Mobile Ad hoc Networking (MANET)                                   J. Yi
Internet-Draft                                                T. Clausen
Intended status: Informational                  LIX, Ecole Polytechnique
Expires: February 14, March 18, 2015                                       U. Herberg
                                         Fujitsu Laboratories of America
                                                         August 13,
                                                      September 14, 2014

       Security Threats for Simplified Multicast Forwarding (SMF)


   This document analyzes security threats of the Simplified Multicast
   Forwarding (SMF), including the vulnerabilities of duplicate packet
   detection and relay set selection mechanisms.  This document is not
   intended to propose solutions to the threats described.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  SMF Threats Overview . . . . . . . . . . . . . . . . . . . . .  4
   4.  Threats to Duplicate Packet Detection  . . . . . . . . . . . .  5
     4.1.  Threats to Identification-based Duplicate Packet
           Detection  . . . . . . . . . . . . . . . . . . . . . . . .  5
       4.1.1.  Pre-activation Attacks (Pre-Play)  . . . . . . . . . .  6
       4.1.2.  De-activation Attacks (Sequence Number wrangling)  . .  6
     4.2.  Threats to Hash-based Duplicate Packet Detection . . . . .  7
       4.2.1.  Replay Attack  . . . . . . . . . . . . . . . . . . . .  7
       4.2.2.  Attack on Hash-Assistant Value . . . . . . . . . . . .  8
   5.  Threats to Relay Set Selection . . . . . . . . . . . . . . . .  8  9
     5.1.  Relay Set Selection Common Threats . . . . . . . . . . . .  9
     5.2.  Threats to E-CDS Algorithm . . . . . . . . . . . . . . . .  9
       5.2.1.  Link Spoofing  . . . . . . . . . . . . . . . . . . . .  9 10
       5.2.2.  Identity Spoofing  . . . . . . . . . . . . . . . . . .  9 10
     5.3.  Threats to S-MPR Algorithm . . . . . . . . . . . . . . . . 10
     5.4.  Threats to MPR-CDS Algorithm . . . . . . . . . . . . . . . 10 11
   6.  Future Work  . . . . . . . . . . . . . . . . . . . . . . . . . 11
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   7. 12
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   8. 12
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     8.1. 12
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     8.2. 12
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 11 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12 13

1.  Introduction

   This document analyzes security threats of the Simplified Multicast
   Forwarding (SMF) mechanism [RFC6621].  SMF aims at providing basic
   Internet Protocol (IP) multicast forwarding, in a way which is
   suitable for limited wireless mesh and Mobile Ad hoc NETworks
   (MANET).  SMF is constituted of two major functional components:
   Duplicate Packet Detection and Relay Set Selection.

   SMF is typically used in decentralized wireless environments, and is
   potentially exposed to different kinds of attacks and
   misconfigurations.  Some of the threats are of particular
   significance as compared to wired networks.  In [RFC6621], SMF does
   not define any explicit security measures for protecting the
   integrity of the protocol.

   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 suitable to deploy common IP protection mechanisms
   (e.g., because of limited resources of MANET routers to support the
   IPsec stack).  The document analyzes possible attacks on and mis-
   configurations of SMF and outlines the consequences of such attacks/
   mis-configurations to the state maintained by SMF in each router
   (and, thus, made available to protocols using this state). router.

   This document aims at analyzing and describing the potential
   vulnerabilities of and attack vecors for SMF.  While completeness in
   such an analysis always is a goal, no claims of being complete are
   made.  The goal of this document is to be helpful for when deploying
   SMF in a network and needing to understand the risks thereby incurred
   - as wll as for providing a reference and documented experience with
   SMF as input for possibly future developments of SMF.

   This document is not intended to propose solutions to the threats
   described.  [RFC7182]  [RFC7183] provides a framework, which can be used with
   SMF, and which - depending on how it is used - may offer some degree
   of protection against the threats described in this document related
   to identity spoofing.

2.  Terminology

   This document uses the terminology and notation defined in [RFC2119],
   [RFC5444], [RFC6621] and [RFC4949].

   Additionally, this document introduces the following terminology:

   SMF router:  A MANET router, running SMF as specified in [RFC6621].

