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
AbstractThis 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.
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. 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 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
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. 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. 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
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
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. 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.
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.
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. 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.
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. [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>. [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>.
[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>.