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rfc:rfc7492

Internet Engineering Task Force (IETF) M. Bhatia Request for Comments: 7492 Ionos Networks Category: Informational D. Zhang ISSN: 2070-1721 Huawei

                                                       M. Jethanandani
                                                     Ciena Corporation
                                                            March 2015
   Analysis of Bidirectional Forwarding Detection (BFD) Security

According to the Keying and Authentication for Routing Protocols (KARP)

                         Design Guidelines

Abstract

 This document analyzes the Bidirectional Forwarding Detection (BFD)
 protocol according to the guidelines set forth in Section 4.2 of RFC
 6518, "Keying and Authentication for Routing Protocols (KARP) Design
 Guidelines".

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/rfc7492.

Bhatia, et al. Informational [Page 1] RFC 7492 BFD Gap Analysis March 2015

Copyright Notice

 Copyright (c) 2015 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
 to this document.  Code Components extracted from this document must
 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  . . . . . . . . . . . . . . . . . . . . . . . .   2
 2.  Requirements to Meet  . . . . . . . . . . . . . . . . . . . .   3
 3.  Current State of Security Methods . . . . . . . . . . . . . .   3
 4.  Impacts of BFD Replays  . . . . . . . . . . . . . . . . . . .   5
 5.  Impact of New Authentication Requirements . . . . . . . . . .   6
 6.  Considerations for Improvement  . . . . . . . . . . . . . . .   7
 7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
 8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
   8.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
   8.2.  Informative References  . . . . . . . . . . . . . . . . .   8
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   9
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

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 checks.  Moving the routing

Bhatia, et al. Informational [Page 2] RFC 7492 BFD Gap Analysis March 2015

 protocols to a stronger algorithm while using a weaker algorithm for
 BFD would allow 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.

2. Requirements to Meet

 There are several requirements described in Section 4 of [RFC6862]
 that BFD, as defined in BFD [RFC5880], 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.
    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" [BFD-CRYPTO] and
    "Authenticating BFD using HMAC-SHA-2 procedures" [BFD-HMAC]
    together propose a solution to support Hashed Message
    Authentication Code (HMAC) with the SHA-2 family of hash functions
    for BFD.
    Preventing 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.  There is
    currently no lightweight mechanism within BFD to address this
    issue and is one of the reasons BFD authentication is still not
    widely deployed in the field.
 The remainder of this document explains the details of how these
 requirements fail to be met and proposes mechanisms for addressing
 them.

3. Current State of Security Methods

 BFD [RFC5880] describes five authentication mechanisms for the
 integrity protection of BFD control packets: Simple Password, Keyed
 MD5 [RFC1321], Meticulous Keyed MD5, Keyed SHA-1, and Meticulous
 Keyed 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

Bhatia, et al. Informational [Page 3] RFC 7492 BFD Gap Analysis March 2015

 Keyed MD5 and the Meticulous Keyed MD5 mechanisms, BFD nodes use
 shared 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 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
 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 a limited period.  For
 instance, if a sequence number is increased by one every 3.3
 milliseconds, 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 drops all packets
 that fall outside the limited range (3 times the 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 the detection 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 randomly selects a new discriminator for a
 new session and uses authentication mechanisms 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 reason (e.g., reboot), the

Bhatia, et al. Informational [Page 4] RFC 7492 BFD Gap Analysis March 2015

 discriminators used to identify the new session will be identical to
 the ones used in the previous session.  This makes performing a
 replay 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.
 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 that 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.  This kind of attack assumes
 that the attacker has access to the link when the BFD session is on a
 point-to-point link or can inject packets for a BFD session with
 multiple hops.
 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 an accepted
 packet is down, the receiver of the packet needs to transfer its
 state to down as well.  Therefore, a carefully selected replayed
 packet can cause a serious denial-of-service attack.

Bhatia, et al. Informational [Page 5] RFC 7492 BFD Gap Analysis March 2015

 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 interconnection
 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.

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
 the 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.  The
 computation problem becomes worse if hundred or thousands of sessions
 require the hash to be recomputed every few milliseconds.
 Smaller and cheaper boxes that have to support a few hundred BFD
 sessions are boxes that also use a slower CPU.  The CPU is used for
 running the entire control plane software in addition to supporting
 the BFD sessions.  As a general rule, no more than 40-45% of the CPU
 can be dedicated towards supporting BFD.  Adding computation of the
 hash for every BFD session can easily cause the CPU to exceed the
 40-45% limit even with a few tens of sessions.  On higher-end boxes
 with faster and more CPU cores, the expectation is that the number of

