Security Threats to the Optimized Link State Routing Protocol Version 2 (OLSRv2)
draft-ietf-manet-olsrv2-sec-threats-04

Network Working Group                                         T. Clausen
Internet-Draft                                                U. Herberg
Intended status: Informational
Expires: July 16, 2017                                             J. Yi
                                                     Ecole Polytechnique
                                                        January 12, 2017

Security Threats to the Optimized Link State Routing Protocol version 2
                                (OLSRv2)
                 draft-ietf-manet-olsrv2-sec-threats-04

Abstract

   This document analyzes common security threats that might apply to
   the Optimized Link State Routing Protocol version 2 (OLSRv2) and
   describes their potential impacts on Mobile Ad Hoc Network (MANET)
   operations.  It then 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

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   This Internet-Draft will expire on July 16, 2017.

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   to this document.  Code Components extracted from this document must
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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  OLSRv2 Overview  . . . . . . . . . . . . . . . . . . . . .  4
       1.1.1.  Neighborhood Discovery . . . . . . . . . . . . . . . .  4
       1.1.2.  MPR Selection  . . . . . . . . . . . . . . . . . . . .  5
       1.1.3.  Link State Advertisement . . . . . . . . . . . . . . .  5
     1.2.  Link State Vulnerability Taxonomy  . . . . . . . . . . . .  5
     1.3.  OLSRv2 Attack Vectors  . . . . . . . . . . . . . . . . . .  6
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  Topology Map Acquisition . . . . . . . . . . . . . . . . . . .  7
     3.1.  Attack on Jittering  . . . . . . . . . . . . . . . . . . .  7
     3.2.  Hop-count and Hop-limit Attacks  . . . . . . . . . . . . .  7
       3.2.1.  Modifying the Hop Limit  . . . . . . . . . . . . . . .  8
       3.2.2.  Modifying the Hop Count  . . . . . . . . . . . . . . .  8
   4.  Effective Topology . . . . . . . . . . . . . . . . . . . . . .  9
     4.1.  Incorrect Forwarding . . . . . . . . . . . . . . . . . . . 10
     4.2.  Wormholes  . . . . . . . . . . . . . . . . . . . . . . . . 10
     4.3.  Sequence Number Attacks  . . . . . . . . . . . . . . . . . 11
       4.3.1.  Message Sequence Number  . . . . . . . . . . . . . . . 11
       4.3.2.  Advertised Neighbor Sequence Number (ANSN) . . . . . . 12
     4.4.  Indirect Jamming . . . . . . . . . . . . . . . . . . . . . 12
   5.  Inconsistent Topology  . . . . . . . . . . . . . . . . . . . . 14
     5.1.  Identity Spoofing  . . . . . . . . . . . . . . . . . . . . 14
     5.2.  Link Spoofing  . . . . . . . . . . . . . . . . . . . . . . 16
       5.2.1.  Inconsistent Topology Maps due to Link State
               Advertisements . . . . . . . . . . . . . . . . . . . . 16
   6.  Mitigation of Security Vulnerabilities for OLSRv2  . . . . . . 17
     6.1.  Inherent OLSRv2 Resilience . . . . . . . . . . . . . . . . 18
     6.2.  Resilience by using RFC7183 with OLSRv2  . . . . . . . . . 18
       6.2.1.  Topology Map Acquisition . . . . . . . . . . . . . . . 19
       6.2.2.  Effective Topology . . . . . . . . . . . . . . . . . . 19
       6.2.3.  Inconsistent Topology  . . . . . . . . . . . . . . . . 20
     6.3.  Correct Deployment . . . . . . . . . . . . . . . . . . . . 20
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 21
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 21
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 21
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 21
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23

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1.  Introduction

   The Optimized Link State Routing Protocol version 2 (OLSRv2)
   [RFC7181] is a successor to OLSR [RFC3626] as a routing protocol for
   MANETs (Mobile Ad hoc NETworks).  OLSRv2 retains the same basic
   algorithms as its predecessor, however 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 with the assumption, that participants
   can be "trusted" to behave in a non-destructive way, is utopian.
   With deployent in the wider Internet, with 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 as 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.  This document
   therefore analyzes OLSRv2, to understand its inherent vulnerabilities
   and resiliences.  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 first 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 then analyzes which of
   these security vulnerabilities can be mitigated when using the

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   mandatory-to-implement security mechanisms for OLSRv2, and how the
   vulnerabilities are mitigated.  This separation is important since
   security mechanisms other than the mandatory-to-implement ones may be
   used in a deployment, 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).

