| Rfc | 8116 |
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| Title | Security Threats to the Optimized Link State Routing Protocol
Version 2 (OLSRv2) |
|---|
| Author | T. Clausen, U. Herberg, J. Yi |
|---|
| Date | May 2017 |
|---|
| Format: | TXT, HTML |
|---|
| Status: | INFORMATIONAL |
|---|
|
Internet Engineering Task Force (IETF) T. Clausen
Request for Comments: 8116
Category: Informational U. Herberg
ISSN: 2070-1721
J. Yi
Ecole Polytechnique
May 2017
Security Threats to the
Optimized Link State Routing Protocol Version 2 (OLSRv2)
Abstract
This document analyzes common security threats to the Optimized Link
State Routing Protocol version 2 (OLSRv2) and describes their
potential impacts on Mobile Ad Hoc Network (MANET) operations. It
also analyzes which of these security vulnerabilities can be
mitigated when using the mandatory-to-implement security mechanisms
for OLSRv2 and how the vulnerabilities are mitigated.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc8116.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. OLSRv2 Overview . . . . . . . . . . . . . . . . . . . . . 5
1.1.1. Neighborhood Discovery . . . . . . . . . . . . . . . 5
1.1.2. MPR Selection . . . . . . . . . . . . . . . . . . . . 6
1.1.3. Link State Advertisement . . . . . . . . . . . . . . 6
1.2. Link State Vulnerability Taxonomy . . . . . . . . . . . . 6
1.3. OLSRv2 Attack Vectors . . . . . . . . . . . . . . . . . . 7
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 7
3. Topology Map Acquisition . . . . . . . . . . . . . . . . . . 7
3.1. Attack on Jittering . . . . . . . . . . . . . . . . . . . 8
3.2. Hop Count and Hop Limit Attacks . . . . . . . . . . . . . 8
3.2.1. Modifying the Hop Limit . . . . . . . . . . . . . . . 8
3.2.2. Modifying the Hop Count . . . . . . . . . . . . . . . 9
4. Effective Topology . . . . . . . . . . . . . . . . . . . . . 10
4.1. Incorrect Forwarding . . . . . . . . . . . . . . . . . . 10
4.2. Wormholes . . . . . . . . . . . . . . . . . . . . . . . . 11
4.3. Sequence Number Attacks . . . . . . . . . . . . . . . . . 12
4.3.1. Message Sequence Number . . . . . . . . . . . . . . . 12
4.3.2. Advertised Neighbor Sequence Number (ANSN) . . . . . 12
4.4. Indirect Jamming . . . . . . . . . . . . . . . . . . . . 12
5. Inconsistent Topology . . . . . . . . . . . . . . . . . . . . 15
5.1. Identity Spoofing . . . . . . . . . . . . . . . . . . . . 15
5.2. Link Spoofing . . . . . . . . . . . . . . . . . . . . . . 17
5.2.1. Inconsistent Topology Maps Due to Link State
Advertisements . . . . . . . . . . . . . . . . . . . 18
6. Mitigation of Security Vulnerabilities for OLSRv2 . . . . . . 19
6.1. Inherent OLSRv2 Resilience . . . . . . . . . . . . . . . 19
6.2. Resilience by Using RFC 7183 with OLSRv2 . . . . . . . . 20
6.2.1. Topology Map Acquisition . . . . . . . . . . . . . . 21
6.2.2. Effective Topology . . . . . . . . . . . . . . . . . 21
6.2.3. Inconsistent Topology . . . . . . . . . . . . . . . . 22
6.3. Correct Deployment . . . . . . . . . . . . . . . . . . . 22
7. Security Considerations . . . . . . . . . . . . . . . . . . . 22
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
8.1. Normative References . . . . . . . . . . . . . . . . . . 23
8.2. Informative References . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
1. Introduction
The Optimized Link State Routing Protocol version 2 (OLSRv2)
[RFC7181] is a successor to OLSR [RFC3626] as a routing protocol for
Mobile Ad Hoc Networks (MANETs). OLSRv2 retains the same basic
algorithms as its predecessor; however, it offers various
improvements, e.g., a modular and flexible architecture allowing
extensions (such as for security) to be developed as add-ons to the
basic protocol. Such building blocks and modules include [RFC5148],
[RFC5444], [RFC5497], [RFC6130], [RFC7182], [RFC7183], [RFC7187],
[RFC7188], [RFC7466], etc.
The developments reflected in OLSRv2 have been motivated by increased
real-world deployment experiences, e.g., from networks such as
FunkFeuer [FUNKFEUER], and the requirements to be addressed for
continued successful operation of these networks. With participation
in such networks increasing (the FunkFeuer community network has,
e.g., roughly 400 individual participants at the time of publication
of this document), operating under the assumption that participants
can be "trusted" to behave in a non-destructive way, is naive. With
deployment in the wider Internet, and a resultant increase in user
numbers, an increase in attacks and abuses has followed necessitating
a change in recommended practices. For example, SMTP servers, which
were initially available for use by everyone on the Internet, require
authentication and accounting for users today [RFC5068].
As OLSRv2 is often used in wireless environments, it is potentially
exposed to different kinds of security threats, some of which are of
greater significance when compared to wired networks. As radio
signals can be received as well as transmitted by any compatible
wireless device within radio range, there are commonly no physical
constraints on the conformation of nodes and communication links that
make up the network (as could be expected for wired networks).
A first step towards hardening against attacks disrupting the
connectivity of a network is to understand the vulnerabilities of the
routing protocol managing the connectivity. Therefore, this document
analyzes OLSRv2 in order to understand its inherent vulnerabilities
and resilience. The authors do not claim completeness of the
analysis but hope that the identified attacks, as presented, form a
meaningful starting point for developing and deploying increasingly
well-secured OLSRv2 networks.
