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Versions: 00 01 02 draft-ietf-karp-ospf-analysis

Network Working Group                                         S. Hartman
Internet-Draft                                         Painless Security
Intended status: Informational                                  D. Zhang
Expires: June 19, 2011                                            Huawei
                                                       December 16, 2010

        Analysis of OSPF Security According to KARP Design Guide


   This document analyzes OSPFv2 and OSPFv3 according to the guidelines
   set forth in section 4.2 of draft-ietf-karp-design-guide.

Status of this Memo

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   This Internet-Draft will expire on June 19, 2011.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Requirements to Meet . . . . . . . . . . . . . . . . . . .  3
     1.2.  Requirements notation  . . . . . . . . . . . . . . . . . .  4
   2.  Current State  . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.1.  OSPFv2 . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.2.  OSPFv3 . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  Impacts of OSPF Replays  . . . . . . . . . . . . . . . . . . .  8
   4.  Gap Analysis and Specific Requirements . . . . . . . . . . . . 10
   5.  Solution Work  . . . . . . . . . . . . . . . . . . . . . . . . 11
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 13
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 14
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 14
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16

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1.  Introduction

   This document performs an initial analysis of the current states of
   OSPFv2 and OSPFv3 according to the requirements of
   [I-D.ietf-karp-design-guide].  This draft is built upon several
   previous analysis efforts in routing security.  The OPSEC working
   group puts together an analysis of cryptographic issues with routing
   protocols in[RFC6039] .  Earlier, the RPSEC working group puts
   together a detailed analysis of OSPF vulnerabilities in
   [I-D.ietf-rpsec-ospf-vuln] .

   OSPF meets many of the requirements expected from a manually keyed
   routing protocol.  Integrity protection is provided with modern
   cryptographic algorithms.  Algorithm agility is provided: the
   algorithm can be changed as part of re-keying an interface or peer.
   Intra-connection re-keying is provided by the specifications,
   although apparently some implementations have trouble with this in
   practice.  OSPFv2 security does not interfere with prioritization of

   However, some gaps remain between the current state and the
   requirements for manually keyed routing security expressed in
   [I-D.ietf-karp-threats-reqs] .  This document explores these gaps and
   proposes directions for addressing the gaps.

1.1.  Requirements to Meet

   The requirements described in section 3 of
   [I-D.ietf-karp-threats-reqs] that OSPF does not currently meet

   Secure Simple PSKs:  Today, OSPF directly uses the key as specified.
      Related key attacks such as those described in section 4.1 of
      [I-D.hartman-karp-ops-model] are possible.

   Replay Protection:  OSPFv3 has no replay protection at all.  Although
      OSPFv2 has most of the mechanisms necessary for intra-connection
      replay protection, it does not securely identify the neighbor with
      whom replay protection state is associated in all cases.  This
      weakness can be used to create significant denial-of-service
      issues using intra-connection replays.  In addition, OSPFv2
      provides no inter-connection replay protection; this creates
      significant denial-of-service opportunities.

   Packet Prioritization:  OSPFv3 uses IPsec to secure the packet
      transportation.  This complicates implementations that wish to
      process certain packets such as hellos and acknowledgements above
      others.  In addition, if IPsec replay mechanisms were used,

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      packets would need to be processed at least by IPsec even if they
      were low priority.

   Neighbor Identification:  In some cases, OSPF identifies a neighbor
      based on the IP address.  This is never protected with OSPFv2 and
      is not typically protected with OSPFv3.

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

1.2.  Requirements notation

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

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2.  Current State

   This section describes the security mechanisms built into OSPFv2 and
   OSPFv3.  There are two goals to this section.  First, this section
   gives a brief explanation of the OSPF security mechanisms to those
   familiar with connectionless integrity mechanisms but not with OSPF.
   Second, this section explains the background necessary to understand
   how OSPF fails to meet some of the requirements proposed for routing

2.1.  OSPFv2

   Appendix D of [RFC2328] describes the basic procedure for
   cryptographic authentication in OSPFv2.  An authentication data field
   in the OSPF packet header contains a key ID, the length of the
   authentication data and a sequence number.  A message authentication
   code (MAC) is appended to the OSPF packet.  This code protects all
   fields of the packet including the sequence number but not the IP

   RFC 2328 defined the use of a keyed-MD5 MAC.  While MD5 has not been
   broken as a MAC, it is not the algorithm recommended any more.

   However, [RFC5709] adds support for the SHA [FIPS180] family of
   hashes to OSPFv2.  The cryptographic authentication described in RFC
   5709 meets modern standards for per-packet integrity protection.
   Thus, OSPFv2 meets the requirement for strong algorithms.  Since
   multiple algorithms are defined and a new algorithm can be selected
   with each key, OSPFv2 meets the requirement for algorithm agility.
   In order to provide cryptographic algorithms believed to have a
   relatively long useful life, RFC 5709 mandates the support for SHA-2
   rather than SHA-1.

   Above security methodologies provide integrity protection on each
   packet.  In addition, limited replay detection is provided.  RFC2328
   [RFC2328] describes the adoption of non-decreasing sequence numbers.
   Once a router has increased its sequence number, an attacker cannot
   replay an old packet without being detected.  Unfortunately, sequence
   numbers are not required to increase for each packet.  For instance,
   because existing OSPF security solutions do not specify how to set
   the sequence number, it is possible that some implementation use,
   e.g., "seconds since reboot" as their sequence numbers.  The sequence
   numbers is thus only increased by every second.  Also, no mechanism
   is provided to deal with the loss of anti-replay state.  Therefore,
   if sequence numbers are reused when a router reboots, inter-
   connection replays are quite straight forward.  Also, because the IP
   header is not protected, the sequence number may not be associated
   with the right neighbor; this opens up opportunities for outsiders to

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   perform replay attacks.  See Section 3 for detailed analysis of these

   The mechanism presented in [RFC5709] provides good support for key
   rollover.  There is a key ID; in addition mechanisms are described
   for managing key lifetimes and starting the use of a new key in an
   orderly manner.  Performing orderly key rollover requires that
   implementations support accepting a new key for received packets
   before using that key to generate packets.  Section D.3 of RFC 2328
   requires this support in the form of four configurable lifetimes for
   each key: two lifetimes control the beginning and ending period for
   acceptance while two lifetimes control the beginning and ending
   period for generation.  This provides a superset of the functionality
   in the key table [I-D.ietf-karp-crypto-key-table] regarding lifetime.

