





Internet Engineering Task Force (IETF)                             J. Yi
Request for Comments: 7186                      LIX, Ecole Polytechnique
Category: Informational                                       U. Herberg
ISSN: 2070-1721                          Fujitsu Laboratories of America
                                                              T. Clausen
                                                LIX, Ecole Polytechnique
                                                              April 2014


    Security Threats for the Neighborhood Discovery Protocol (NHDP)

Abstract

   This document analyzes common security threats of the Neighborhood
   Discovery Protocol (NHDP) and describes their potential impacts on
   Mobile Ad Hoc Network (MANET) routing protocols using NHDP.  This
   document is not intended to propose solutions to the threats
   described.

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 5741.

   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/rfc7186.

















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Copyright Notice

   Copyright (c) 2014 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
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  NHDP Threat Overview  . . . . . . . . . . . . . . . . . . . .   4
   4.  Detailed Threat Description . . . . . . . . . . . . . . . . .   5
     4.1.  Jamming . . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  Denial-of-Service Attack  . . . . . . . . . . . . . . . .   5
     4.3.  Eavesdropping and Traffic Analysis  . . . . . . . . . . .   6
     4.4.  Incorrect HELLO Message Generation  . . . . . . . . . . .   7
       4.4.1.  Identity Spoofing . . . . . . . . . . . . . . . . . .   7
       4.4.2.  Link Spoofing . . . . . . . . . . . . . . . . . . . .   8
     4.5.  Replay Attack . . . . . . . . . . . . . . . . . . . . . .   9
     4.6.  Message Timing Attacks  . . . . . . . . . . . . . . . . .   9
       4.6.1.  Interval Time Attack  . . . . . . . . . . . . . . . .  10
       4.6.2.  Validity Time Attack  . . . . . . . . . . . . . . . .  10
     4.7.  Indirect Channel Overloading  . . . . . . . . . . . . . .  10
     4.8.  Attack on Link Quality Update . . . . . . . . . . . . . .  11
   5.  Impact of Inconsistent Information Bases on Protocols using
       NHDP  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  12
     5.1.  MPR Calculation . . . . . . . . . . . . . . . . . . . . .  12
       5.1.1.  Flooding Disruption due to Identity Spoofing  . . . .  12
       5.1.2.  Flooding Disruption due to Link Spoofing  . . . . . .  13
       5.1.3.  Broadcast Storm . . . . . . . . . . . . . . . . . . .  14
     5.2.  Routing Loops . . . . . . . . . . . . . . . . . . . . . .  15
     5.3.  Invalid or Nonexistent Paths to Destinations  . . . . . .  16
     5.4.  Data Sinkhole . . . . . . . . . . . . . . . . . . . . . .  16
   6.  Future Work . . . . . . . . . . . . . . . . . . . . . . . . .  16
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  18
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  18
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  18



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1.  Introduction

   The Neighborhood Discovery Protocol (NHDP) [RFC6130] allows routers
   to acquire topological information up to two hops away from
   themselves, by way of periodic HELLO message exchanges.  The
   information acquired by NHDP is used by other protocols, such as the
   Optimized Link State Routing Protocol version 2 (OLSRv2) [RFC7181]
   and Simplified Multicast Forwarding (SMF) [RFC6621].  The topology
   information, acquired by way of NHDP, serves these routing protocols
   by detecting and maintaining local 1-hop and 2-hop neighborhood
   information.

   As NHDP is typically used in wireless environments, it is potentially
   exposed to different kinds of security threats, some of which are of
   particular 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 is commonly no physical
   protection as otherwise known for wired networks.  NHDP does not
   define any explicit security measures for protecting the integrity of
   the information it acquires; however, it suggests that the integrity
   protection be addressed in a fashion appropriate to the deployment of
   the network.

   This document is based on the assumption that no additional security
   mechanism such as IPsec is used in the IP layer, as not all MANET
   deployments may be able to accommodate such common IP protection
   mechanisms (e.g., because of limited resources of MANET routers).
   The document analyzes possible attacks on and misconfigurations of
   NHDP and outlines the consequences of such attacks/misconfigurations
   to the state maintained by NHDP in each router (and, thus, made
   available to protocols using this state).

   This document is not intended to propose solutions to the threats
   described.  [RFC7185] provides further information on how to enable
   integrity protection to NHDP, which can help mitigating the threats
   described related to identity spoofing.