This document describes security vulnerabilities of OLSRv2 when it is
used without the mandatory-to-implement security mechanisms, as
specified in Section 23.5 of [RFC7181]. It also analyzes which of
these security vulnerabilities can be mitigated when using the
mandatory-to-implement security mechanisms for OLSRv2 and how the
vulnerabilities are mitigated. This separation is important since,
as explicitly stated in [RFC7181]:
Any deployment of OLSRv2 SHOULD use the security mechanism
specified in [RFC7183] but MAY use another mechanism if more
appropriate in an OLSRv2 deployment. For example, for longer-term
OLSRv2 deployments, alternative security mechanisms (e.g.,
rekeying) SHOULD be considered.
Moreover, this document is also based on the assumption that no
additional security mechanism such as IPsec is used in the IP layer,
or other mechanisms on lower layers, as not all MANET deployments may
be able to accommodate such common protection mechanisms (e.g.,
because of limited resources of MANET routers).
As NHDP is a fundamental component of OLSRv2, the vulnerabilities of
NHDP, discussed in [RFC7186], also apply to OLSRv2.
It should be noted that many OLSRv2 implementations are configurable,
and so an attack on the configuration system (such as [RFC7939] and
[RFC7184]) can be used to adversely affect the operation of an OLSRv2
implementation.
1.1. OLSRv2 Overview
OLSRv2 contains three basic processes: neighborhood discovery,
Multipoint Relay (MPR) selection, and Link State Advertisements
(LSAs). They are described in the sections below with sufficient
details to allow elaboration of the analyses in this document.
1.1.1. Neighborhood Discovery
Neighborhood discovery is the process whereby each router discovers
the routers that are in direct communication range of itself (1-hop
neighbors) and detects with which of these it can establish
bidirectional communication. Each router sends HELLO messages
periodically, listing the identifiers of all the routers from which
it has recently received a HELLO message as well as the "status" of
the link (heard or verified bidirectional). A router A receiving a
HELLO message from a neighbor router B, in which B indicates it has
recently received a HELLO message from A, considers the link A-B to
be bidirectional. As B lists identifiers of all its neighbors in its
HELLO message, A learns the "neighbors of its neighbors" (2-hop
neighbors) through this process. HELLO messages are sent
periodically; however, certain events may trigger non-periodic
HELLOs. OLSRv2 [RFC7181] uses NHDP [RFC6130] as its neighborhood
discovery mechanism. The vulnerabilities of NHDP are analyzed in
[RFC7186].
1.1.2. MPR Selection
Multipoint Relay (MPR) selection is the process whereby each router
is able to identify a set of relays for efficiently conducting
network-wide broadcasts. Each router designates, from among its
bidirectional neighbors, a subset (the "MPR set") such that an
OLSRv2-specific multicast message transmitted by the router and
relayed by the MPR set can be received by all its 2-hop neighbors.
MPR selection is encoded in outgoing NHDP HELLO messages.
In their HELLO messages, routers may express their "willingness" to
be selected as an MPR using an integer between 0 and 7 ("will never"
to "will always"). This is taken into consideration for the MPR
calculation and is useful, for example, when an OLSRv2 network is
"planned". The set of routers having selected a given router as an
MPR is the MPR selector set of that router. A study of the MPR
flooding algorithm can be found in [MPR-FLOODING].
1.1.3. Link State Advertisement
Link State Advertisement (LSA) is the process whereby routers
determine which link state information to advertise through the
network. Each router must advertise, at least, all links between
itself and its MPR selectors in order to allow all routers to
calculate shortest paths. Such LSAs are carried in Topology Control
(TC) messages, which are broadcast through the network using the MPR
flooding process described in Section 1.1.2. As a router selects
MPRs only from among bidirectional neighbors, links advertised in TC
are also bidirectional and routing paths calculated by OLSRv2 contain
only bidirectional links. TCs are sent periodically; however,
certain events may trigger non-periodic TCs.
1.2. Link State Vulnerability Taxonomy
Proper functioning of OLSRv2 assumes that:
o each router signals its presence in the network and the topology
information that it obtained correctly;
o each router can acquire and maintain a topology map that
accurately reflects the effective network topology; and,
o that the network converges, i.e., that all routers in the network
will have sufficiently identical topology maps.
An OLSRv2 network can be disrupted by breaking any of these
assumptions, specifically that (a) routers may be prevented from
acquiring a topology map of the network, (b) routers may acquire a
topology map that does not reflect the effective network topology,
and (c) two or more routers may acquire inconsistent topology maps.
1.3. OLSRv2 Attack Vectors
Besides "radio jamming", attacks on OLSRv2 consist of a compromised
OLSRv2 router injecting apparently correct, but invalid, control
traffic (TCs, HELLOs) into the network. A compromised OLSRv2 router
can either (a) advertise erroneous information about itself (its
identification and its willingness to serve as an MPR), henceforth
called identity spoofing, or (b) advertise erroneous information
about its relationship to other routers (pretend existence of links
to other routers), henceforth called link spoofing. Such attacks may
disrupt the LSA process by targeting the MPR flooding mechanism or by
causing incorrect link state information to be included in TCs,
causing routers to have incomplete, inaccurate, or inconsistent
topology maps. In a different class of attacks, a compromised OLSRv2
router injects control traffic designed so as to cause an in-router
resource exhaustion, e.g., by causing the algorithms calculating
routing tables or MPR sets to be invoked continuously, preventing the
internal state of a router from converging, which depletes the energy
of battery-driven routers, etc.