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

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

   Legitimate SMF router:  An SMF router, which is not a compromised SMF

3.  SMF Threats Overview

   SMF requires an external dynamic neighborhood discovery mechanism in
   orde to maintain suitable topological information describing its
   immediate neighborhood, and thereby allowing it to select reduced
   relay sets for forwarding multicast data traffic.  Such an external
   dynamic neighborhood discovery mechanism MAY be provided by lower-
   layer interface information, by a concurrently operating MANET
   routing protocol which already maintains such information such as
   [RFC7181], or by explicitly using MANET Neighborhood Discovery
   Protocol (NHDP) [RFC6130].  If NHDP is used for neighborhood
   discovery by SMF, SMF implicitly inherits the vulnerabilities of
   NHDP, as discussed in [RFC7186].  This document assumes that NHDP is

   Based on neighborhood discovery mechanisms, SMF specified two major
   functional components: Duplicate Packet Detection (DPD) and Relay Set
   Selection (RSS).

   DPD is required by SMF in order to be able to detect duplicate
   packets and eliminate their redundant forwarding.  An Attacker has
   several ways in which to harm the DPD mechanisms:

   o  It can "deactivate" DPD, so as to make it such that duplicate
      packets are not correctly detected, and that as a consequence they
      are (redundantly) transmitted, increasing the load on the network,
      draing the batteries of the routers involved, etc.

   o  It can "pre-activate" DPD, so as to make DPD detect a later
      arriving (valid) packet as being a duplicate, which therefore
      won't be forwarded"

   The attacks on DPD are detailed in Section 4.

   RSS produces a reduced relay set forforwarding multicast data packets
   across the MANET.  SMF supports the use of several relay set
   algorithms, including E-CDS (Essential Connected Dominating Set),
   S-MPR (Source-based Multi-point Relay, as known from [RFC3626] and
   [RFC7181]), or MPR-CDS.  An Attacker can disrupt the RSS algorithm,
   by degrading it to classical flooding, or by "masking" certain part
   of the routers from the multicasting domain.  The attacks to RSS
   algorithms are illustrated in Section 5.

4.  Threats to Duplicate Packet Detection

   Duplicate Packet Detection (DPD) is required for packet dissemination
   in MANET because the packets may be transmitted via the same physical
   interface as the one over which they were received.  A router may
   also receive multiple copies of the same packets from different
   neighbors.  DPD is thus used to check if an incoming packet has been
   received or not.

   DPD is achieved by a router maintaining a record of recently
   processed multicast packets, and comparing later received multicast
   herewith.  A duplicate packet detected is silently dropped, and is
   not inserted into the forwarding path of that router, nor is it
   delivered to an application.  DPD, as proposed by SMF, supports both
   IPv4 and IPv6 and for each suggests two duplicate packet detection
   mechanisms: 1) header content identification-based DPD (I-DPD), using
   packet headers, in combination with flow state, to estimate temporal
   uniqueness of a packet, and 2) hash-based DPD (H-DPD), employing
   hashing of selected header fields and payload for the same effect.

   As they are distinct mechanisms, the threats to I-DPD and H-DPD are
   discussed separately.

4.1.  Threats to Identification-based Duplicate Packet Detection

   I-DPD uses a specific DPD identifier in the packet header to identify
   a packet.  By default, such packet identification is not provided by
   the IP packet header (for both IPv4 and IPv6).  Therefore, additional
   identification header, such as the fragment header, a hop-by-hop
   header option, or IPSec sequencing, must be employed in order to
   support I-DPD.  The uniqueness of a packet can then be identified by
   the [source IP address] of the packet originator, and the [sequence
   number] (from the fragment header, hop-by-hop header option, or
   IPsec).  By doing so, each intermediate router can keep a record of
   recently received packet, and determine the coming packet has been
   received or not.

4.1.1.  Pre-activation Attacks (Pre-Play)

   In a wireless environment, or across any other shared channel, a
   compromised SMF router can perceive the identification tuple [source
   IP, sequence number] of a packet.  If sequence number progression is
   predictable, then it is trivial to generate aand and inject invalid
   packets with "future" identification information into the network.
   If these invalid packets arrive before the legitimate packets that
   they're spoofing, the latter will be treated as a duplicates and
   discarded.  This can prevent multicast packets from reaching parts of
   the network.