   It should be noted that many NHDP implementations are configurable,
   and so an attack on the configuration system (such as [RFC6779]) can
   be used to adversely affect the operation of an NHDP implementation.

   The NHDP MIB module [RFC6779] might help monitoring some of the
   security attacks mentioned in this document.  [MGMT-SNAP] provides a
   snapshot of OLSRv2-routed MANET management as currently deployed,
   while [MANET-MGMT] is intended to provide specific guidelines on
   MANET network management considering the various MIB modules that
   have been written.




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2.  Terminology

   This document uses the terminology and notation defined in
   "Generalized Mobile Ad Hoc Network (MANET) Packet/Message Format"
   [RFC5444], "Mobile Ad Hoc Network (MANET) Neighborhood Discovery
   Protocol (NHDP)" [RFC6130], and "Internet Security Glossary, Version
   2" [RFC4949].

   Additionally, this document introduces the following terminology:

   NHDP router:  A MANET router, running NHDP as specified in [RFC6130].

   Attacker:  A device that is present in the network and intentionally
      seeks to compromise the information bases in NHDP routers.

   Compromised NHDP router:  An attacker that is present in the network
      and generates syntactically correct NHDP control messages.
      Control messages emitted by a compromised NHDP router may contain
      additional information, or omit information, as compared to a
      control message generated by a non-compromised NHDP router located
      in the same topological position in the network.

   Legitimate NHDP router:  An NHDP router that is not a compromised
      NHDP router.

3.  NHDP Threat Overview

   NHDP defines a HELLO messages exchange, enabling each NHDP router to
   acquire topological information describing its 1-hop and 2-hop
   neighbors, and specifies information bases for recording this
   information.

   An NHDP router periodically transmits HELLO messages using a link-
   local multicast on each of its interfaces with a hop-limit of 1
   (i.e., HELLOs are never forwarded).  In these HELLO messages, an NHDP
   router announces the IP addresses as heard, symmetric, or lost
   neighbor interface addresses.

   An Attacker has several ways of harming this neighbor discovery
   process: it can announce "wrong" information about its identity,
   postulate nonexistent links, and replay HELLO messages.  These
   attacks are presented in detail in Section 4.

   The different ways of attacking an NHDP deployment may eventually
   lead to inconsistent information bases, not accurately reflecting the
   correct topology of the MANET.  The consequence is that protocols
   using NHDP will base their operation on incorrect information,
   causing routing protocols to not be able to calculate correct (or



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   any) paths, degrade the performance of flooding operations based on
   reduced relay sets, etc.  These consequences to protocols using NHDP
   are described in detail in Section 5.

4.  Detailed Threat Description

   For each threat, a description of the mechanism of the corresponding
   attack is given, followed by a description of how the attack affects
   NHDP.  The impacts from each attack on protocols using NHDP are given
   in Section 5.

   For simplicity in the description, the examples given assume that
   NHDP routers have a single interface with a single IP address
   configured.  All the attacks apply, however, for NHDP routers with
   multiple interfaces and multiple addresses as well.

4.1.  Jamming

   One vulnerability, common for all protocols operating a wireless ad
   hoc network, is that of "jamming", i.e., that a device generates
   massive amounts of interfering radio transmissions, which will
   prevent legitimate traffic (e.g., control traffic as well as data
   traffic) on part of a network.  Jamming is a form of interference and
   overload with the threat consequence of disruption [RFC4593].

   Depending on lower layers, this may not affect transmissions: HELLO
   messages from an NHDP router with "jammed" interfaces may be received
   by other NHDP routers.  As NHDP identifies whether a link to a
   neighbor is unidirectional or bidirectional, a routing protocol that
   uses NHDP for neighborhood discovery may ignore a link from a jammed
   NHDP router to a non-jammed NHDP router.  The jammed router (a router
   with jammed carrier) would appear simply as "disconnected" for the
   unjammed part of the network, which is able to maintain accurate
   topology maps.

   If a considerable amount of HELLO messages are lost or corrupted due
   to collisions caused by a jamming attack, neighbor NHDP routers are
   not able to establish links between themselves any more.  Thus, NHDP
   will present empty information bases to the protocols using it.

4.2.  Denial-of-Service Attack

   A denial-of-service (DoS) attack can be a result of misconfiguration
   of legitimate NHDP routers (e.g., very short HELLO transmission
   interval) or malicious behavior of compromised NHDP routers
   [ACCT2012], so-called Byzantine routers [RFC4593].  DoS is a form of
   interference and overload with the threat consequence of disruption
   [RFC4593].