2. Terminology
This document uses the terminology and notation defined in [RFC5444],
[RFC6130], and [RFC7181]. Additionally, it defines the following
terminology:
Compromised OLSRv2 router: An attacker that eavesdrops on the
network traffic and/or generates syntactically correct OLSRv2
control messages. Control messages emitted by a compromised
OLSRv2 router may contain additional information or omit
information, as compared to a control message generated by a non-
compromised OLSRv2 router located in the same topological position
in the network.
Legitimate OLSRv2 router: An OLSRv2 router that is not a compromised
OLSRv2 router.
3. Topology Map Acquisition
Topology Map Acquisition relates to the ability for any given router
in the network to acquire a representation of the network
connectivity. A router that is unable to acquire a topology map is
incapable of calculating routing paths and participating in
forwarding data. Topology map acquisition can be hindered by (i) TCs
not being delivered to (all) routers in the network, such as what
happens in case of flooding disruption, or (ii) in case of "jamming"
of the communication channel.
The jamming and flooding disruption due to identity spoofing and link
spoofing have been discussed in [RFC7186].
3.1. Attack on Jittering
OLSRv2 incorporates a jittering mechanism: a random, but bounded,
delay on outgoing control traffic [RFC5148]. This may be necessary
when link layers (such as 802.11 [IEEE802.11]) are used that do not
guarantee collision-free delivery of frames and where jitter can
reduce the probability of collisions of frames on lower layers.
In OLSRv2, TC forwarding is jittered by a value between 0 and
MAX_JITTER. In order to reduce the number of transmissions, when a
control message is due for transmission, OLSRv2 piggybacks all queued
messages into a single transmission. Thus, if a compromised OLSRv2
router sends many TCs within a very short time interval, the jitter
time of the attacked router tends towards 0. This renders jittering
ineffective and can lead to collisions on the link layer.
In addition to causing more collisions, forwarding a TC with little
or no jittering can make sure that the TC message forwarded by a
compromised router arrives before the message forwarded by legitimate
routers. The compromised router can thus inject malicious content in
the TC: for example, if the message identification is spoofed, the
legitimate message will be discarded as a duplicate message. This
preemptive action is important for some of the attacks introduced in
the following sections.
3.2. Hop Count and Hop Limit Attacks
The hop count and hop limit fields are the only parts of a TC that
are modified when forwarding; therefore, they are not protected by
integrity check mechanisms. A compromised OLSRv2 router can modify
either of these when forwarding TCs.
3.2.1. Modifying the Hop Limit
A compromised OLSRv2 router can decrease the hop limit when
forwarding a TC. This will reduce the scope of forwarding for the
message and may lead to some routers in the network not receiving
that TC. Note that this is not necessarily the same as not relaying
the message (i.e., setting the hop limit to 0), as is illustrated in
Figure 1.
.---.
| X |
--'---' __
/ \
/ \
.---. .---.
TC -----> | A | | C |
'---' '---'
\ .---. /
\-- | B |__/
'---'
Figure 1: Hop Limit Attack
A TC arrives at and is forwarded by router A such that it is received
by both B and the malicious X. X can forward the TC without any
delay (including without jitter) such that its transmissions arrive
before that of B at C. Before forwarding, it significantly reduces
the hop limit of the message. Router C receives the TC, processes
(and forwards) it, and marks it as already received -- causing it to
discard further copies received from B. Thus, if the TC is forwarded
by C, it has a very low hop limit and will not reach the whole
network.
3.2.2. Modifying the Hop Count
A compromised OLSRv2 router can modify the hop count when forwarding
a TC. This may have two consequences: (i) if the hop count is set to
the maximum value, then the TC will be forwarded no further or (ii)
artificially manipulating the hop count may affect the validity time
as calculated by recipients, when using distance-dependent validity
times as defined in [RFC5497] (e.g., as part of a Fish Eye extension
to OLSR2 [OLSR-FSR] [OLSR-FSR-Scaling]).
v_time(3hops)=9s v_time(4hops)=12s v_time(5hops)=15s
.---. .---. .---. .---.
| A |-- ... --> | B | -------> | X |---------->| C |
`---' `---' `---' `---'
Figure 2: Different Validity Times Based on the Distance in Hops
In Figure 2, router A sends a TC with a validity time of 9 seconds
for routers in a 3-hop distance, 12 seconds for routers in a 4-hop
distance, and 15 seconds in a 5-hop distance. If X is a compromised
OLSRv2 router and modifies the hop count (say, by decreasing it to
3), then C will calculate the validity time of received information
to 9 seconds -- after which it expires unless refreshed. If TCs from
A are sent less frequently than that up to 4 hops, this causes links
advertised in such TCs to be only intermittently available to C.
4. Effective Topology
Link state protocols assume that each router can acquire an accurate
topology map that reflects the effective network topology. This
implies that the routing protocol is able to identify a path from a
source to a destination, and this path is valid for forwarding data
traffic. If an attacker disturbs the correct protocol behavior, the
perceived topology map of a router can permanently differ from the
effective topology.
Consider the example in Figure 3(a), which illustrates the topology
map as acquired by router S. This topology map indicates that the
routing protocol has identified that for S, a path exists to D via B,
which it therefore assumes can be used for transmitting data. If B
does not forward data traffic from S, then the topology map in S does
not accurately reflect the effective network topology. Rather, the
effective network topology from the point of view of S would be as
indicated in Figure 3(b): D is not part of the network reachable from
router S.
.---. .---. .---. .---. .---.
| S |----| B |----| D | | S |----| B |
`---' `---' `---' `---' `---'
(a) (b)
Figure 3: Incorrect Data Traffic Forwarding
Some of the attacks related to NHDP, such as message timing attacks
and indirect channel overloading, are discussed in [RFC7186]. Other
threats specific to OLSRv2 are further detailed in this section.
