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Analysis of Bidirectional Forwarding Detection (BFD) Security According to KARP Design Guide

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 7492.
Authors Manav Bhatia , Dacheng Zhang , Mahesh Jethanandani
Last updated 2013-03-11
Replaces draft-bhatia-zhang-karp-bfd-analysis
RFC stream Internet Engineering Task Force (IETF)
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Network Working Group                                        M.B. Bhatia
Internet-Draft                                            Alcatel-Lucent
Intended status: Informational                                  D. Zhang
Expires: September 13, 2013                Huawei Technologies co., LTD.
                                                       M.J. Jethanandani
                                                       Ciena Corporation
                                                          March 12, 2013

Analysis of Bidirectional Forwarding Detection (BFD) Security According
                          to KARP Design Guide


   This document analyzes the Bidirectional Forwarding Detection
   protocol (BFD) according to the guidelines set forth in section 4.2
   of KARP Design Guidelines [RFC6518].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 13, 2013.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

1.  Introduction

   This document performs a gap analysis of the current state of
   Bidirectional Forwarding Detection [RFC5880] according to the
   requirements of KARP Design Guidelines [RFC6518].  Previously, the
   OPSEC working group has provided an analysis of cryptographic issues
   with BFD in Issues with Existing Cryptographic Protection Methods for
   Routing Protocols [RFC6039].

   The existing BFD specifications provide a basic security solution.
   Key ID is provided so that the key used in securing a packet can be
   changed on demand.  Two cryptographic algorithms (MD5 and SHA-1) are
   supported for integrity protection of the control packets; the
   algorithms are both demonstrated to be subject to collision attacks.
   Routing protocols like RIPv2 Cryptographic Authentication [RFC4822],
   IS-IS Generic Cryptographic Authentication [RFC5310] and OSPFv2 HMAC-
   SHA Cryptographic Authentication [RFC5709] have started to use BFD
   for liveliness check.  Moving the routing protocols to a stronger
   algorithm while using weaker algorithm for BFD would require the
   attacker to bring down BFD in order to bring down the routing
   protocol.  BFD therefore needs to match the routing protocols in its
   strength of algorithm.

   While BFD uses a non-decreasing per-packet sequence number to protect
   itself from intra-connection replay attacks, it still leaves the
   protocol vulnerable to the inter-session replay attacks.

1.1.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Requirements to Meet

   There are several requirements described in section 3 of The Threat
   Analysis and Requirements for Cryptographic Authentication of Routing
   Protocols' Transports [I-D.ietf-karp-threats-reqs] that BFD does not
   currently meet:

      Replay Protection: BFD provides an incomplete intra-session and no
      inter-session replay attack protection; this creates significant
      denial-of-service opportunities.

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      Strong Algorithms: the cryptographic algorithms adopted for
      message authentication in BFD are MD5 or SHA-1 based.  However,
      both algorithms are known to be vulnerable to collision attacks.
      BFD Generic Cryptographic Authentication
      [I-D.ietf-bfd-generic-crypto-auth] and Authenticating BFD using
      HMAC-SHA-2 procedures [I-D.ietf-bfd-hmac-sha] together propose a
      solution to support HMAC with the SHA-2 family of hash functions
      for BFD.

      DoS Attacks: BFD packets can be sent at millisecond intervals (the
      protocol uses timers at microsecond intervals).  When malicious
      packets are sent at short intervals, with the authentication bit
      set, it can cause a DoS attack.

   The remainder of this document explains the details of how these
   requirements fail to be met and proposes mechanisms for addressing

3.  Current State of Security Methods

   BFD [RFC5880] describes five authentication mechanisms for the
   integrity protection of BFD control packets: Simple Password, Keyed
   MD5 The MD5 Message-Digest Algorithm [RFC1321], Meticulous Keyed MD5,
   Keyed SHA-1 and Meticulous SHA-1.  In the simple password mechanism,
   every control packet is associated with a password transported in
   plain text; attacks eavesdropping the network traffic can easily
   learn the password and compromise the security of the corresponding
   BFD session.  In the Keyed MD5 and the Meticulous Keyed MD5
   mechanisms, BFD nodes use share secret keys to generate keyed MD5
   digests for control packets.  Similarly, in the Keyed SHA-1 and the
   Meticulous Keyed SHA-1 mechanisms, BFD nodes use shared secret keys
   to generate keyed SHA-1 digests for control packets.  Note that in
   the keyed authentication mechanisms, every BFD control packet is
   associated with a non-decreasing 32-bit sequence number to resist
   replay attacks.  In the Keyed MD5 and the Keyed SHA-1 mechanisms, the
   sequence member is only required to increase occasionally.  However,
   in the Meticulous Keyed MD5 and the Meticulous Keyed SHA-1
   mechanisms, the sequence member is required to monotonically increase
   with each successive packet.