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   By transmitting a huge amount of HELLO messages in a short period of
   time, NHDP routers can increase channel occupation as described in
   Section 4.1.  Furthermore, a compromised NHDP router can spoof a
   large amount of different IP addresses and send HELLOs to its
   neighbors to fill their Link/Neighbor Sets.  This may result in
   memory overflow, and it makes the processing of legitimate HELLO
   messages impossible.  A compromised NHDP router can also use link
   spoofing in its HELLO messages, generating huge 2-hop Sets in
   adjacent NHDP routers and therefore potentially a memory overflow.
   Moreover, protocols such as SMF and OLSRv2, using the 2-hop
   information for multipoint relay (MPR) calculation, may exhaust the
   available computational resources of the router if the Neighbor Set
   and 2-hop Sets have too many entries.

   By exhausting the memory, CPU, and/or channel resources of a router
   in a DoS attack or a misconfiguration, NHDP routers may not be able
   to accomplish their specified tasks of exchanging 1-hop and 2-hop
   neighborhood information, and thereby disturbing the operation of
   routing protocols using NHDP.

   In some MANETs, the routers are powered by battery.  Another
   consequence of a DoS attack in such networks is that the power will
   be drained quickly by unnecessary processing, transmitting, and
   receiving of messages.

4.3.  Eavesdropping and Traffic Analysis

   Eavesdropping, sometimes referred to as sniffing, is a common and
   easy passive attack in a wireless environment.  Once a packet is
   transmitted, any adjacent NHDP router can potentially obtain a copy,
   for immediate or later processing.  Neither the source nor the
   intended destination can detect this.  A malicious NHDP router can
   eavesdrop on the NHDP message exchange and thus learn the local
   topology.  It may also eavesdrop on data traffic to learn source and
   destination addresses of data packets, or other header information,
   as well as the packet payload.

   Eavesdropping does not pose a direct threat to the network or to
   NHDP, in as much as that it does not alter the information recorded
   by NHDP in its information bases and presented to other protocols.
   However, eavesdropping can provide network information required for
   enabling other attacks, such as the identity of communicating NHDP
   routers, detection of link characteristics, and NHDP router
   configuration.  The compromised NHDP routers may use the obtained
   information to launch subsequent attacks, and they may also share
   NHDP routing information with other NHDP or non-NHDP entities.
   [RFC4593] would categorize the threat consequence as disclosure.




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   Traffic analysis normally follows eavesdropping, which is the process
   of intercepting messages in order to deduce information from
   communication patterns.  It can be performed even when HELLO messages
   are encrypted (encryption is not a part of NHDP), for example:

   o  Triggered HELLO messages: an attacker could figure out that
      messages are triggered and determine that there was a change of
      symmetric neighbors of an NHDP router sending the HELLO (as well
      get the frequency).

   o  Message size: the message grows exactly by x bytes per neighbor.
      Depending on which cipher is used for the encryption, some
      information about the size could be inferred, and thus the number
      of neighbors could be guessed.

   [RFC4593] would categorize the threat consequence as disclosure.

4.4.  Incorrect HELLO Message Generation

   An NHDP router performs two distinct tasks: it periodically generates
   HELLO messages, and it processes incoming HELLO messages from
   neighbor NHDP routers.  This section describes security attacks
   involving the HELLO generation.

4.4.1.  Identity Spoofing

   Identity spoofing implies that a compromised NHDP router sends HELLO
   messages, pretending to have the identity of another NHDP router, or
   even a router that does not exist in the networks.  A compromised
   NHDP router can accomplish this by using an IP address, which is not
   its own, in an address block of a HELLO message, and associating this
   address with a LOCAL_IF Address Block TLV [IJNSIA2010].

   An NHDP router receiving that HELLO message from a neighbor will
   assume that it originated from the NHDP router with the spoofed
   interface address.  As a consequence, it will add a Link Tuple to
   that neighbor with the spoofed address, and include it in its next
   HELLO messages as a heard neighbor (and possibly as a symmetric
   neighbor after another HELLO exchange).

   Identity spoofing is particularly harmful if a compromised NHDP
   router spoofs the identity of another NHDP router that exists in the
   same routing domain.  With respect to NHDP, such a duplicated,
   spoofed address can lead to an inconsistent state up to two hops from
   an NHDP router.  [RFC4593] would categorize the threat consequences
   as disclosure and deception.