4.1. Incorrect Forwarding
OLSRv2 routers exchange information using link-local transmissions
(link-local multicast or limited broadcast) for their control
messages, with the routing process in each router retransmitting
received messages destined for network-wide diffusion. Thus, if the
operating system in a router is not configured to enable forwarding,
this will not affect the operating of the routing protocol or the
topology map acquired by the routing protocol. It will, however,
cause a discrepancy between the effective topology and the topology
map, as indicated in Figures 3(a) and 3(b).
This situation is not hypothetical. A common error seen when
deploying OLSRv2-based networks using a Linux-based computer as a
router is to neglect enabling IP forwarding, which effectively
becomes an accidental attack of this type.
4.2. Wormholes
A wormhole, depicted in the example in Figure 4, may be established
between two collaborating devices that are connected by an out-of-
band channel. These devices send traffic through the "tunnel" to
their alter ego, which "replays" the traffic. Thus, routers D and S
appear as if direct neighbors and are reachable from each other in 1
hop through the tunnel, with the path through the MANET being 100
hops long.
.---. .---.
| S |---- ....100-hop-long path ... ---| D |
`---. / `---'
\ /
\ /
\X=============================X
1-hop path via wormhole
Figure 4: Wormholing between Two Collaborating Devices Not
Participating in the Routing Protocol
The consequences of such a wormhole in the network depend on the
detailed behavior of the wormhole. If the wormhole relays only
control traffic, but not data traffic, the same considerations as in
Section 4.1 apply. If, however, the wormhole relays all traffic
(control and data alike), it is identical, connectivity wise, to a
usable link - and the routing protocol will correctly generate a
topology map reflecting the effective network topology. The
efficiency of the topology obtained depends on (i) the wormhole
characteristics, (ii) how the wormhole presents itself, and (iii) how
paths are calculated.
Assuming that paths are calculated with unit cost for all links,
including the "link" presented by the wormhole, if the real
characteristics of the wormhole are as if it were a path of more than
100 hops (e.g., with respect to delay, bandwidth, etc.), then the
presence of the wormhole results in a degradation in performance as
compared to using the non-wormhole path. Conversely, if the "link"
presented by the wormhole has better characteristics, the wormhole
results in improved performance.
If paths are calculated using non-unit-costs for all links, and if
the cost of the "link" presented by the wormhole correctly represents
the actual cost (e.g., if the cost is established through
measurements across the wormhole), then the wormhole may, in the
worst case, cause no degradation in performance or, in the best case,
improve performance by offering a better path. If the cost of the
"link" presented by the wormhole is misrepresented, then the same
considerations as for unit-cost links apply.
An additional consideration with regard to wormholes is that they may
present topologically attractive paths for the network; however, it
may be undesirable to have data traffic transit such a path. An
attacker could, by virtue of introducing a wormhole, acquire the
ability to record and inspect transiting data traffic.
4.3. Sequence Number Attacks
OLSRv2 uses two different sequence numbers in TCs to (i) avoid
processing and forwarding the same message more than once (Message
Sequence Number) and to (ii) ensure that old information, arriving
late due to, e.g., long paths or other delays, is not allowed to
overwrite more recent information generated (Advertised Neighbor
Sequence Number (ANSN)).
4.3.1. Message Sequence Number
An attack may consist of a compromised OLSRv2 router spoofing the
identity of another router in the network and transmitting a large
number of TCs, each with different Message Sequence Numbers.
Subsequent TCs with the same sequence numbers, originating from the
router whose identity was spoofed, would hence be ignored until
eventually information concerning these "spoofed" TCs expires.
4.3.2. Advertised Neighbor Sequence Number (ANSN)
An attack may consist of a compromised OLSRv2 router spoofing the
identity of another router in the network and transmitting a single
TC with an ANSN significantly larger than that which was last used by
the legitimate router. Routers will retain this larger ANSN as "the
most recent information" and discard subsequent TCs with lower
sequence numbers as being "old".
4.4. Indirect Jamming
Indirect jamming is an attack in which a compromised OLSRv2 router
is, by its actions, causing legitimate routers to generate inordinate
amounts of control traffic, thereby increasing both channel
occupation and the overhead incurred in each router for processing
this control traffic. This control traffic will be originated from
legitimate routers; thus, to the wider network, the malicious device
may remain undetected.
The general mechanism whereby a malicious router can cause indirect
jamming is for it to participate in the protocol by generating
plausible control traffic and to tune this control traffic to in turn
trigger receiving routers to generate additional traffic. For
OLSRv2, such an indirect attack can be directed at the neighborhood
discovery mechanism and the LSA mechanism, respectively.
One efficient indirect jamming attack in OLSRv2 is to target control
traffic destined for network-wide diffusion. This is illustrated in
Figure 5.
The malicious router X selects router A as an MPR at time t0 in a
HELLO. This causes X to appear as MPR selector for A and,
consequently, A sets X to be advertised in its "Neighbor Set" and
increments the associated "Advertised Neighbor Sequence Number"
(ANSN). Router A must then advertise the link between itself and X
in subsequent outgoing TCs (t1), also including the ANSN in such TCs.
