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Network Working Group C. Vogt
Request for Comments: 4832 Universitaet Karlsruhe (TH)
Category: Informational J. Kempf
DoCoMo USA Labs
April 2007
Security Threats to Network-Based Localized
Mobility Management (NETLMM)
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document discusses security threats to network-based localized
mobility management. Threats may occur on two interfaces: the
interface between a localized mobility anchor and a mobile access
gateway, as well as the interface between a mobile access gateway and
a mobile node. Threats to the former interface impact the localized
mobility management protocol itself.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Threats to Interface between LMA and MAG . . . . . . . . . . . 3
2.1. LMA Compromise or Impersonation . . . . . . . . . . . . . 3
2.2. MAG Compromise or Impersonation . . . . . . . . . . . . . 4
2.3. Man-in-the-Middle Attack . . . . . . . . . . . . . . . . . 6
3. Threats to Interface between MAG and Mobile Node . . . . . . . 6
3.1. Mobile Node Compromise or Impersonation . . . . . . . . . 7
3.2. Man-in-the-Middle Attack . . . . . . . . . . . . . . . . . 9
4. Threats from the Internet . . . . . . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References . . . . . . . . . . . . . . . . . . . 10
7.2. Informative References . . . . . . . . . . . . . . . . . . 10
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1. Introduction
The network-based localized mobility management (NETLMM) architecture
[1] supports movement of IPv6 mobile nodes locally within a domain
without requiring mobility support in the mobile nodes' network
stacks. A mobile node can keep its IP address constant as it moves
from link to link, avoiding the signaling overhead and latency
associated with changing the IP address. Software specifically for
localized mobility management is not required on the mobile node,
whereas IP-layer movement detection software may be necessary, and
driver software for link-layer mobility is prerequisite.
The IP addresses of mobile nodes have a prefix that routes to a
localized mobility anchor (LMA) [3]. The LMA maintains an individual
route for each registered mobile node. Any particular mobile node's
route terminates at a mobile access gateway (MAG) [3], to which the
mobile node attaches at its current access link. MAGs are
responsible for updating the mobile node's route on the LMA as the
mobile node moves. A MAG detects the arrival of a mobile node on its
local access link based on handoff signaling that the mobile node
pursues. The MAG may additionally monitor connectivity of the mobile
node in order to recognize when the mobile node has left the local
access link. The localized mobility management architecture
therefore has two interfaces:
1. The interface between a MAG and an LMA where route update
signaling occurs.
2. The interface between a mobile node and its current MAG where
handoff signaling and other link maintenance signaling occur.
The localized mobility management architecture demands no specific
protocol for a MAG to detect the arrival or departure of mobile nodes
to and from its local access link and accordingly initiate route
update signaling with an LMA. An appropriate mechanism may be
entirely implemented at the link layer, such as is common for
cellular networks. In that case, the IP layer never detects any
movement, even when a mobile node moves from one link to another
handled by a different MAG. If the link layer does not provide the
necessary functionality, the mobile node must perform IP-layer
movement detection and auto-configuration signaling, thereby
providing the trigger for the MAG to update its route on the LMA. A
mobile node identity, established by the localized mobility
management domain when the mobile node initially connects and
authenticates, enables the MAG to ascribe the decisive link- or IP-
layer signaling to the correct mobile node. Some wireless access
technologies may require the mobile node identity to be reestablished
on every link-layer handoff.
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Vulnerabilities in either interface of the localized mobility
management architecture may entail new security threats that go
beyond those that already exist in IPv6. Potential attack objectives
may be to consume network services at the cost of a legitimate mobile
node, interpose in a mobile node's communications and possibly
impersonate the mobile node from a position off-link, operate under
the disguise of a false or non-existing identity, or cause denial of
service to a mobile node or to the localized mobility management
domain as a whole. This document identifies and discusses security
threats on both interfaces of the localized mobility management
architecture. It is limited to threats that are peculiar to
localized mobility management; threats to IPv6 in general are
documented in [4].