   Figure 1 gives an example of pre-activation attack.  A, B, and C are
   legitimate SMF routers, and X is the compromised SMF router.  The
   line between the routers presents the packet forwarding.  Router A is
   the source and originates a multicast packet with sequence number n.
   When router X receives the packet, it generates an invalid packet
   with the the source address of A, and sequence number n.  If the
   invalid packet arrives at router C before the forwarding of router B,
   the valid packet will be dropped by C as duplicate packet.  In a
   wireless environment, jitter is commonly used to avoid systematic
   collisions at MAC layer [RFC5148], thus an attacker can increase the
   probability that its invalid packets arrive first by retransmitting
   them without jittering.

                                | X |
                              --'---' __
       packet with seq=n     /          \  invalid packet with seq=n
                            /            \
                        .---.              .---.
                        | A |              | C |
                        '---'              '---'
       packet with seq=n    \    .---.   /
                             \-- | B |__/  valid packet with seq=n

                                 Figure 1

4.1.2.  De-activation Attacks (Sequence Number wrangling)

   A compromised SMF router can also seek to de-activate DPD, by
   modifying the sequence number in packets that it forwards.  Thus,
   routers will not be able to detect an actual duplicate packet as a
   duplicate - rather, they will treat them as new packets, i.e.,
   process and forward them.  This is similar to DoS attack.  The
   consequence of this attack is an increased channel load, the origin
   of which appears to be a router other than the compromised SMF

   Given the topology shown in Figure 1, on receiving packet with seq=n,
   the attacker X can forward the packet with modified sequence number
   n+i.  This has two consequences: firstly, router C will not be able
   to detect the packet forwarded by X is a duplicate packet; secondly,
   the consequent packet with seq=n+i generated by router A probably
   will be treated as duplicate packet, and dropped by router C.

4.2.  Threats to Hash-based Duplicate Packet Detection

   When it is not feasible to have explicit sequence numbers in packet
   headers, hash-based DPD can be used.  A hash of the non-mutable
   fields in the header of and the data payload can be generated, and
   recorded at the intermediate routers.  A packet can thus be uniquely
   identified by the source IP address of the packet, and its hash-

   The hash algorithm used by SMF is being applied only to provide a
   reduced probability of collision and is not being used for
   cryptographic or authentication purposes.  Consequently, a digest
   collision is still possible.  In case the source router or gateway
   identifies that it recently has generated or injected a packet with
   the same hash-value, it inserts a "Hash-Assist Value (HAV)" IPv6
   header option into the packet, such that calculating the hash also
   over this HAV will render the resulting value unique.

4.2.1.  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.

   One possible replay attack is based on the Time-to-Live (TTL, for
   IPv4) or hop limit (for IPv6) field.  As routers only forward packets
   with TTL > 1, a compromised SMF router can forward an otherwise valid
   packet, while drastically reducing the TTL hereof.  This will inhibit
   recipient routers from later forwarding the same multicast packet,
   even if received with a different TTL - essentially a compromised SMF
   router thus can instruct its neighbors to block forwarding of valid
   multicast packets.

   For example, given the example in Figure 2, router A forwards a
   multicast packet with a TTL of 64 to the network.  A, B, and C are
   legitimate SMF routers, and X is the compromised SMF router.  Router
   X forwards the packet without jittering, and reduces the TTL to 1.
   Router C thus records the hash value of the packets, but stops
   forwarding it to the next hops because of the TTL value.  When the
   same packet with normal TTL value (63 in this case) arrives from
   router B, it will be discarded as duplicate packet.

                                    | X |
                                  --'---' __
           packet with TTL=64    /          \  packet with TTL=1
                                /            \
                            .---.              .---.
                            | A |              | C |
                            '---'              '---'
           packet with TTL=64   \    .---.   /
                                 \-- | B |__/  packet with TTL=63

                                 Figure 2

   As the TTL of a packet is intended to be manipulated by
   intermediaries forwarding it, classic methods such as integrity check
   values (e.g., digital signatures) are typically calculated with
   setting TTL fields to some pre-determined value (e.g., 0) - such is
   for example the case for IPsec Authentication Headers - rendering
   such an attack more difficult to both detect and counter.  If the
   compromised SMF router has access to a "wormhole" through the network
   (a directional antenna, a tunnel to a collaborator or a wired
   connection, allowing it to bridge parts of a network otherwise
   distant) it can make sure that the packets with such an artificially
   reduced TTL arrive before their unmodified counterparts.