Upon X having received this TC, it declares the link between itself
and A as no longer valid (t2) in a HELLO (indicating the link to A as
LOST). Since only symmetric links are advertised by OLSRv2 routers,
A will (upon receipt hereof) remove X from the set of advertised
neighbors and increment the ANSN. Router A will then, in subsequent
TCs, advertise the remaining set of advertised neighbors (i.e., with
X removed) and the corresponding ANSN (t3). Upon X having received
this information in another TC from A, it may repeat this cycle,
alternating advertising the link A-X as "LOST" and as "MPR".
broadcast TC ANS={} TC:()
(X-A) ANSN ANSN++ ANSN
.---. .---. .---. .---.
| A | | A | | A | | A |
'---' '---' '---' '---'
^ | ^ |
| | | |
| select | |indicate |
| as MPR | |as LOST |
.---. .---. .---. .---.
| X | | X | | X | | X |
'---' '---' '---' '---'
t0 t1 t2 t3
Description: The malicious X flips between link status MPR and LOST.
Figure 5: Indirect Jamming in Link State Advertisement
Routers receiving a TC message will parse and process this message,
specifically updating their topology map as a consequence of
successful receipt. If the ANSN between two successive TCs from the
same router has incremented, then the topology has changed and
routing sets are to be recalculated. This has the potential to be a
computationally costly operation.
A compromised OLSRv2 router may chose to conduct this attack against
all its neighbors, thus maximizing its disruptive impact on the
network with relatively little overhead of its own: other than
participating in the neighborhood discovery procedure, the
compromised OLSRv2 router will monitor TCs generated by its neighbors
and alternate the advertised status for each such neighbor between
"MPR" and "LOST". The compromised OLSRv2 router will indicate its
willingness to be selected as an MPR as 0 (thus avoiding selection as
an MPR) and may ignore all other protocol operations while still
remaining effective as an attacker.
The basic operation of OLSRv2 employs periodic message emissions, and
by this attack it can be ensured that each such periodic message will
entail routing table recalculation in all routers in the network.
If the routers in the network have "triggered TCs" enabled, this
attack may also cause an increased TC frequency. Triggered TCs are
intended to allow a (stable) network to have relatively low TC
emission frequencies yet still allow link breakage or link emergence
to be advertised through the network rapidly. A minimum message
interval (typically much smaller than the regular periodic message
interval) is imposed to rate-limit worst-case message emissions.
This attack can cause the TC interval to permanently become equal to
the minimum message interval. [RFC7181] proposes as default that the
minimum TC interval be 0.25 x TC_INTERVAL (TC_INTERVAL being the
maximum interval between two TC messages from the same OLSRv2
router).
Indirect jamming by a compromised OLSRv2 router can thus have two
effects: (i) it may cause increased frequency of TC generation and
transmission, and (ii) it will cause additional routing table
recalculation in all routers in the network.
5. Inconsistent Topology
Inconsistent topology maps can occur by a compromised OLSRv2 router
employing either identity spoofing or link spoofing for conducting an
attack against an OLSRv2 network. The threats related to NHDP, such
as identity spoofing in NHDP, link spoofing in NHDP, and creating
loops, have been illustrated in [RFC7186]. This section mainly
addresses the vulnerabilities in [RFC7181].
5.1. Identity Spoofing
Identity spoofing can be employed by a compromised OLSRv2 router via
the neighborhood discovery process and via the LSA process. Either
of them causes inconsistent topology maps in routers in the network.
The creation of inconsistent topology maps due to neighborhood
discovery has been discussed in [RFC7186]. For OLSRv2, the attack on
the LSA process can also cause inconsistent topology maps.
An inconsistent topology map may occur when the compromised OLSRv2
router takes part in the LSA process by selecting a neighbor as an
MPR, which in turn advertises the spoofed identities of the
compromised OLSRv2 router. This attack will alter the topology maps
of all routers of the network.
A -- B -- C -- D -- E -- F -- X
(X spoofs A)
Description: A compromised OLSRv2 router X spoofs the identity of A,
leading to a wrongly perceived topology.
Figure 6: Identity Spoofing
In Figure 6, router X spoofs the address of router A. If X selects F
as an MPR, all routers in the network will be informed about the link
F-A by the TCs originating from F. Assuming that (the real) A
selects B as an MPR, the link B-A will also be advertised in the
network.
When calculating paths, B and C will calculate paths to A via B, as
illustrated in Figure 7(a); for these routers, the shortest path to A
is via B. E and F will calculate paths to A via F, as illustrated in
Figure 7(b); for these routers, the shortest path to A is via the
compromised OLSRv2 router X, and these are thus disconnected from the
real A. D will have a choice, as the path calculated to A via B is
of the same length as the path via the compromised OLSRv2 router X,
as illustrated in Figure 7(c).
In general, the following observations can be made:
o The network will be split in two, with those routers closer to B
than to X reaching A, whereas those routers closer to X than to B
will be unable to reach A.
o Routers beyond B, i.e., routers beyond 1 hop away from A, will be
unable to detect this identity spoofing.
The identity spoofing attack via the LSA procedure has a higher
impact than the attack on the neighborhood discovery procedure since
it alters the topology maps of all routers in the network and not
only in the 2-hop neighborhood. However, the attack is easier to
detect by other routers in the network. Since the compromised OLSRv2
router is advertised in the whole network, routers whose identities
are spoofed by the compromised OLSRv2 router can detect the attack.
For example, when A receives a TC from F advertising the link F-A, it
can deduce that some entity is injecting incorrect link state
information as it does not have F as one of its direct neighbors.
(X spoofs A)
A < ---- B < ---- C E ----> F ----> X
(a) Routers B and C (b) Routers E and F
A < --- B < --- C < --- D ---> E ---> F ----> X
(X spoofs A)
Description: These paths appear as calculated by the different
routers in the network in presence of a compromised OLSRv2 router X,
spoofing the address of A.
Figure 7: Routing Paths towards A
As the compromised OLSRv2 router X does not itself send the TCs, but
rather, by virtue of MPR selection, ensures that the addresses it
spoofs are advertised in TCs from its MPR selector F, the attack may
be difficult to counter. Simply ignoring TCs that originate from F
may also suppress the link state information for other, legitimate,
MPR selectors of F.