   The threats related to NHDP (Neighborhood Discovery Protocol
   [RFC6130]) have been discussed in [RFC7186].  As NHDP is a
   fundamental block of OLSRv2, the vulnerabilities of NHDP apply also
   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, MPR
   Selection and Link State Advertisements.  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 which are in direct communication range of itself (1-hop
   neighbors), and detects with which of these it can establish bi-
   directional 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 bi-directional).  A router A receiving a
   HELLO message from a neighbor B, in which B indicates to have
   recently received a HELLO message from A, considers the link A-B to
   be bi-directional.  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

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   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

   Multi Point 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 bi-
   directional neighbors, a subset (MPR set) such that a 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.

   Routers may express, in their HELLO messages, their "willingness" (an
   integer between 0 "will never" and 7 "will always") to be selected as
   MPR, which is taken into consideration for the MPR calculation, and
   which is useful for example when an OLSRv2 network is "planned".  The
   set of routers having selected a given router as 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, broadcast through the network using the MPR flooding
   process described above.  As a router selects MPRs only from among
   bi-directional neighbors, links advertised in TC are also bi-
   directional and routing paths calculated by OLSRv2 contain only bi-
   directional 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, accurately
      reflecting the effective network topology;

   o  the network converges, i.e., that all routers in the network will
      have sufficiently identical topology maps.

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   An OLSRv2 network can be disrupted by breaking any of these
   assumptions, specifically (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, its willingness to serve as MPR), henceforth
   identified as Identity Spoofing, or (b) advertise erroneous
   information about its relationship to other routers (pretend
   existence of links to other routers), henceforth identified as Link
   Spoofing.  Such attacks may disrupt the LSA process, through
   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, depleting 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 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.

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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, 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 to 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, and are therefore not protected by
   integrity check mechanisms.  A compromised OLSRv2 router can modify
   either of these when forwarding TCs.

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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 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 by, 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]).

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            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 that are 3 hops away, 12 seconds for routers in a 4-hop
   distance and 15 seconds in a 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, reflecting the effective network topology.  This
   implies that the routing protocol, through its message exchange,
   identifies 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.

   Considering 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, effectively, 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 attack,

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   indirect channel overloading have been 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 Figure 3(a) and Figure 3(b).

   This situation is not hypothetical.  A common error seen when
   deploying OLSRv2-based networks using Linux-based computers as 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, 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 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 depends on the
   detailed behavior of the wormhole.  If the wormhole relays only
   control traffic, but not data traffic, the same considerations as in

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   Section 4.1 applies.  If, however, the wormhole relays all traffic,
   control and data alike, it is connectivity-wise identical to a usable
   link - and the routing protocol will correctly generate a topology
   map reflecting the effective network topology.  The efficiency of the
   topology so 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 was 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, 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 regards to wormholes is, that such
   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 (ii) to 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

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   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, respectively, the
   Neighborhood Discovery mechanism and the LSA mechanism.

   The most 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 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".

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              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

   Figure 5: Indirect Jamming in Link State Advertisement: the malicious
                 X flips between link status MPR and LOST.

   Routers receiving a TC 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 is a potentially
   computationally costly operation.

   A compromised OLSRv2 router may chose to conduct this attack against
   all its neighbors, thus attaining maximum 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 zero (thus, avoid being
   selected as 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

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   to the minimum message interval.  [RFC7181] proposes as default that
   the minimum TC interval be 0.25 x TC interval.

   Indirect Jamming by a compromised OLSRv2 router can thus have two
   effects: it may cause increased frequency of TC generation and
   transmission, and 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 of 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
   LSAs can also cause inconsistent topology maps.

   An inconsistent topology map may occur when the compromised OLSRv2
   router takes part in the LSA procedure, by selecting a neighbor as
   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)

    Figure 6: Identity Spoofing: compromised OLSRv2 router X spoofs the
          identity of A, leading to a wrongly perceived topology.

   In Figure 6, router X spoofs the address of router A. If X selects F
   as 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 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

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   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: 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 one 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)

     Figure 7: Routing paths towards A, as calculated by the different
   routers in the network in presence of a compromised OLSRv2 router X,
                        spoofing the address of 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

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   may also suppress the link state information for other, legitimate,
   MPR selectors of F.

   Identity spoofing by a compromised OLSRv2 router, participating in
   the LSA process by selecting MPRs only, thus, 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 which are, either,
   not present in the network, or which 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 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 a 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)

   Figure 8: Link Spoofing: 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.

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   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 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

      Figure 9: Routing paths towards router A, as calculated by the
   different routers in the network in presence of a compromised OLSRv2
                  router X, spoofing a link to router A.

   In general, the following observations can be made:

   o  The network will be separated in two, with those routers closer to
      B than to X reaching A, whereas those routers closer to X than to
      B unable to reach A.

   o  Routers beyond B, i.e., routers beyond one 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], as well as the protection that

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   [RFC7183] provides when used in a deployment.