4.2.2.  Attack on Hash-Assistant Value

   The HAV header is helpful when a digest collision happens.  However,
   it also introduces a potential vulnerability.  As the HAV option is
   only added when the source or the ingressing SMF router detects that
   the coming packet has digest collision with previously generated
   packets, it actually can be regarded as a "flag" of potential digest
   collision.  A compromised SMF router can discover the HAV header, and
   be able to conclude a hash collision is possible if the HAV header is
   removed.  By doing so, other SMF routers receiving the modified
   packet will be treated as duplicate packet, and be dropped because it
   has the same hash value with precedent packet.

   In the example of Figure Figure 2, 3, Router A and B are legitimate SMF
   routers, X is a compromised SMF router.  A generate two packets P1
   and P2, with the same hash value h(P1)=h(P2)=x.  Based on SMF
   specification, a hash-assistant value (HAV) is added to the latter
   packet P2, so that h(P2+HAV)=x', to avoid digest collision.  When the
   attacker X detects the HAV of P2, it is able to conclude that a
   collision is possible by removing the HAV header.  By doing so,
   packet P2 will be treated as duplicate packet by router B, and be

              P2            P1                P2         P1
   .---.  h(P2+HAV)=x'    h(P1)=x    .---.  h(P2)=x     h(P1)=x    .---.
   | A |---------------------------> | X | ----------------------> | B |
   `---'                             `---'                         `---'

                                 Figure 2 3

5.  Threats to Relay Set Selection

   A framework for RSS mechanism, rather than a specific RSS algorithm
   is provided by SMF.  It is normally achieved by distributed
   algorithms that can dynamically generate a topological Connected
   Dominating Set based on 1-hop and 2-hop neighborhood information.  In
   this section, the common threats to the RSS framework are first
   discussed.  Then the three commonly used algorithms: Essential
   Connection Dominating Set (E-CDS) algorithm, Source-based Multipoint
   Relay (S-MPR) and Multipoint Relay Connected Dominating Set (MPR-CDS)
   are analyzed.

5.1.  Relay Set Selection Common Threats

   The common threats to RSS algorithms, including Denial of Service
   attack, eavesdropping, message timing attack and broadcast storm have
   been discussed in [RFC7186].

5.2.  Threats to E-CDS Algorithm

   The "Essential Connected Dominating Set" (E-CDS) algorithm [RFC5614]
   forms a single CDS mesh for the SMF operating region.  It requires
   2-hop neighborhood information (the identify of the neighbors, the
   link to the neighbors and neighbors' priority information) collected
   through NHDP or another process.

   An SMF Router select itself as a relay, if:

   o  The SMF Router has a higher priority than all of its symmetric
      neighbors, or

   o  There does not exist a path from the neighbor with largest
      priority to any other neighbor, via neighbors with greater

   A Compromised SMF Router can disrupt the E-CDS algorithm by link
   spoofing or identity spoofing.

5.2.1.  Link Spoofing

   Link spoofing implies that a Compromised SMF Router advertises non-
   existing links to another router (present in the network or not).

   A Compromised SMF Router can declare itself with high route priority,
   and spoofs the links to as many Legitimate SMF Routers as possible to
   declare high connectivity.  By doing so, it can prevent Legitimate
   SMF Routers from self-selecting as relays.  As the "super" relay in
   the network, the Compromised SMF Router can manipulate the traffic
   relayed by it.

5.2.2.  Identity Spoofing

   Identity spoofing implies that a compromised SMF router determines
   and makes use of the identity of other legitimate routers, without
   being authorised to do so.  The identity of other routers can be
   obtained by overhearing the control messages or source/destination
   address from datagram.  The compromised SMF router can then generate
   control or datagram traffic, pretending to be a legitimate router.

   Because E-CDS self-selection is based on the router priority value, a
   compromised SMF router can spoof the identity of other legitimate
   routers, and declares a different router priority value.  If it
   declares a higher priority of a spoofed router, it can prevent other
   routers from selecting themselves as relays.  On the other hand, if
   the compromised router declares lower priority of a spoofed router,
   it can enforces other routers to selecting themselves as relays, to
   degrade the multicast forwarding to classical flooding.