Thus, identity spoofing by a compromised OLSRv2 router, participating
in the LSA process by selecting MPRs only, creates a situation
wherein two or more routers have substantially inconsistent topology
maps: traffic for an identified destination is, depending on where in
the network it appears, delivered to different routers.
5.2. Link Spoofing
Link spoofing is a situation in which a router advertises non-
existing links to another router (possibly not present in the
network). Essentially, TCs and HELLOs both advertise links to direct
neighbor routers with the difference being the scope of the
advertisement. Thus, link spoofing consists of a compromised OLSRv2
router reporting that it has neighbors routers that are either not
present in the network or are effectively not neighbors of the
compromised OLSRv2 router.
It can be noted that a situation similar to link spoofing may occur
temporarily in an OLSR or OLSRv2 network without compromised OLSRv2
routers: if A was, but is no more, a neighbor of B, then A may still
be advertising a link to B for the duration of the time it takes for
the neighborhood discovery process to determine this changed
neighborhood.
In the context of this document, link spoofing refers to a persistent
situation where a compromised OLSRv2 router intentionally advertises
links to other routers for which it is not a direct neighbor.
5.2.1. Inconsistent Topology Maps Due to Link State Advertisements
Figure 8 illustrates a network in which the compromised OLSRv2 router
X spoofs links to an existing router A by participating in the LSA
process and including this non-existing link in its advertisements.
A --- B --- C --- D --- E --- F --- G --- H --- X
(X spoofs the link to A)
Description: The compromised OLSRv2 router X advertises a spoofed
link to A in its TCs; thus, all routers will record both of the links
X-A and B-A.
Figure 8: Link Spoofing
As TCs are flooded through the network, all routers will receive and
record information describing a link X-A in this link state
information. If A has selected router B as an MPR, B will likewise
flood this link state information through the network; thus, all
routers will receive and record information describing a link B-A.
When calculating routing paths, B, C, and D will calculate paths to A
via B, as illustrated in Figure 9(a); for these routers, the shortest
path to A is via B. F and G will calculate paths to A via X, as
illustrated in Figure 9(b); for these routers, the shortest path to A
is via X, and these are thus disconnected from the real router A. E
will have a choice: the path calculated to A via B is of the same
length as the path via X, as illustrated in Figure 9(b).
A < --- B < --- C < --- D F ---> G ---> X ---> A
(a) Routers B, C, and D (b) Routers F and G
A < --- B < --- C < --- D < --- E ---> F ---> G ---> X ---> A
(c) Router E
Description: These paths appear as calculated by the different
routers in the network in the presence of a compromised OLSRv2 router
X, spoofing a link to router A.
Figure 9: Routing Paths towards Router A
In general, the following observations can be made:
o The network will be separated in two: routers closer to B than X
will reach A, whereas routers closer to X than B will be unable to
reach A.
o Routers beyond B, i.e., routers beyond 1 hop away from A, will be
unable to detect this link spoofing.
6. Mitigation of Security Vulnerabilities for OLSRv2
As described in Section 1, [RFC7183] specifies a security mechanism
for OLSRv2 that is mandatory to implement. However, deployments may
choose to use different security mechanisms if more appropriate.
Therefore, it is important to understand both the inherent resilience
of OLSRv2 against security vulnerabilities when not using the
mechanisms specified in [RFC7183] and the protection that [RFC7183]
provides when used in a deployment.
6.1. Inherent OLSRv2 Resilience
OLSRv2 (even when used without the mandatory-to-implement security
mechanisms in [RFC7183]) provides some inherent resilience against
part of the attacks described in this document. In particular, it
provides the following resilience:
o Sequence numbers: OLSRv2 employs message sequence numbers, which
are specific per the router identity and message type. Routers
keep an "information freshness" number (ANSN) incremented each
time the content of an LSA from a router changes. This allows
rejecting both "old" information and duplicate messages, and it
provides some protection against "message replay". However, this
also presents an attack vector (Section 4.3).
o Ignoring unidirectional links: The neighborhood discovery process
detects and admits only bidirectional links for use in MPR
selection and LSA. Jamming attacks may affect only reception of
control traffic; however, OLSRv2 will correctly recognize, and
ignore, such a link that is not bidirectional.
o Message interval bounds: The frequency of control messages, with
minimum intervals imposed for HELLO and TCs. This may limit the
impact from an indirect jamming attack (Section 4.4).
o Additional reasons for rejecting control messages: The OLSRv2
specification includes a list of reasons for which an incoming
control message should be rejected as malformed -- and allows that
a protocol extension may recognize additional reasons for OLSRv2
to consider a message malformed. Together with the flexible
message format [RFC5444], this allows addition of security
mechanisms, such as digital signatures, while remaining compliant
with the OLSRv2 standard specification.
6.2. Resilience by Using RFC 7183 with OLSRv2
[RFC7183] specifies mechanisms for integrity and replay protection
for NHDP and OLSRv2 using the generalized packet/message format
described in [RFC5444] and the TLV definitions in [RFC7182]. The
specification describes how to add an Integrity Check Value (ICV) in
a TLV to each control message, providing integrity protection of the
content of the message using Hashed Message Authentication Code
(HMAC) / SHA-256. In addition, a timestamp TLV is added to the
message prior to creating the ICV, enabling replay protection of
messages. The document specifies how to sign outgoing messages and
how to verify incoming messages, as well as under which circumstances
an invalid message is rejected. Because of the HMAC/SHA-256 ICV, a
shared key between all routers in the MANET is assumed. A router
without valid credentials is not able to create an ICV that can be
correctly verified by other routers in the MANET; therefore, such an
incorrectly signed message will be rejected by other MANET routers,
and the router cannot participate in the OLSRv2 routing process
(i.e., the malicious router will be ignored by other legitimate
routers). [RFC7183] does not address the case where a router with
valid credentials has been compromised. Such a compromised router
will not be excluded from the routing process, and other means of
detecting such a router are necessary if required in a deployment:
for example, using an asymmetric key extension to [RFC7182] that
allows revocation of the access of one particular router.