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   Figure 1 depicts a simple example.  In that example, NHDP router A is
   in radio range of NHDP router C, but not of the compromised NHDP
   router X.  If X spoofs the address of A, that can lead to conflicts
   for a routing protocol that uses NHDP, and therefore for wrong path
   calculations as well as incorrect data traffic forwarding.

   .---.    .---.    .---.
   | A |----| C |----| X |
   '---'    '---'    '---'

                                 Figure 1

   Figure 2 depicts another example.  In this example, NHDP router A is
   two hops away from NHDP router C, reachable through NHDP router B.
   If the compromised NHDP router X spoofs the address of A, NHDP router
   D will take A as its 1-hop neighbor, and C may think that A is indeed
   reachable through D.

   .---.    .---.    .---.    .---.    .---.
   | A |----| B |----| C |----| D |----| X |
   '---'    '---'    '---'    '---'    '---'

                                 Figure 2

4.4.2.  Link Spoofing

   Similar to identity spoofing, link spoofing implies that a
   compromised NHDP router sends HELLO messages, signaling an incorrect
   set of neighbors.  This is sometimes referred to as falsification
   [RFC4593], and in NHDP it may take either of two forms:

   o  A compromised NHDP router can postulate addresses of non-present
      neighbor NHDP routers in an address block of a HELLO, associated
      with LINK_STATUS TLVs.

   o  A compromised NHDP router can "ignore" otherwise existing
      neighbors by not advertising them in its HELLO messages.

   The effect of link spoofing with respect to NHDP are twofold,
   depending on the two cases mentioned above:

   o  If the compromised NHDP router ignores existing neighbors in its
      advertisements, links will be missing in the information bases
      maintained by other routers, and there may not be any connectivity
      for these NHDP routers to or from other NHDP routers in the MANET.






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   o  On the other hand, if the compromised NHDP router advertises
      nonexistent links, this will lead to inclusion of topological
      information in the information base, describing nonexistent links
      in the network (which, then, may be used by other protocols using
      NHDP in place of other, existing, links).

   [RFC4593] would categorize the threat consequences as usurpation,
   deception, and disruption.

4.5.  Replay Attack

   A replay attack implies that control traffic from one region of the
   network is recorded and replayed in a different region at (almost)
   the same time, or in the same region at a different time.  This may,
   for example, happen when two compromised NHDP routers collaborate on
   an attack, one recording traffic in its proximity and tunneling it to
   the other compromised NHDP router, which replays the traffic.  In a
   protocol where links are discovered by testing reception, this will
   result in extraneous link creation (basically, a "virtual" link
   between the two compromised NHDP routers will appear in the
   information bases of neighboring NHDP routers).  [RFC4593] would
   categorize this as a falsification and interference threat with
   threat consequences of usurpation, deception, and disruption.

   While this situation may result from an attack, it may also be
   intentional: if data traffic is also relayed over the "virtual" link,
   the link being detected is indeed valid for use.  This is, for
   instance, used in wireless repeaters.  If data traffic is not carried
   over the virtual link, an imaginary, useless link between the two
   compromised NHDP routers has been advertised and is being recorded in
   the information bases of their neighboring NHDP routers.

   Compared to incorrect HELLO message attacks described in Section 4.4,
   the messages used in replay attacks are legitimate messages sent out
   by (non-malicious) NHDP routers and replayed at a later time or
   different locality by malicious routers.  This makes this kind of
   attack harder to be detect and to counteract; integrity checks cannot
   help in this case, as the original message's Integrity Check Value
   (ICV) was correctly calculated.

4.6.  Message Timing Attacks

   In NHDP, each HELLO message contains a "validity time" (the amount of
   time that information in that control message should be considered
   valid before being discarded) and may contain an "interval time"
   field (the amount of time until the next control message of the same
   type should be expected) [RFC5497].




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4.6.1.  Interval Time Attack

   A use of the expected interval between two successive HELLO messages
   is for determining the link quality in NHDP: if messages are not
   received within the expected intervals (e.g., a certain fraction of
   messages are missing), then this may be used to exclude a link from
   being considered as useful, even if (some) bidirectional
   communication has been verified.  If a compromised NHDP router X
   spoofs the identity of an existing NHDP router A and sends HELLOs
   indicating a low interval time, an NHDP router B receiving this HELLO
   will expect the following HELLO to arrive within the interval time
   indicated.  If that expectation is not met, the link quality for the
   link A-B will be decreased.  Thus, X may cause NHDP router B's
   estimate of the link quality for the link A-B to fall below the
   minimum considered useful, so the link would not be used
   [CPSCOM2011].  [RFC4593] would categorize the threat consequence as
   usurpation.