   Additionally, limited key updating functionality is provided.  There
   is a Key ID in every authenticated BFD control packet, indicating the
   key used to hash the packet.  However, there is no mechanism
   described to provide a smooth key rollover that the BFD routers can
   use when moving from one key to the other.

   The BFD session timers are defined with the granularity of
   microseconds, and it is common in practice to send BFD packets at

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   millisecond intervals.  Since the cryptographic sequence number space
   is only 32 bits, a sequence number used in a BFD session may reach
   its maximum value and roll over within limited period.  For instance,
   if a sequence number is increased by one every 3.3 millisecond, then
   it will reach its maximum value in less than 24 weeks.  This can
   result in potential inter-session replay attacks especially when BFD
   uses the non-meticulous authentication modes.

   Note that when using authentication mechanisms, BFD requests the
   sequence of a received BFD packets drops with a limited range (3*
   Detection time multiplier).  Therefore, when meticulous
   authentication modes are used, a replayed BFD packet will be rejected
   if it cannot fit into a relatively short window (3 times of the
   detect interval of the session).  This introduces some difficulties
   for replaying packets.  However, in a non-meticulous authentication
   mode, such windows can be large as sequence numbers are only
   increased occasionally, thus making it easier to perform replay
   attacks .

   In a BFD session, each node needs to select a 32-bit discriminator to
   identify itself.  Therefore, a BFD session is identified by two
   discriminators.  If a node will randomly select a new discriminator
   for a new session and use authentication mechanism to secure the
   control packets, inter-session replay attacks can be mitigated to
   some extent.  However, in existing BFD demultiplexing mechanisms, the
   discriminators used in a new BFD session may be predictable.  In some
   deployment scenarios, the discriminators of BFD routers may be
   decided by the destination and source addresses.  So, if the sequence
   number of a BFD router rolls over for some reasons (e.g., reboot),
   the discriminators used to identify the new session will be identical
   to the ones used in the previous session.  This makes performing a
   reply attack relatively simple.

   BFD allows a mode called the echo mode.  Echo packets are not defined
   in the BFD specification, though they can keep the BFD session up.
   The format of the echo packet is local to the sending side and there
   are no guidelines on the properties of these packets beyond the
   choice of the source and destination addresses.  While the BFD
   specification recommends applying security mechanisms to prevent
   spoofing of these packets, there are no guidelines on what type of
   mechanisms are appropriate.

4.  Impacts of BFD Replays

   As discussed, BFD cannot meet the requirements of inter-session or
   intra-session replay protection.  This section discusses the impacts
   of BFD replays.

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   When cryptographic authentication mechanisms are adopted for BFD, a
   non-decreasing 32-bit long sequence number is used.  In the Keyed MD5
   and the Keyed SHA-1 mechanisms, the sequence member is not required
   to increase for every packet.  Therefore an attacker can keep
   replaying the packets with the latest sequence number until the
   sequence number is updated.  This issue is eliminated in the
   Meticulous Keyed MD5 and the Meticulous Keyed SHA-1 mechanisms.
   However, note that a sequence number may reach its maximum and be
   rolled over in a session.  In this case, without the support from a
   automatic key management mechanism, the BFD session will be
   vulnerable to replay attacks performed by sending the packets before
   the roll over of the sequence number.  For instance, an attacker can
   replay a packet with a sequence number which is larger than the
   current one.  If the replayed packet is accepted, the victim will
   reject the legal packets whose sequence members are less than the one
   in the replayed packet.  Therefore, the attacker can get a good
   chance to bring down the BFD session.

   Additionally, the BFD specification allows for the change of
   authentication state based on the state of a received packet.  For
   instance, according to BFD [RFC5880], if the state of a accepted
   packet is down, the receiver of the packet needs to transfer its
   state to down as well.  Therefore, an elaborately selected replayed
   packet can cause a serious denial-of-service attack.

   BFD does not provide any solution to deal with inter-session replay
   attacks.  If two subsequent BFD sessions adopt an identical
   discriminator pair and use the same cryptographic key to secure the
   control packets, it is intuitive to use a malicious authenticated
   packet (stored from the past session) to perform inter-connection
   replay attacks.