In the following sections, each of the vulnerabilities described
earlier in this document will be evaluated in terms of whether OLSRv2
with the mechanisms in [RFC7183] provides sufficient protection
against the attack. It is implicitly assumed in each of the
following sections that [RFC7183] is used with OLSRv2.
6.2.1. Topology Map Acquisition
Attack on Jittering: As only OLSRv2 routers with valid credentials
can participate in the routing process, a malicious router cannot
reduce the jitter time of an attacked router to 0 by sending many
TC messages in a short time. The attacked router would reject all
the incoming messages as "invalid" and not forward them. The same
applies for the case where a malicious router wants to assure that
by forcing a 0 jitter interval, the message arrives before the
same message forwarded by legitimate routers.
Modifying the Hop Limit and the Hop Count: As the hop limit and hop
count are not protected by [RFC7183] (since they are mutable
fields that change at every hop), this attack is still feasible.
It is possible to apply [RFC5444] packet-level protection by using
ICV Packet TLV defined in [RFC7182] to provide hop-by-hop
integrity protection -- at the expense of a requirement of
pairwise trust between all neighbor routers.
6.2.2. Effective Topology
Incorrect Forwarding: As only OLSRv2 routers with valid credentials
can participate in the routing process, a malicious router will
not be part of the topology of other legitimate OLSRv2 routers.
Therefore, no data traffic will be sent to the malicious router
for forwarding.
Wormholes: Since a wormhole consists of at least two devices
forwarding (unmodified) traffic, this attack is still feasible and
undetectable by the OLSRv2 routing process since the attack does
not involve the OLSRv2 protocol itself (but rather lower layers).
By using [RFC7183], it can at least be assured that the content of
the control messages is not modified while being forwarded via the
wormhole. Moreover, the timestamp TLV assures that the forwarding
can only be done in a short time window after the actual TC
message has been sent.
Message Sequence Number: As the message sequence number is included
in the ICV calculation, OLSRv2 is protected against this attack.
Advertised Neighbor Sequence Number (ANSN): As the ANSN is included
in the ICV calculation, OLSRv2 is protected against this attack.
Indirect Jamming: Since the control messages of a malicious router
will be rejected by other legitimate OLSRv2 routers in the MANET,
this attack is mitigated.
6.2.3. Inconsistent Topology
Identity Spoofing: Since the control messages of a malicious router
will be rejected by other legitimate OLSRv2 routers in the MANET,
a router without valid credentials may spoof its identity (e.g.,
IP source address or message originator address), but the messages
will be ignored by other routers. As the mandatory mechanism in
[RFC7183] uses shared keys amongst all MANET routers, a single
compromised router may spoof its identity and cause harm to the
network stability. Removing this one malicious router, once
detected, implies rekeying all other routers in the MANET.
Asymmetric keys, particularly when using identity-based signatures
(such as those specified in [RFC7859]), may give the possibility
of revoking single routers and verifying their identity based on
the ICV itself.
Link Spoofing: Similar to identity spoofing, a malicious router
without valid credentials may spoof links, but its control
messages will be rejected by other routers, thereby mitigating the
attack.
Inconsistent Topology Maps Due to LSAs: The same considerations for
link spoofing apply.
6.3. Correct Deployment
Other than implementing OLSRv2, including appropriate security
mechanisms, the way in which the protocol is deployed is also
important to ensure proper functioning and threat mitigation. For
example, Section 4.1 discussed considerations due to an incorrect
forwarding-policy setting, and Section 4.2 discussed considerations
for when intentional wormholes are present in a deployment.
7. Security Considerations
This document does not specify a protocol or a procedure but reflects
on security considerations for OLSRv2 and for its constituent parts,
including NHDP. The document initially analyses threats to topology
map acquisition, with the assumption that no security mechanism
(including the mandatory-to-implement mechanisms from [RFC7182] and
[RFC7183]) is in use. Then, it proceeds to discuss how the use of
[RFC7182] and [RFC7183] mitigate the identified threats. When
[RFC7183] is used with routers using a single shared key, the
protection offered is not effective if a compromised router has valid
credentials.
8. References
8.1. Normative References
[RFC6130] Clausen, T., Dearlove, C., and J. Dean, "Mobile Ad Hoc
Network (MANET) Neighborhood Discovery Protocol (NHDP)",
RFC 6130, DOI 10.17487/RFC6130, April 2011,
<http://www.rfc-editor.org/info/rfc6130>.
[RFC7181] Clausen, T., Dearlove, C., Jacquet, P., and U. Herberg,
"The Optimized Link State Routing Protocol Version 2",
RFC 7181, DOI 10.17487/RFC7181, April 2014,
<http://www.rfc-editor.org/info/rfc7181>.
[RFC7186] Yi, J., Herberg, U., and T. Clausen, "Security Threats for
the Neighborhood Discovery Protocol (NHDP)", RFC 7186,
DOI 10.17487/RFC7186, April 2014,
<http://www.rfc-editor.org/info/rfc7186>.
8.2. Informative References
[FUNKFEUER]
Funkfeuer, "Funkfeuer", <https://www.funkfeuer.at/>.