4.6.2.  Validity Time Attack

   A compromised NHDP router X can spoof the identity of an NHDP router
   A and send a HELLO using a low validity time (e.g., 1 ms).  A
   receiving NHDP router B will discard the information upon expiration
   of that interval, i.e., a link between NHDP router A and B will be
   "torn down" by X.  The sending of a low validity time can be caused
   by intended malicious behaviors or simply misconfiguration in the
   NHDP routers.  [RFC4593] would categorize the threat consequence as
   usurpation.

4.7.  Indirect Channel Overloading

   Indirect Channel Overloading is when a compromised NHDP router X by
   its actions causes other legitimate NHDP routers to generate
   inordinate amounts of control traffic.  This increases channel
   occupation and the overhead in each receiving NHDP router that
   processes this control traffic.  With this traffic originating from
   legitimate NHDP routers, the malicious device may remain undetected
   in the wider network.  It is a form of interference and overload with
   the threat consequence of disruption [RFC4593].

   Figure 3 illustrates Indirect Channel Overloading with NHDP.  A
   compromised NHDP router X advertises a symmetric spoofed link to the
   nonexistent NHDP router B (at time t0).  Router A selects X as MPR
   upon reception of the HELLO then triggers a HELLO at t1.  Overhearing
   this triggered HELLO, the attacker sends another HELLO at t2,
   advertising the link to B as lost; this causes NHDP router A to





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   deselect the attacker as MPR, and to send another triggered message
   at t3.  The cycle may be repeated, where the link X-B is advertised
   alternately as LOST and SYM.

                MPRs(X)                   MPRs()
   .---.        .---.        .---.        .---.
   | A |        | A |        | A |        | A |
   '---'        '---'        '---'        '---'
     |            |            |            |
     | SYM(B)     |            | LOST(B)    |
     |            |            |            |
   .---.        .---.        .---.        .---.
   | X |        | X |        | X |        | X |
   '---'        '---'        '---'        '---'
     .            .
     .            .
     .            .
   .....        .....
   . B .        . B .
   .....        .....

    t0           t1           t2           t3

                                 Figure 3

4.8.  Attack on Link Quality Update

   According to NHDP [RFC6130]:

      Link quality is a mechanism whereby a router MAY take
      considerations other than message exchange into account for
      determining when a link is and is not a candidate for being
      considered as HEARD or SYMMETRIC.  As such, it is a "link
      admission" mechanism.

   Section 14.4 of NHDP [RFC6130] then lists several examples of which
   information can be used to update link quality.  One of the listed
   examples uses packet exchanges between neighbor routers (as described
   in [RFC5444]), e.g., an NHDP router may update the link quality of a
   neighbor based on receipt or loss of packets if they include a
   sequential packet sequence number.

   NHDP does not specify how to acquire link quality updates
   normatively; however, attack vectors may be introduced if an
   implementation chooses to calculate link quality based on packet
   sequence numbers.  The consequences of such threats would depend on
   specific implementations.  For example, if the link quality update is
   based on a sequential packet sequence number from neighbor routers, a



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   compromised NHDP router can spoof packets appearing to be from
   another legitimate NHDP router that skips some packet sequence
   numbers.  The NHDP router receiving the spoofed packets may degrade
   the link quality as it appears that several packets have been
   dropped.  Eventually, the router may remove the neighbor when the
   link quality drops below HYST_REJECT.

5.  Impact of Inconsistent Information Bases on Protocols using NHDP

   This section describes the impact on protocols that use NHDP when
   NHDP fails to obtain and represent accurate information, possibly as
   a consequence of the attacks described in Section 4.  This
   description emphasizes the impacts on the MANET protocols OLSRv2
   [RFC7181] and SMF [RFC6621].

5.1.  MPR Calculation

   MPR selection (as used in [RFC7181] and [RFC6621], for example) uses
   information about a router's 1-hop and 2-hop neighborhood, assuming
   that (i) this information is accurate, and (ii) each 1-hop neighbor
   is apt to act as MPR, depending on the willingness it reports.  Thus,
   a compromised NHDP router may seek to manipulate the 1-hop and 2-hop
   neighborhood information in a router so as to cause the MPR selection
   to fail, leading to a flooding disruption of traffic control
   messages.  This can result in incomplete topology advertisement or
   can degrade the optimized flooding to classical flooding.