1.1. Terminology
The terminology in this document follows the definitions in [2], with
those revisions and additions from [1]. In addition, the following
definition is used:
Mobile Node Identity
An identity established for the mobile node when initially
connecting to the localized mobility management domain. It allows
the localized mobility management domain to definitively and
unambiguously identify the mobile node upon handoff for route
update signaling purposes. The mobile node identity is
conceptually independent of the mobile node's IP or link-layer
addresses, but it must be securely bound to the mobile node's
handoff signaling.
2. Threats to Interface between LMA and MAG
The localized mobility management protocol executed on the interface
between an LMA and a MAG serves to establish, update, and tear down
routes for data plane traffic of mobile nodes. Threats to this
interface can be separated into compromise or impersonation of a
legitimate LMA, compromise or impersonation of a legitimate MAG, and
man-in-the-middle attacks.
2.1. LMA Compromise or Impersonation
A compromised LMA can ignore route updates from a legitimate MAG in
order to deny service to a mobile node. It may also be able to trick
a legitimate MAG into creating a new, incorrect route, thereby
preparing the MAG to receive redirected traffic of a mobile node; it
may cause the traffic forwarded by a MAG to be redirected to a
different LMA; or it may simply have the MAG drop an existing route
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in order to deny the mobile node service. Since data plane traffic
for mobile nodes routes through the LMA, a compromised LMA can also
intercept, inspect, modify, or drop such traffic, or redirect it to a
destination in collusion with the attacker. The attack can be
conducted transiently to selectively disable traffic for any
particular mobile node or MAG at particular times.
Moreover, a compromised LMA may manipulate its routing table such
that all packets are directed towards a single MAG. This may result
in a denial-of-service attack against that MAG and its attached
access link.
These threats also emanate from an attacker which tricks a MAG into
believing that it is a legitimate LMA. This attacker can cause the
MAG to conduct route update signaling with the attacker instead of
with the legitimate LMA, enabling it to ignore route updates from the
MAG, or induce incorrect route changes at the MAG as described above,
in order to redirect or deny a mobile node's traffic. The attacker
does not necessarily have to be on the original control plane path
between the legitimate LMA and the MAG, provided that it can somehow
make its presence known to the MAG. Failure to mutually authenticate
when establishing an association between an LMA and a MAG would allow
an attacker to establish itself as a rogue LMA.
The attacker may further be able to intercept, inspect, modify, drop,
or redirect data plane traffic to and from a mobile node. This is
obvious if the attacker is on the original data plane path between
the legitimate LMA and the mobile node's current MAG, which may
happen independently of whether the attacker is on the original
control plane path. If the attacker is not on this path, it may be
able to leverage the localized mobility management protocol to
redefine the prefix that the mobile node uses in IP address
configuration. The attacker can then specify a prefix that routes to
itself. Whether or not outgoing data plane packets sourced by the
mobile node can be interfered with by an attacker off the original
data plane path depends on the specific data plane forwarding
mechanism within the localized mobility management domain. For
example, if IP-in-IP encapsulation or an equivalent approach is used
for outbound data plane packets, the packets can be forced to be
routed through the attacker. On the other hand, standard IP routing
may cause the packets to be relayed via a legitimate LMA and hence to
circumvent the attacker.
2.2. MAG Compromise or Impersonation
A compromised MAG can redirect a mobile node's traffic onto its local
access link arbitrarily, without authorization from the mobile node.
This threat is similar to an attack on a typical routing protocol
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where a malicious stub router injects a bogus host route for the
mobile node. In general, forgery of a subnet prefix in link state or
distance vector routing protocols requires support of multiple
routers in order to obtain a meaningful change in forwarding
behavior. But a bogus host route is likely to take precedence over
the routing information advertised by legitimate routers, which is
usually less specific; hence, the attack should succeed even if the
attacker is not supported by other routers. A difference between
redirection in a routing protocol and redirection in localized
mobility management is that the former impacts the routing tables of
multiple routers, whereas the latter involves only the compromised
MAG and an LMA.
Moreover, a compromised MAG can ignore the presence of a mobile node
on its local access link and refrain from registering the mobile node
at an LMA. The mobile node then loses its traffic. The compromised
MAG may further be able to cause interruption to a mobile node by
deregistering the mobile node at the serving LMA, pretending that the
mobile node has powered down. The mobile node then needs to
reinitiate the network access authentication procedure, which the
compromised MAG may prevent repeatedly until the mobile node moves to
a different MAG. The mobile node should be able to handle this
situation, but the recovery process may be lengthy and hence impair
ongoing communication sessions to a significant extent.