6.1.  Inherent OLSRv2 Resilience

   OLSRv2 (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,
      specific per router identity and message type.  Routers keep an
      "information freshness" number (ANSN), incremented each time the
      content of a LSA from a router changes.  This allows rejecting
      "old" information and duplicate messages, and provides some
      protection against "message replay".  This, however, also presents
      an attack vector (Section 4.3).

   o  Ignoring uni-directional links: The Neighborhood Discovery process
      detects and admits only bi-directional 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 as not bi-directional.

   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.  This allows - together with the
      flexible message format [RFC5444] - addition of security
      mechanisms, such as digital signatures, while remaining compliant
      with the OLSRv2 standard specification.

6.2.  Resilience by using RFC7183 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 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 a non-valid message is rejected.  Because

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   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 to revoke access to 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 zero 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, changing 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.

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   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, in particular when using identity based
      signatures, such as specified in [RFC7859] may give possibility of
      revoking single routers and to verify their identity based on the
      ICV itself.

   Link Spoofing -  Similar to identity spoofing, a malicious router
      without valid credential 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 as
      for link spoofing apply.

6.3.  Correct Deployment

   Other than implementing OLSRv2 itself and corresponding security
   mechanisms, deploying the protocol correctly is also important to
   guarantee the protocol functioning and mitigate security

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   vulnerabilities.  For example, Section 4.1 and Section 4.2 discussed
   the vulnerabilities because of incorrect forwarding policy setting
   and wormholes.  This requires the routers are deployed with IP
   forwarding enabled and the wormholes (if exist) are managed
   appropriately.

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], [RFC7183]) is in use - then, 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.  IANA Considerations

   This document has no actions for IANA.

9.  References

9.1.  Normative References

   [RFC6130]  Clausen, T., Dearlove, C., and J. Dean, "Mobile Ad Hoc
              Network (MANET) Neighborhood Discovery Protocol (NHDP)",
              RFC 6130, April 2011.

   [RFC7181]  Clausen, T., Dearlove, C., Jacquet, P., and U. Herberg,
              "The Optimized Link State Routing Protocol Version 2",
              RFC 7181, April 2014.

   [RFC7186]  Yi, J., Herberg, U., and T. Clausen, "Security Threats for
              the Neighborhood Discovery Protocol (NHDP)", RFC 7186,
              April 2014.

9.2.  Informative References

   [FUNKFEUER]
              "http://www.funkfeuer.at/".

   [IEEE802.11]

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              "IEEE 802.11: Wireless LAN Medium Access Control (MAC) and
              Physical Layer (PHY) Spec.", 2007.

   [MPR-FLOODING]
              Qayyum, A., Viennot, L., and A. Laouiti, "Multipoint
              relaying: An efficient technique for flooding in mobile
              wireless networks.", 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 Wireless Networks", 2004.

   [RFC3626]  Clausen, T. and P. Jacquet, "The Optimized Link State
              Routing Protocol", RFC 3626, October 2003.

   [RFC5068]  Hutzler, C., Crocker, D., Resnick, P., Allman, E., and T.
              Finch, "Email Submission Operations: Access and
              Accountability Requirements", RFC 5068, BCP 134,
              October 2007.

   [RFC5148]  Clausen, T., Dearlove, C., and B. Adamson, "Jitter
              Considerations in Mobile Ad Hoc Networks (MANETs)",
              RFC 5148, February 2008.

   [RFC5444]  Clausen, T., Dearlove, C., Dean, J., and C. Adjih,
              "Generalized MANET Packet/Message Format", RFC 5444,
              February 2009.

   [RFC5497]  Clausen, T. and C. Dearlove, "Representing Multi-Value
              Time in Mobile Ad Hoc Networks (MANETs)", RFC 5497,
              March 2009.

   [RFC7182]  Herberg, U., Clausen, T., and C. Dearlove, "Integrity
              Check Value and Timestamp TLV Definitions for Mobile Ad
              Hoc Networks (MANETs)", RFC 7182, April 2014.

   [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, April 2014.

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   [RFC7184]  Herberg, U., Cole, R., and T. Clausen, "Definition of
              Managed Objects for the Optimized Link State Routing
              Protocol Version 2", RFC 7184, April 2014.

   [RFC7187]  Dearlove, C. and T. Clausen, "Routing Multipoint Relay
              Optimization for the Optimized Link State Routing Protocol
              Version 2 (OLSRv2)", RFC 7187, April 2014.

   [RFC7188]  Dearlove, C. and T. Clausen, "Optimized Link State Routing
              Protocol Version 2 (OLSRv2) and MANET Neighborhood
              Discovery Protocol (NHDP) Extension TLVs", RFC 7188,
              April 2014.

   [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

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   Jiazi Yi
   Ecole Polytechnique
   91128 Palaiseau Cedex,
   France

   Phone: +33 1 77 57 80 85
   Email: jiazi@jiaziyi.com
   URI:   http://www.jiaziyi.com/

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