5.1.1.  Flooding Disruption due to Identity Spoofing

   A compromised NHDP router can spoof the identify of other routers in
   order to disrupt the MPR selection, so as to prevent certain parts of
   the network from receiving flooded traffic [IJNSIA2010].

   In Figure 4, a compromised NHDP router X spoofs the identity of B.
   The link between X and C is correctly detected and listed in X's
   HELLOs.  Router A will receive HELLOs indicating links from B:{B-E},
   X:{X-C, X-E}, and D:{D-E, D-C}, respectively.  For router A, X and D
   are equal candidates for MPR selection.  To make sure the X can be
   selected as MPR for router A, X can set its willingness to the
   maximum value.











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   .---.    .---.    .---.
   | E |----| D |----| C |
   '---'    '---'    '---'
     |        |        .
     |        |        .
   .---.    .---.    .---.
   | B |----| A |----| X |
   '---'    '---'    '---'
                     spoofs B

                                 Figure 4

   If B and X (i) accept MPR selection and (ii) forward flooded traffic
   as if they were both B, identity spoofing by X is harmless.  However,
   if X does not forward flooded traffic (i.e., does not accept MPR
   selection), its presence entails flooding disruption: selecting B
   over D renders C unreachable by flooded traffic.

                      .---.
                      | D |
                      '---'
                        |
                        |
    .---.    .---.    .---.    .---.    .---.
    | X |----| A |----| B |----| C |----| E |...
    '---'    '---'    '---'    '---'    '---'
   spoofs E

                                 Figure 5

   In Figure 5, the compromised NHDP router X spoofs the identity of E,
   i.e., routers A and C both receive HELLOs from a router identifying
   itself as E.  For router B, routers A and C present the same neighbor
   sets and are equal candidates for MPR selection.  If router B selects
   only router A as MPR, C will not relay flooded traffic from B or
   transiting via B, and router X (and routers to the "right" of it)
   will not receive flooded traffic.

5.1.2.  Flooding Disruption due to Link Spoofing

   A compromised NHDP router can also spoof links to other NHDP routers,
   thereby making itself appear as the most appealing candidate to be
   MPR for its neighbors, possibly to the exclusion of other NHDP
   routers in the neighborhood.  (In particular, this can occur if the
   compromised NHDP router spoofs links to all other NHDP routers in the
   neighborhood, plus to one NHDP router outside the neighborhood.)  By
   thus excluding other legitimate NHDP routers from being selected as
   MPR, the compromised NHDP router will receive and be expected to



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   relay all flooded traffic (e.g., traffic control messages in OLSRv2
   or data traffic in SMF) that it can then drop or otherwise
   manipulate.

   In the network in Figure 6, the compromised NHDP router X spoofs
   links to the existing router C, as well as to a fictitious W.  Router
   A receives HELLOs from X and B, reporting X: {X-C, X-W}, B: {B-C}.
   All else being equal, X appears a better choice for MPR than B, as X
   appears to cover all neighbors of B, plus W.

                               ,---.    .....
                               | S |    . C .
                               '---'    .....
                                 |        .
                                 |        .
    .---.    .---.    .---.    .---.    .---.
    | D |----| C |----| B |----| A |----| X |
    '---'    '---'    '---'    '---'    '---'
                                          .
                                          .
                                        .....
                                        . W .
                                        .....

                                 Figure 6

   As router A will not select B as MPR, B will not relay flooded
   messages received from router A.  The NHDP routers on the left of B
   (starting with C) will, thus, not receive any flooded messages from
   router A or transiting router A (e.g., a message originating from S).

5.1.3.  Broadcast Storm

   A compromised NHDP router may attack the network by attempting to
   degrade the performance of optimized flooding algorithms so as to be
   equivalent to classic flooding.  This can be achieved by forcing an
   NHDP router into choosing all its 1-hop neighbors as MPRs.  In
   MANETs, a broadcast storm caused by classic flooding is a serious
   problem that can result in redundancy, contention, and collisions
   [MOBICOM99].

   As shown in Figure 7, the compromised NHDP router X spoofs the
   identity of NHDP router B and, spoofs a link to router Y {B-Y} (Y
   does not have to exist).  By doing so, the legitimate NHDP router A
   has to select the legitimate NHDP router B as its MPR in order for it
   to reach all its 2-hop neighbors.  The compromised NHDP router Y can





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   perform this identity-and-link spoofing for all of NHDP router A's
   1-hop neighbors, thereby forcing NHDP router A to select all its
   neighbors as MPR and disabling the optimization sought by the MPR
   mechanism.