Denial of service against an LMA is another threat of MAG subversion.
The compromised MAG can trick an LMA into believing that a high
number of mobile nodes have attached to the MAG. The LMA will then
establish a routing table entry for each of the non-existing mobile
nodes. The unexpected growth of the routing table may eventually
cause the LMA to reject legitimate route update requests. It may
also decrease the forwarding speed for data plane packets due to
higher route lookup latencies, and it may, for the same reason, slow
down the responsiveness to control plane packets. Another adverse
side effect of a high number of routing table entries is that the
LMA, and hence the localized mobility management domain as a whole,
becomes more susceptible to flooding packets from external attackers
(see Section 4). The high number of superfluous routes increase the
probability that a flooding packet, sent to a random IP address
within the localized mobility management domain, matches an existing
routing table entry at the LMA and gets tunneled to a MAG, which in
turn performs address resolution on the local access link. At the
same time, fewer flooding packets can be dropped directly at the LMA
on the basis of a nonexistent routing table entry.
All of these threats apply not just to a compromised MAG, but also to
an attacker that manages to counterfeit the identity of a legitimate
MAG in interacting with both mobile nodes and an LMA. Such an
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attacker can behave towards mobile nodes like an authorized MAG and
engage an LMA in route update signaling. In a related attack, the
perpetrator eavesdrops on signaling packets exchanged between a
legitimate MAG and an LMA, and replays these packets at a later time.
These attacks may be conducted transiently, to selectively disable
traffic for any particular mobile node at particular times.
2.3. Man-in-the-Middle Attack
An attacker that manages to interject itself between a legitimate LMA
and a legitimate MAG can act as a man in the middle with respect to
both control plane signaling and data plane traffic. If the attacker
is on the original control plane path, it can forge, modify, or drop
route update packets so as to cause the establishment of incorrect
routes or the removal of routes that are in active use. Similarly,
an attacker on the original data plane path can intercept, inspect,
modify, drop, and redirect data plane packets sourced by or destined
to a mobile node.
A compromised switch or router located between an LMA and a MAG can
cause similar damage. Any switch or router on the control plane path
can forge, modify, or drop control plane packets, and thereby
interfere with route establishment. Any switch or router on the data
plane path can intercept, inspect, modify, and drop data plane
packets, or rewrite IP headers so as to divert the packets from their
original path.
An attacker between an LMA and a MAG may further impersonate the MAG
towards the LMA, and vice versa in route update signaling. The
attacker can interfere with a route establishment even if it is not
on the original control plane path between the LMA and the MAG. An
attacker off the original data plane path may undertake the same to
cause inbound data plane packets destined to the mobile node to be
routed first from the LMA to the attacker, then to the mobile node's
MAG, and finally to the mobile node itself. As explained in
Section 2.1, here, too, it depends on the specific data plane
forwarding mechanism within the localized mobility management domain
whether or not the attacker can influence the route of outgoing data
plane packets sourced by the mobile node.
3. Threats to Interface between MAG and Mobile Node
A MAG monitors the arrival and departure of mobile nodes to and from
its local access link based on link- or IP-layer mechanisms.
Whatever signaling on the access link is thereby decisive must be
securely bound to the mobile node identity. A MAG uses this binding
to ascribe the signaling to the mobile node and accordingly initiate
route update signaling with an LMA. The binding must be robust to
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spoofing because it would otherwise facilitate impersonation of the
mobile node by a third party, denial of service, or man-in-the-middle
attacks.
3.1. Mobile Node Compromise or Impersonation
An attacker that is able to forge the mobile node identity of a
mobile node can trick a MAG into redirecting data plane packets for
the mobile node to the attacker. The attacker can launch such an
impersonation attack against a mobile node that resides on the same
link as the attacker, or against a mobile node on a different link.
If the attack is on-link, the redirection of packets from the mobile
node to the attacker is internal to the MAG, and it involves no route
update signaling between the MAG and an LMA. On-link attacks are
possible in a regular IPv6 network [4] that does not use Secure
Neighbor Discovery [5].