Bhatia, et al. Informational [Page 6] RFC 7492 BFD Gap Analysis March 2015

 sessions that need to be supported are in the thousands, but the
 number of BFD sessions with authentication that CPU can support is
 still in the hundreds.
 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.
 The security risks brought by SHA-1 and MD5 have been well
 understood.  However, when using a stronger digest algorithm, e.g.,
 SHA-2, the imposed computing overhead will seriously affect the
 performance of BFD implementation.  In order to make the trade-off
 between the strong algorithm requirement and the imposed overhead,
 Galois Message Authentication Code (GMAC) can be a candidate option.
 This algorithm is relatively effective and has been supported by
 IPsec for data origin authentication.  More detailed information can
 be found in "The Use of Galois Message Authentication Code (GMAC) in
 IPsec ESP and AH" [RFC4543].
 There has been some hallway conversation around the idea of using BFD
 cryptographic authentication only when some data in the BFD payload
 changes.  The other BFD packets can be transmitted and received
 without authentication enabled.  The bulk of the BFD packets that are
 transmitted and received have no state change associated with them.
 Limiting authentication to BFD packets that affect a BFD session
 state allows for more sessions to be supported for authentication.
 This change can significantly help the routers since they don't have
 to compute and verify the authentication digest for the BFD packets
 coming at the millisecond intervals.  This proposal needs some more
 discussion in the BFD working group and is certainly a direction that
 BFD could look at.

7. Security Considerations

 This document discusses vulnerabilities in the existing BFD protocol
 and suggests possible mitigations.
 In analyzing the improvements for BFD, the ability to repel a replay
 attack is discussed.  For example, increasing the sequence number to
 a 64-bit value makes the wrap-around time much longer, and a replay
 attack can be easily prevented.

Bhatia, et al. Informational [Page 7] RFC 7492 BFD Gap Analysis March 2015

 Mindful of the impact that stronger algorithms can have on the
 performance of BFD, the document suggests GMAC as a possible
 candidate for MAC function.

8. References

8.1. Normative References

 [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
            April 1992, <http://www.rfc-editor.org/info/rfc1321>.
 [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
            (BFD)", RFC 5880, June 2010,
            <http://www.rfc-editor.org/info/rfc5880>.
 [RFC6039]  Manral, V., Bhatia, M., Jaeggli, J., and R. White, "Issues
            with Existing Cryptographic Protection Methods for Routing
            Protocols", RFC 6039, October 2010,
            <http://www.rfc-editor.org/info/rfc6039>.

8.2. Informative References

 [BFD-CRYPTO]
            Bhatia, M., Manral, V., Zhang, D., and M. Jethanandani,
            "BFD Generic Cryptographic Authentication", Work in
            Progress, draft-ietf-bfd-generic-crypto-auth-06, April
            2014.
 [BFD-HMAC] Zhang, D., Bhatia, M., Manral, V., and M. Jethanandani,
            "Authenticating BFD using HMAC-SHA-2 procedures", Work in
            Progress, draft-ietf-bfd-hmac-sha-05, July 2014.
 [RFC4543]  McGrew, D. and J. Viega, "The Use of Galois Message
            Authentication Code (GMAC) in IPsec ESP and AH", RFC 4543,
            May 2006, <http://www.rfc-editor.org/info/rfc4543>.
 [RFC4822]  Atkinson, R. and M. Fanto, "RIPv2 Cryptographic
            Authentication", RFC 4822, February 2007,
            <http://www.rfc-editor.org/info/rfc4822>.
 [RFC5310]  Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
            and M. Fanto, "IS-IS Generic Cryptographic
            Authentication", RFC 5310, February 2009,
            <http://www.rfc-editor.org/info/rfc5310>.

Bhatia, et al. Informational [Page 8] RFC 7492 BFD Gap Analysis March 2015

 [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,
            <http://www.rfc-editor.org/info/rfc5709>.
 [RFC6518]  Lebovitz, G. and M. Bhatia, "Keying and Authentication for
            Routing Protocols (KARP) Design Guidelines", RFC 6518,
            February 2012, <http://www.rfc-editor.org/info/rfc6518>.
 [RFC6862]  Lebovitz, G., Bhatia, M., and B. Weis, "Keying and
            Authentication for Routing Protocols (KARP) Overview,
            Threats, and Requirements", RFC 6862, March 2013,
            <http://www.rfc-editor.org/info/rfc6862>.

Acknowledgements

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

Authors' Addresses

 Manav Bhatia
 Ionos Networks
 Bangalore
 India
 EMail: manav@ionosnetworks.com
 Dacheng Zhang
 Huawei
 EMail: dacheng.zhang@gmail.com
 Mahesh Jethanandani
 Ciena Corporation
 3939 North 1st Street
 San Jose, CA  95134
 United States
 Phone: 408.904.2160
 Fax:   408.436.5582
 EMail: mjethanandani@gmail.com

Bhatia, et al. Informational [Page 9]

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