   .---.
   | B |
   '---'
     |
     |
   .---.    .---.      .....
   | A |----| X | .  . . Y .
   '---'    '---'      .....
            spoofs B

                                 Figure 7

5.2.  Routing Loops

   Inconsistent information bases, provided by NHDP to other protocols,
   can also cause routing loops.  In Figure 8, the compromised NHDP
   router X spoofs the identity of NHDP router E.  NHDP router D has
   data traffic to send to NHDP router A.  The topology recorded in the
   information base of router D indicates that the shortest path to
   router A is {D->E->A}, because of the link {A-E} reported by X.
   Therefore, the data traffic will be routed to NHDP router E.  As the
   link {A-E} does not exist in NHDP router E's information bases, it
   will identify the next hop for data traffic to NHDP router A as being
   NHDP router D.  A loop between the NHDP routers D and E is thus
   created.

         .---.    .---.    .---.    .---.    .---.
         | A |----| B |----| C |----| D |----| E |
         '---'    '---'    '---'    '---'    '---'
           |
           |
         .---.
         | X |
         '---'
         spoofs E

                                 Figure 8









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5.3.  Invalid or Nonexistent Paths to Destinations

   By reporting inconsistent topology information in NHDP, the invalid
   links and routers can be propagated as link state information with
   traffic control messages and results in route failure.  As
   illustrated in Figure 8, if NHDP router B tries to send data packets
   to NHDP router E, it will choose router A as its next hop, based on
   the information about the nonexistent link {A-E} reported by the
   compromised NHDP router X.

5.4.  Data Sinkhole

   With the ability to spoof multiple identities of legitimate NHDP
   routers (by eavesdropping, for example), the compromised NHDP router
   can represent a "data sinkhole" for its 1-hop and 2-hop neighbors.
   Data packets that come across its neighbors may be forwarded to the
   compromised NHDP router instead of to the real destination.  The
   packet can then be dropped, manipulated, duplicated, etc., by the
   compromised NHDP router.  As shown in Figure 8, if the compromised
   NHDP router X spoofs the identity of NHDP router E, all the data
   packets to E that cross NHDP routers A and B will be sent to NHDP
   router X, instead of to E.

6.  Future Work

   This document does not propose solutions to mitigate the security
   threats described in Section 4.  However, this section aims at
   driving new work by suggesting which threats discussed in Section 4
   could be addressed by deployments or applications.

   o  Section 4.1: Jamming - If a single router or a small area of the
      MANET is jammed, protocols could be specified that increase link
      metrics in NHDP for the jammed links.  When a routing protocol
      such as OLSRv2 uses NHDP for neighborhood discovery, other paths
      leading "around" the jammed area would be preferred, and therefore
      would mitigate the threat to some extent.

   o  Section 4.2: DoS - A DoS attack using a massive amount of HELLO
      messages can be mitigated by admitting only trusted routers to the
      network.  [RFC7185] specifies a mechanism for adding Integrity
      Check Values (ICVs) to HELLO messages and therefore providing an
      admittance mechanism for NHDP routers to a MANET.  (Note that
      adding ICVs creates a new DoS attack vector, as ICV verification
      requires CPU and memory resources.)  However, using ICVs does not
      address the problem of compromised routers.  Detecting compromised
      routers could be addressed in new work.  [RFC7185] mandates
      implementation of a security mechanism that is based on shared
      keys and makes excluding single compromised routers difficult;



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      work could be done to facilitate revocation mechanisms in certain
      MANET use cases where routers have sufficient capabilities to
      support asymmetric keys.

   o  Section 4.3: Eavesdropping - [RFC7185] adds ICVs to HELLO messages
      but does not encrypt them.  Therefore, eavesdropping of control
      traffic is not mitigated.  Future work could provide encryption of
      control traffic for sensitive MANET topologies.  Note that, other
      than using a single shared secret key, providing encryption of
      traffic among a set of neighbors (when that set is potentially
      undetermined) is nontrivial, especially without multiplying
      overheads.  With traffic analysis, attackers could still deduce
      the network information like HELLO message triggering and HELLO
      message size, even though the HELLO messages are encrypted.