Off-link impersonation requires the attacker to fabricate handoff
signaling of the mobile node and thus trick the MAG into believing
that the mobile node has handed over onto the MAG's access link. The
attack is conceivable both if the attacker and the mobile node are on
separate links that connect to different MAGs, as well as if they are
on separate, possibly virtual per-mobile-node links that connect to
the same MAG. In the former case, two MAGs would think they see the
mobile node and both would independently perform route update
signaling with the LMA. In the latter case, route update signaling
is likely to be performed only once, and the redirection of packets
from the mobile node to the attacker is internal to the MAG. The
mobile node can always recapture its traffic back from the attacker
through another run of handoff signaling. But standard mobile nodes
are generally not prepared to counteract this kind of attack, and
even where network stacks include suitable functionality, the attack
may not be noticeable early enough at the link or IP layer to quickly
institute countermeasures. The attack is therefore disruptive at a
minimum, and may potentially persist until the mobile node initiates
signaling again upon a subsequent handoff.
Impersonation attacks can be prevented at the link layer,
particularly with cellular technologies where the handoff signaling
between the mobile node and the network must be authenticated and is
completely controlled by the wireless link layer. Cellular access
technologies provide a variety of cryptographic and non-cryptographic
attack barriers at the link layer, which makes mounting an
impersonation attack, both on-link and off-link, very difficult.
However, for non-cellular technologies that do not require link-layer
authentication and authorization during handoff, impersonation
attacks may be possible.
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An attacker that can forge handoff signaling may also cause denial of
service against the localized mobility management domain. The
attacker can trick a MAG into believing that a large number of mobile
nodes have attached to the local access link and thus induce it to
initiate route update signaling with an LMA for each mobile node
assumed on link. The result of such an attack is both superfluous
signaling overhead on the control plane as well as a high number of
needless entries in the LMA's and MAG's routing tables. The
unexpected growth of the routing tables may eventually cause the LMA
to reject legitimate route update requests, and it may cause the MAG
to ignore handoffs of legitimate mobile nodes onto its local access
link. It may also decrease the LMA's and MAG's forwarding speed for
inbound and outbound data plane packets due to higher route lookup
latencies, and it may for the same reason slow down their
responsiveness to control plane packets. An adverse side effect of
this attack is that the LMA, and hence the localized mobility
management domain as a whole, becomes more susceptible to flooding
packets from external attackers (see Section 4). The high number of
superfluous routes increases the probability that a flooding packet,
sent to a random IP address within the localized mobility management
domain, matches an existing routing table entry at the LMA and gets
tunneled to a MAG, which in turn performs address resolution on the
local access link. At the same time, fewer flooding packets can be
dropped directly at the LMA on the basis of a nonexistent routing
table entry.
A threat related to the ones identified above, but not limited to
handoff signaling, is IP spoofing [6]. Attackers use IP spoofing
mostly for reflection attacks or to hide their identities. The
threat can be reasonably contained by a wide deployment of network
ingress filtering [7] in routers, especially within access networks.
This technique prevents IP spoofing to the extent that it ensures
topological correctness of IP source address prefixes in to-be-
forwarded packets. Where the technique is deployed in an access
router, packets are forwarded only if the prefix of their IP source
address is valid on the router's local access link. An attacker can
still use a false interface identifier in combination with an on-link
prefix. But since reflection attacks typically aim at off-link
targets, and the enforcement of topologically correct IP address
prefixes also limits the effectiveness of identity concealment,
network ingress filtering has proven adequate so far. On the other
hand, prefixes are not limited to a specific link in a localized
mobility management domain, so merely ensuring topological
correctness through ingress filtering becomes insufficient. An
additional mechanism for IP address ownership verification is
necessary to prevent an attacker from sending packets with an off-
link IP source address.
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3.2. Man-in-the-Middle Attack
An attacker that can interpose between a mobile node and a MAG during
link- and/or IP-layer handoff signaling may be able to mount a man-
in-the-middle attack on the mobile node, spoofing the mobile node
into believing that it has a legitimate connection with the localized
mobility management domain. The attacker can thus intercept,
inspect, modify, or drop data plane packets sourced by or destined to
the mobile node.
