Care-of Test and binding update
messages, the ability to remain stateless until a valid binding
Nikander, et al. Expires December 29, 2003 [Page 30]
Internet-Draft Mobile IPv6 RO Security Design June 2003
update is received, and the limiting of the binding life times to a
few minutes. Next we briefly discuss some of the remaining threats
and other problems inherent to the design.
Nikander, et al. Expires December 29, 2003 [Page 31]
Internet-Draft Mobile IPv6 RO Security Design June 2003
5. Security considerations
In this section we give a brief analysis of the security design,
mostly in the light of what was know at the time the design was
completed in fall 2002. It should be noted that this section does
*not* present a proper security analysis of the protocol, but merely
discusses a few issues that were known at the time the design was
completed.
It should be kept in mind that the MIPv6 RO security design was never
intended to be fully secure. Instead, as we stated earlier, to goal
was to be roughly as secure as non-mobile IPv4 was known to be at the
time of the design. As it turns out, the result is slightly less
secure than IPv4, but the difference is small and most likely to be
insignificant in real life.
The known difference to IPv4, a time shifting problem, is discussed
in Section 5.4 discusses the special case of two mobile nodes
conversing with each other.
5.1 Time shifting attacks
As we mentioned in Section 4.2, the lifetime of a binding represents
a potential time shift in an attack. That is, an attacker that is
able to create a false binding is able to reap the benefits of the
binding as long as the binding lasts, or, alternatively, is able to
delay a return-to-the-home flooding attack Section 3.2.2) until the
binding expires. This is a difference from IPv4 where an attacker may
continue an attack only as long as it is at the path between the two
hosts.
Since the binding lifetimes are severely restricted in the current
design, the ability to do a time shifting attack is respectively
restricted.
5.2 Interaction with IPsec
A major motivation behind the current binding update design was
scalability, the ability to run the protocol without any existing
security infrastructure. An alternative would have been to rely on
existing trust relationships, perhaps in the form of a special
purpose Public Key Infrastructure and IPsec. That would have limited
scalability, making route optimization available in environments
where it is possible to create appropriately authorized IPsec
security associations between the mobile nodes and the corresponding
nodes.
There clearly are situations where there exists an appropriate
Nikander, et al. Expires December 29, 2003 [Page 32]
Internet-Draft Mobile IPv6 RO Security Design June 2003
relationship between a mobile node and the correspondent node. For
example, if the correspondent node is a server that has
pre-established keys with the mobile node, that would be the case.
However, entity authentication or an authenticated session key is not
necessarily sufficient for accepting Binding Updates. If one wants
to replace the home address check with some cryptographic
credentials, the credentials *must* carry proper *authorization* for
the specific home address. For example, if the mobile nodes hands out
a certificate to the correspondent node and they consequently create
a pair of IPsec security associations, it is not necessarily clear
that those security associations could be used to replace the home
address check. Instead, if and only if the certificate explicitly
states what the mobile node's home address is and that the mobile
node is *authorized* to create bindings for its home address, home
address checks may be dropped. Furthermore, care must be taken to
make sure that the issuer of the certificate is entitled to express
such authorization.
In practise, it seems highly unlikely that the nodes were ever able
to replace the care-of address check with credentials. The care-of
addresses are ephemeral, and it is highly unlikely that a mobile node
would be able to present credentials that show it *authorized* to use
the care of address without any check.
The current specification does not specify how to use IPsec together
with the mobility procedures between the mobile node and
correspondent node. Hence, currently there are no standard way of
replacing the home address check. On the other hand, the
specification is carefully written to allow the creation of the
binding management key Kbm through some different means.
5.3 Pretending to be your neighbor
One possible attack against the security design is to pretend to be a
neighboring node. To launch this attack, the mobile nodes establishes
route optimization with some arbitrary correspondent node. While
performing the return routability tests and creating the binding
management key Kbm, the attacker uses its real home address but a
faked care-of address. Indeed, the care-of address would be the
address of the neighboring node on the local link. The attacker is
able to create the binding since it receives a valid Home Test
normally, and it is able to eavesdrop the Care-of Test as it appears
on the local link.
This attack would allow the mobile node to divert unwanted traffic
towards the neighboring node, resulting in an flooding attack.
However, this attack is not very serious in practise. Firstly, it is
Nikander, et al. Expires December 29, 2003 [Page 33]
Internet-Draft Mobile IPv6 RO Security Design June 2003
limited in the terms of location, since it is only possible against
neighbors. Secondly, the attack works also against the attacker,
since it is sharing the local link with the target. Thirdly, a
similar attack can be worked out with Neighbor Discovery spoofing.
5.4 Two mobile nodes talking to each other
When two mobile nodes want to establish route optimization with each
other, some care must be exercised in order not to reveal the reverse
tokens to an attacker. In this situation, both mobile nodes act
simultaneously in the mobile node and the correspondent node roles.