   Any security issues in the BFD echo mode will directly affect the BFD
   protocol and session states, and hence the network stability.  For
   instance, any replay attacks would be indistinguishable from normal
   forwarding of the tested router.  An attack would still cause a
   faulty link to be believed to be up, but there is little that can be
   done about it.  However, if the echo packets are guessable, it may be
   possible to spoof from an external source and cause BFD to believe
   that a one-way link is really bidirectional.  As a result, it is
   important that the echo packets contain random material that is also
   checked upon reception.

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5.  Impact of New Authentication Requirements

   BFD can be run in software or hardware.  Hardware implementations run
   BFD at a much smaller timeout, typically in the order of few
   milliseconds.  For instance with a timeout of 3.3 milliseconds, a BFD
   session is required to send or receive 3 packets every 10
   milliseconds.  Software implementations typically run with a timeout
   in hundreds of milliseconds.

   Additionally, it is not common to find hardware support for computing
   the authentication data for the BFD session in hardware or software.
   In the keyed MD5 and Keyed SHA-1 implementation where the sequence
   number does not increase with every packet, software can be used to
   compute the authentication data.  This is true if the time between
   increasing sequence number is long enough to compute the data in
   software.  The ability to compute the hash in software is difficult
   with Meticulous Keyed MD5 and Meticulous Keyed SHA-1 if the time
   interval between transmits or between receives is small.

   Implementors should assess the impact of authenticating BFD sessions
   on their platform.

6.  Considerations for improvement

   This section suggests changes that can be adopted to improve the
   protection of BFD.

   As mentioned in section 3, a 32 bit sequence number space can wrap
   around in less than 24 weeks when set for the minimum time interval
   of 3.3 milliseconds.  To prevent a replay attack the sequence number
   can be tied to notion of real time where part of the sequence number
   reflects say the UTC time.  A replay attack therefore can easily be
   detected.  However, it does require that the two stations exchanging
   BFD packets are synchorizied with respect to time.  Alternatively,
   the sequence number can be a nonce number generated using the shared
   key.  But nonce numbers will also run out in 24 weeks.

   Increasing the sequence number space to 64 bits makes the wrap around
   time be a little less than 2 million years.  Combined with nonce or
   part of the number reflecting real time would make replay attacks
   difficult if not impossible.

7.  IANA Considerations

   This document makes no request of IANA.

   Note to RFC Editor: this section may be removed on publication as an

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8.  Security Considerations

9.  Acknowledgements

   We would like to thank Alexander Vainshtein for his comments on this

10.  References

10.1.  Normative References

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              April 1992.

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

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, June 2010.

   [RFC6039]  Manral, V., Bhatia, M., Jaeggli, J., and R. White, "Issues
              with Existing Cryptographic Protection Methods for Routing
              Protocols", RFC 6039, October 2010.

10.2.  Informative References

              Bhatia, M., Manral, V., and D. Zhang, "BFD Generic
              Cryptographic Authentication", draft-ietf-bfd-generic-
              crypto-auth-03 (work in progress), October 2012.

              Zhang, D., Bhatia, M., and V. Manral, "Authenticating BFD
              using HMAC-SHA-2 procedures", draft-ietf-bfd-hmac-sha-02
              (work in progress), October 2012.

              Lebovitz, G., Bhatia, M., and B. Weis, "Keying and
              Authentication for Routing Protocols (KARP) Overview,
              Threats, and Requirements", draft-ietf-karp-threats-
              reqs-07 (work in progress), December 2012.

   [RFC4822]  Atkinson, R. and M. Fanto, "RIPv2 Cryptographic
              Authentication", RFC 4822, February 2007.

   [RFC5310]  Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
              and M. Fanto, "IS-IS Generic Cryptographic
              Authentication", RFC 5310, February 2009.

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   [RFC5709]  Bhatia, M., Manral, V., Fanto, M., White, R., Barnes, M.,
              Li, T., and R. Atkinson, "OSPFv2 HMAC-SHA Cryptographic
              Authentication", RFC 5709, October 2009.

   [RFC6518]  Lebovitz, G. and M. Bhatia, "Keying and Authentication for
              Routing Protocols (KARP) Design Guidelines", RFC 6518,
              February 2012.

Authors' Addresses

   Manav Bhatia


   Dacheng Zhang
   Huawei Technologies co., LTD.


   Mahesh Jethanandani
   Ciena Corporation
   1741 Technology Drive, #400
   San Jose, CA  95110

   Phone: 408.436.3313
   Fax:   408.436.5582

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