5.3.  Threats to S-MPR Algorithm

   The source-based multipoint relay (S-MPR) set selection algorithm
   enables individual routers, using 2-hop topology information, to
   select relays from their set of neighboring routers.  MPRs are
   selected so that forwarding to the router's complete 2-hop neighbor
   set is covered.

   An SMF router forwards a multicast packet if and only if:

   o  the packet is not received before, and

   o  the neighbor from which the packet was received has selected the
      router as MPR.

   Because MPR calculation is based on the willingness declared by the
   SMF routers, and the connectivity of the routers, it can be disrupted
   by both link spoofing and identity spoofing.  The threats and its
   impacts have been illustrated in section 5.1 of [RFC7186].

5.4.  Threats to MPR-CDS Algorithm

   MPR-CDS is a derivative from S-MPR.  The main difference between
   S-MPR and MPR-CDS is that while S-MPR forms a different broadcast
   tree for each source in the network, MPR-CDS forms a unique broadcast
   tree for all sources in the network.

   As MPR-CDS combines E-CDS and S-MPR, the vulnerabilities of E-CDS and
   S-MPR that discussed in Section 5.2 and Section 5.3 apply to MPR-CDS

6.  Future Work

   Neither [RFC6621] nor this document propose mechanisms to secure the
   SMF protocol.  However, this section aims at discussing possibilities
   to secure the protocol in the future and driving new work by
   suggesting which threats discussed in the previous sections could be

   If NHDP is used as the neighborhood discovery protocol, [RFC7183] is
   recommended to be implemented to enable integrity protection to NHDP,
   which can help mitigating the threats related to identity spoofing
   through the exchange of HELLO messages.

   [RFC7183] provides certain protection against identity spoofing by
   admitting only trusted routers to the network using Integrity Check
   Values (ICVs) in HELLO messages.  However, using ICVs does not
   address the problem of compromised routers that can generate valid
   ICVs.  Detecting such compromised routers could be studied in new
   work.  [RFC7183] mandates implementation of a security mechanism that
   is based on shared keys and makes excluding single compromised
   routers difficult.  Work could be done to facilitate revocation
   mechanisms in certain MANET use cases where routers have sufficient
   capabilities to support asymmetric keys.

   As [RFC7183] does not protect the integrity of the datagram, and no
   mechanism is specified to do that for SMF yet, the duplicate packet
   detection is still vulnerable to the threats introduced in Section 4.

   If pre-activation/de-activation attacks and attack on hash-assistant
   value are to be mitigated, a datagram-level integrity protection
   mechanism is desired, by taking consideration of the identity field
   or hash-assistant value.  However, this would not be helpful for the
   attacks on the TTL (or hop limit for IPv6) field, because the mutable
   fields are generally not considered when ICV is calculated.

7.  Security Considerations

   This document does not specify a protocol or a procedure.  The
   document, however, reflects on security considerations for SMF for
   packet dissemination in MANETs.


8.  IANA Considerations

   This document contains no actions for IANA.


9.  References


9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5614]  Ogier, R. and P. Spagnolo, "Mobile Ad Hoc Network (MANET)
              Extension of OSPF Using Connected Dominating Set (CDS)
              Flooding", RFC 5614, August 2009.

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

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


9.2.  Informative References

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

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

   [RFC6130]  Clausen, T., Dean, J., and C. Dearlove, "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.


   [RFC7183]  Herberg, U., Clausen, T., and C. Dearlove, "Integrity
              Check Value C., and Timestamp TLV Definitions T. Clausen, "Integrity
              Protection for Mobile Ad
              Hoc Networks (MANETs)", the Neighborhood Discovery Protocol (NHDP)
              and Optimized Link State Routing Protocol Version 2
              (OLSRv2)", RFC 7182, 7183, April 2014.

Authors' Addresses

   Jiazi Yi
   LIX, Ecole Polytechnique
   91128 Palaiseau Cedex,

   Phone: +33 1 77 57 80 85

   Thomas Heide Clausen
   LIX, Ecole Polytechnique
   91128 Palaiseau Cedex,

   Phone: +33 6 6058 9349
   Ulrich Herberg
   Fujitsu Laboratories of America
   1240 E Arques Ave
   Sunnyvale, CA 94085