   o  Section 4.4.2: Link spoofing - [RFC7185] provides certain
      protection against link spoofing, but an NHDP router has to
      "trust" the originator of a HELLO that the advertised links are
      correct.  For example, if a router A reports a link to B, routers
      receiving HELLOs from A have to trust that B is actually a
      (symmetric) neighbor of A.  New protocol work could address
      protection of links without overly increasing the space and time
      overheads.  An immediate suggestion for deployments is to protect
      routers against being compromised and to distribute keys only to
      trusted routers.

   o  Section 4.5: Replay Attacks - [RFC7185] uses ICVs and timestamps
      to provide some protection against replay attacks.  It is still
      feasible to replay control messages within a limited time.  A
      suggestion for deployments is to provide time synchronization
      between routers.  New work could provide time synchronization
      mechanisms for certain MANET use cases or specify a mechanism
      using nonces instead of timestamps in HELLO messages.

   o  Section 4.4.1: Identity spoofing; Section 4.6: Message timing
      attacks; Section 4.7: Indirect channel overloading; and
      Section 4.8: Attack on link quality update - [RFC7185] provides
      protection against these attacks, assuming the routers are not
      compromised.

7.  Security Considerations

   This document does not specify a protocol or a procedure.  The
   document, however, reflects on security considerations for NHDP and
   MANET routing protocols using NHDP for neighborhood discovery.






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8.  Acknowledgments

   The authors would like to gratefully acknowledge the following people
   for valuable comments and technical discussions: Teco Boot, Henning
   Rogge, Christopher Dearlove, John Dowdell, Joseph Macker, and all the
   other participants of the IETF MANET working group.

9.  References

9.1.  Normative References

   [RFC5444]  Clausen, T., Dearlove, C., Dean, J., and C. Adjih,
              "Generalized Mobile Ad Hoc Network (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.

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

9.2.  Informative References

   [ACCT2012] Jhaveri, R. and S. Patel, "DoS Attacks in Mobile Ad Hoc
              Networks: A Survey", Second International Conference on
              Advanced Computing & Communication Technologies (ACCT),
              January 2012.

   [CPSCOM2011]
              Yi, J., Clausen, T., and U. Herberg, "Vulnerability
              Analysis of the Simple Multicast Forwarding (SMF) Protocol
              for Mobile Ad Hoc Networks", Proceedings of the IEEE
              International Conference on Cyber, Physical, and Social
              Computing (CPSCom), October 2011.

   [IJNSIA2010]
              Herberg, U. and T. Clausen, "Security Issues in the
              Optimized Link State Routing Protocol version 2",
              International Journal of Network Security & Its
              Applications, April 2010.

   [MANET-MGMT]
              Nguyen, J., Cole, R., Herberg, U., Yi, J., and J. Dean,
              "Network Management of Mobile Ad hoc Networks (MANET):
              Architecture, Use Cases, and Applicability", Work in
              Progress, February 2013.



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   [MGMT-SNAP]
              Clausen, T. and U. Herberg, "Snapshot of OLSRv2-Routed
              MANET Management", Work in Progress, February 2014.

   [MOBICOM99]
              Ni, S., Tseng, Y., Chen, Y., and J. Sheu, "The Broadcast
              Storm Problem in a Mobile Ad Hoc Network", Proceedings of
              the 5th annual ACM/IEEE international conference on Mobile
              computing and networking, 1999.

   [RFC4593]  Barbir, A., Murphy, S., and Y. Yang, "Generic Threats to
              Routing Protocols", RFC 4593, October 2006.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2", RFC
              4949, August 2007.

   [RFC6621]  Macker, J., "Simplified Multicast Forwarding", RFC 6621,
              May 2012.

   [RFC6779]  Herberg, U., Cole, R., and I. Chakeres, "Definition of
              Managed Objects for the Neighborhood Discovery Protocol",
              RFC 6779, October 2012.

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

   [RFC7185]  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 7185, April 2014.




















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Authors' Addresses

   Jiazi Yi
   LIX, Ecole Polytechnique
   91128 Palaiseau Cedex
   France

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


   Ulrich Herberg
   Fujitsu Laboratories of America
   1240 E Arques Ave
   Sunnyvale, CA  94085
   USA

   EMail: ulrich@herberg.name
   URI:   http://www.herberg.name/


   Thomas Heide Clausen
   LIX, Ecole Polytechnique
   91128 Palaiseau Cedex
   France

   Phone: +33 6 6058 9349
   EMail: T.Clausen@computer.org
   URI:   http://www.thomasclausen.org/





















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