[IEEE802.11]
IEEE, "IEEE Standard for Information technology -
Telecommunications and information exchange between
systems Local and metropolitan area networks - Specfic
requirements Part 11: Wireless LAN Medium Access Control
and Physical (PHY) Specifications", IEEE Std 802.11-2016,
DOI 10.1109/IEEESTD.2016.7786995, December 2016.
[MPR-FLOODING]
Qayyum, A., Viennot, L., and A. Laouiti, "Multipoint
Relaying: An Efficient Technique for Flooding in Mobile
Wireless Networks", Proceedings of the 35th Annual Hawaii
International Conference on System Sciences (HICSS
'01), IEEE Computer Society, 2001.
[OLSR-FSR] Clausen, T., "Combining Temporal and Spatial Partial
Topology for MANET routing - Merging OLSR and FSR",
Proceedings of the 2003 IEEE Conference of Wireless
Personal Multimedia Communications (WPMC '03), 2003.
[OLSR-FSR-Scaling]
Adjih, C., Baccelli, E., Clausen, T., Jacquet, P., and G.
Rodolakis, "Fish Eye OLSR Scaling Properties", IEEE
Journal of Communication and Networks (JCN), Special Issue
on Mobile Ad Hoc Networks, December 2004.
[RFC3626] Clausen, T., Ed. and P. Jacquet, Ed., "Optimized Link
State Routing Protocol (OLSR)", RFC 3626,
DOI 10.17487/RFC3626, October 2003,
<http://www.rfc-editor.org/info/rfc3626>.
[RFC5068] Hutzler, C., Crocker, D., Resnick, P., Allman, E., and T.
Finch, "Email Submission Operations: Access and
Accountability Requirements", BCP 134, RFC 5068,
DOI 10.17487/RFC5068, November 2007,
<http://www.rfc-editor.org/info/rfc5068>.
[RFC5148] Clausen, T., Dearlove, C., and B. Adamson, "Jitter
Considerations in Mobile Ad Hoc Networks (MANETs)",
RFC 5148, DOI 10.17487/RFC5148, February 2008,
<http://www.rfc-editor.org/info/rfc5148>.
[RFC5444] Clausen, T., Dearlove, C., Dean, J., and C. Adjih,
"Generalized Mobile Ad Hoc Network (MANET) Packet/Message
Format", RFC 5444, DOI 10.17487/RFC5444, February 2009,
<http://www.rfc-editor.org/info/rfc5444>.
[RFC5497] Clausen, T. and C. Dearlove, "Representing Multi-Value
Time in Mobile Ad Hoc Networks (MANETs)", RFC 5497,
DOI 10.17487/RFC5497, March 2009,
<http://www.rfc-editor.org/info/rfc5497>.
[RFC7182] Herberg, U., Clausen, T., and C. Dearlove, "Integrity
Check Value and Timestamp TLV Definitions for Mobile Ad
Hoc Networks (MANETs)", RFC 7182, DOI 10.17487/RFC7182,
April 2014, <http://www.rfc-editor.org/info/rfc7182>.
[RFC7183] Herberg, U., Dearlove, C., and T. Clausen, "Integrity
Protection for the Neighborhood Discovery Protocol (NHDP)
and Optimized Link State Routing Protocol Version 2
(OLSRv2)", RFC 7183, DOI 10.17487/RFC7183, April 2014,
<http://www.rfc-editor.org/info/rfc7183>.
[RFC7184] Herberg, U., Cole, R., and T. Clausen, "Definition of
Managed Objects for the Optimized Link State Routing
Protocol Version 2", RFC 7184, DOI 10.17487/RFC7184, April
2014, <http://www.rfc-editor.org/info/rfc7184>.
[RFC7187] Dearlove, C. and T. Clausen, "Routing Multipoint Relay
Optimization for the Optimized Link State Routing Protocol
Version 2 (OLSRv2)", RFC 7187, DOI 10.17487/RFC7187, April
2014, <http://www.rfc-editor.org/info/rfc7187>.
[RFC7188] Dearlove, C. and T. Clausen, "Optimized Link State Routing
Protocol Version 2 (OLSRv2) and MANET Neighborhood
Discovery Protocol (NHDP) Extension TLVs", RFC 7188,
DOI 10.17487/RFC7188, April 2014,
<http://www.rfc-editor.org/info/rfc7188>.
[RFC7466] Dearlove, C. and T. Clausen, "An Optimization for the
Mobile Ad Hoc Network (MANET) Neighborhood Discovery
Protocol (NHDP)", RFC 7466, DOI 10.17487/RFC7466, March
2015, <http://www.rfc-editor.org/info/rfc7466>.
[RFC7859] Dearlove, C., "Identity-Based Signatures for Mobile Ad Hoc
Network (MANET) Routing Protocols", RFC 7859,
DOI 10.17487/RFC7859, May 2016,
<http://www.rfc-editor.org/info/rfc7859>.
[RFC7939] Herberg, U., Cole, R., Chakeres, I., and T. Clausen,
"Definition of Managed Objects for the Neighborhood
Discovery Protocol", RFC 7939, DOI 10.17487/RFC7939,
August 2016, <http://www.rfc-editor.org/info/rfc7939>.
Authors' Addresses
Thomas Clausen
Phone: +33-6-6058-9349
Email: T.Clausen@computer.org
URI: http://www.thomasclausen.org
Ulrich Herberg
Email: ulrich@herberg.name
URI: http://www.herberg.name
Jiazi Yi
Ecole Polytechnique
91128 Palaiseau Cedex
France
Phone: +33 1 77 57 80 85
Email: jiazi@jiaziyi.com
URI: http://www.jiaziyi.com/