In the correspondent node role, the nodes are vulnerable to attackers
that are co-located at the same link. Such an attacker is able to
learn both the Home Test and Care-of Test sent by the mobile node,
and therefore it is able to spoof the location of the *other* mobile
host to the neighboring one. What is worse is that the attacker can
obtain a valid Care-of Test itself, combine it with the Home Test,
and the claim to the neighboring node that the other node has just
arrived at the same link.
There is an easy way to void this attack. In the correspondent node
role, the mobile node should tunnel the sent Home Test messages
through its home agent. This prevents the co-located attacker from
learning any valid Home Test messages.
Nikander, et al. Expires December 29, 2003 [Page 34]
Internet-Draft Mobile IPv6 RO Security Design June 2003
6. Conclusions
In this document we have discussed the security design rationale for
the Mobile IPv6 Route Optimization. We have tried to describe the
dangers created by Mobile IP Route Optimization, the security goals
and background of the design, and the actual mechanisms employed.
We started the discussion with a background tour to the IP routing
architecture the definition of the mobility problem. After that we
covered the dimensions of the danger: the targets, the time shifting
abilities, and the possible locations of an attacker. We outlined a
number of identified threat scenarios, and discussed how they are
mitigated in the current design. Finally, in Section 4 we gave an
overview of the actual mechanisms employed, and the rational behind
them.
We have also briefly covered some of the known subtleties and
shortcomings, but that discussion cannot be exhaustive. It is quite
probable that new subtle problems will be discovered from the design.
As a consequence, it is most likely that the design needs to be
revised in the light of experience and insights.
Nikander, et al. Expires December 29, 2003 [Page 35]
Internet-Draft Mobile IPv6 RO Security Design June 2003
7. Acknowledgements
Hesham Soliman for reminding us about the threat explained in Section
5.3. Francis Dupont for first discussing the case of two mobile
nodes talking to each other Section 5.4. For sundry discussions in
this problem space, thanks to Erik Nordmark who, along with the
authors of this note, participated in the security design team whose
output and motivations we have summarized here.
Nikander, et al. Expires December 29, 2003 [Page 36]
Internet-Draft Mobile IPv6 RO Security Design June 2003
References (informative)
[1] Aura, T., Roe, M. and J. Arkko, "Security of Internet Location
Management", Proc. 18th Annual Computer Security Applications
Conference, pages 78-87, Las Vegas, NV USA, IEEE Press.,
December 2002.
[2] Campbell, A., Gomez, J., Kim, S., Turanyi, Z., Wan, C-Y. and A.
Valko, "Comparison of IP Micro-Mobility Protocols", IEEE
Wireless Communications Magazine Vol. 9, No. 1, February 2002.
[3] Bush, R. and D. Meyer, "Some Internet Architectural Guidelines
and Philosophy", RFC 3439, December 2002.
[4] Chiappa, J., "Will The Real "End-End Principle" Please Stand
Up?", date unknown.
[5] Savage, S., Cardwell, N., Wetherall, D. and T. Anderson, "TCP
Congestion Control with a Misbehaving Receiver", Computer
Communication Review 29:5, 1999.
[6] Nikander, P., "Denial-of-Service, Address Ownership, and Early
Authentication in the IPv6 World", Security Protocols 9th
International Workshop, Cambridge, UK, April 25-27 2001, LNCS
2467, pages 12-26, Springer, 2002.
[7] Perlman, R., "Network Layer Protocols with Byzantine
Robustness", PhD thesis Department of EECS, MIT, August 1988.
[8] Chiappa, J., "Endpoints and Endpoint Names: A Proposed
Enhancement to the Internet Architecture", date unknown.
[9] Nikander, P., Ylitalo, J. and J. Wall, "Integrating Security,
Mobility, and Multi-Homing in a HIP Way", Proceedings of
Network and Distributed Systems Security Symposium (NDSS'03),
February 6-7, 2003, San Diego, CA, pages 87-99, Internet
Society, February 2003.
Nikander, et al. Expires December 29, 2003 [Page 37]
Internet-Draft Mobile IPv6 RO Security Design June 2003
Authors' Addresses
Pekka Nikander
Ericsson Research Nomadic Lab
JORVAS FIN-02420
FINLAND
Phone: +358 9 299 1
EMail: pekka.nikander@nomadiclab.com
Tuomas Aura
Microsoft Research
Jari Arkko
Ericsson Research Nomadic Lab
Gabriel Montenegro
Sun Microsystems
Erik Nordmark
Sun Microsystems
Nikander, et al. Expires December 29, 2003 [Page 38]
Internet-Draft Mobile IPv6 RO Security Design June 2003
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assignees.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
Nikander, et al. Expires December 29, 2003 [Page 39]
Internet-Draft Mobile IPv6 RO Security Design June 2003
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Nikander, et al. Expires December 29, 2003 [Page 40]