4. Threats from the Internet
A localized mobility management domain uses individual host routes
for data plane traffic of different mobile nodes, each between an LMA
and a MAG. Creation, maintenance, and deletion of these routes cause
control traffic within the localized mobility management domain.
These characteristics are transparent to mobile nodes as well as
external correspondent nodes, but the functional differences within
the domain may influence the impact that a denial-of-service attack
from the outside world can have on the domain.
A denial-of-service attack on an LMA may be launched by sending
packets to arbitrary IP addresses that are potentially in use by
mobile nodes within the localized mobility management domain. Like a
border router, the LMA is in a topological position through which a
substantial amount of data plane traffic goes, so it must process the
flooding packets and perform a routing table lookup for each of them.
The LMA can discard packets for which the IP destination address is
not registered in its routing table. But other packets must be
encapsulated and forwarded. A target MAG as well as any mobile nodes
attached to that MAG's local access link are also likely to suffer
damage because the unrequested packets must be decapsulated and
consume link bandwidth as well as processing capacities on the
receivers. This threat is in principle the same as for denial of
service on a regular IPv6 border router, but because the routing
table lookups may enable the LMA to drop part of the flooding packets
early on or, on the contrary, additional tunneling workload is
required for packets that cannot be dropped, the impact of an attack
against localized mobility management may be different.
In a related attack, the attacker manages to obtain a globally
routable IP address of an LMA or a different network entity within
the localized mobility management domain and perpetrates a denial-of-
service attack against that IP address. Localized mobility
management is, in general, somewhat resistant to such an attack
because mobile nodes need never obtain a globally routable IP address
of any entity within the localized mobility management domain.
Hence, a compromised mobile node cannot pass such an IP address off
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to a remote attacker, limiting the feasibility of extracting
information on the topology of the localized mobility management
domain. It is still possible for an attacker to perform IP address
scanning if MAGs and LMAs have globally routable IP addresses, but
the much larger IPv6 address space makes scanning considerably more
time consuming.
5. Security Considerations
This document describes threats to network-based localized mobility
management. These may either occur on the interface between an LMA
and a MAG, or on the interface between a MAG and a mobile node.
Mitigation measures for the threats, as well as the security
considerations associated with those measures, are described in the
respective protocol specifications [3][8] for the two interfaces.
6. Acknowledgments
The authors would like to thank the NETLMM working group, especially
Jari Arkko, Charles Clancy, Gregory Daley, Vijay Devarapalli,
Lakshminath Dondeti, Gerardo Giaretta, Wassim Haddad, Andy Huang,
Dirk von Hugo, Julien Laganier, Henrik Levkowetz, Vidya Narayanan,
Phil Roberts, and Pekka Savola (in alphabetical order) for valuable
comments and suggestions regarding this document.
7. References
7.1. Normative References
[1] Kempf, J., Ed., "Problem Statement for Network-Based Localized
Mobility Management", RFC 4830, April 2007.
[2] Manner, J. and M. Kojo, "Mobility Related Terminology",
RFC 3753, June 2004.
7.2. Informative References
[3] Levkowetz, H., Ed., "The NetLMM Protocol", Work in Progress,
October 2006.
[4] Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor
Discovery (ND) Trust Models and Threats", RFC 3756, May 2004.
[5] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
Neighbor Discovery (SEND)", RFC 3971, March 2005.
[6] CERT Coordination Center, "CERT Advisory CA-1996-21 TCP SYN
Flooding and IP Spoofing Attacks", September 1996.
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[7] Ferguson, P. and D. Senie, "Network Ingress Filtering: Defeating
Denial of Service Attacks which employ IP Source Address
Spoofing", BCP 38, RFC 2827, May 2000.
[8] Laganier, J., Narayanan, S., and F. Templin, "Network-based
Localized Mobility Management Interface between Mobile Node and
Access Router", Work in Progress, June 2006.
Authors' Addresses
Christian Vogt
Institute of Telematics
Universitaet Karlsruhe (TH)
P.O. Box 6980
76128 Karlsruhe
Germany
EMail: chvogt@tm.uka.de
James Kempf
DoCoMo USA Labs
3240 Hillview Avenue
Palo Alto, CA 94304
USA
EMail: kempf@docomolabs-usa.com
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