   The OSPFv2 replay mechanism does not handle packet priorities as
   described above.  Assume in an implementation, packets are processed
   out-of-order.  Then if the process of a newly accepted packet results
   in the increase of the sequence number, the packets processed later
   will be discarded.

2.2.  OSPFv3

   RFC 4552 [RFC4552] describes how the authentication header and
   encapsulating security payload mechanism can be used to protect
   OSPFv3 packets.  This mechanism provides per-packet integrity and
   optional confidentiality using a wide variety of cryptographic
   algorithms.  Because OSPF uses multicast traffic, only manual key
   management is supported.  This mechanism meets requirements related
   to algorithm selection and agility.

   The Security Parameter Index (SPI) provides an identifier for the
   security association.  This along with other IPsec facilities
   provides a mechanism for moving from one key to another, meeting the
   key rollover requirements.

   Because manual keying is used, no replay protection is provided for
   OSPFv3.  Thus the intra-connection and inter-connection replay
   requirements are not met.

   There is another serious problem with the OSPFv3 security: rather
   than being integrated into OSPF, it is based on IPsec.  In practice,
   this has lead to deployment problems.

   OSPF implementations generally prioritize packets in order to
   minimize disruption when router resources such as CPU or memory
   experience contention.  When IPsec is used with OSPFv3, the offset of
   the packet type, which is used to prioritize packets, depends on what

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   integrity transform is used.  For this reason, prioritizing packets
   may be more complex for OSPFv3.  One approach is to establish per-SPI
   filters to find the packet type and act accordingly.

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3.  Impacts of OSPF Replays

   As discussed, neither version of OSPF meets all the requirements of
   inter-connection or intra-connection replay protection.  This section
   discusses the impacts of OSPF replays.

   In OSPFv2, two facilities limit the scope of replay attacks.  First,
   when cryptographic authentication is used, each packet includes a
   sequence number that is non-decreasing.  In the current
   specifications, the sequence number is remembered as part of an
   adjacency.  Therefore, if an attacker can cause an adjacency to go
   down, the associated replay state will be lost.  Database Description
   packets also include a per-LSA sequence number as a part of the
   information being flooded.  Even if a packet is replayed, the per-LSA
   sequence number will prevent an old LSA from being installed.  Unlike
   the per-packet sequence number, the per-LSA sequence number must
   increase when an LSA is changed.  As a result, replays cannot be used
   to install old routing information.

   While the LSA sequence number provides some defense, there are a
   number of attacks that are possible because of a per-packet replay.
   The RPSEC analysis [I-D.ietf-rpsec-ospf-vuln] describes a number of
   attacks which take advantage of per-packet replay issues.  Amongst
   those attacks, the attacks against Hello packets can be the most
   serious, which may cause an adjacency to fail.  Other attacks may
   cause excessive flooding or excessive use of CPU.

   Another serious attack concerns Database Description packets.  In
   addition to the per-packet sequence number that is part of
   cryptographic authentication for OSPFv2 and the per-LSA sequence
   numbers, Database Description packets also include a Database
   Description sequence number.  If a Database Description packet with
   the incorrect sequence number is received, the database exchange
   process will be restarted.

   The per-packet OSPFv2 sequence number can be used to reduce the
   window in which a replay is valid.  A receiver will harmlessly reject
   a packet whose per-packet sequence number is older than the one most
   recently received from a neighbor.  Replaying the most recent packet
   from a neighbor does not appear to create problems.  So, if the per-
   packet sequence number is incremented on every packet sent, replay
   attacks should not disrupt OSPFv2.  Unfortunately, OSPFv2 does not
   have a procedure for dealing with sequence numbers reaching the
   maximum age.  It may be possible to figure out a set of rules
   sufficient to disrupt the damage of packet replays while minimizing
   the use of the sequence number space.

   As mentioned previously, when an adjacency is dropped, the associated

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   replay state is lost.  So, after rebooting or when all adjacencies
   are lost, a router may allow its sequence number to decrease.  In
   this case, an attacker can cause significant damage by replaying a
   packet captured before the sequence number decreases.  After the
   replayed packet is accepted, the sequence number will be updated.
   However, at this time the legitimate sender will be using a lower
   sequence number.  As a consequence, legitimate packets will be
   rejected.  A similar attack is possible in cases where OSPF
   identifies a neighbor based on source address.  An attacker can
   change the source address of a captured packet and replay it.  If the
   attacker causes a replay from a neighbor with a high sequence number
   to appear to be from a low sequence number neighbor, the connectivity
   with that neighbor will be disrupted until the adjacency fails.

   OSPFv3 lacks the per-packet sequence number but has the per-LSA
   sequence number.  As such, OSPFv3 has little defense against denial
   of service attacks that exploit replay.

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4.  Gap Analysis and Specific Requirements

   The design guide[I-D.ietf-karp-design-guide] requires each design
   team to enumerate a set of requirements for the routing protocol.
   The only concerns identified with OSPF are areas where it fails to
   meet general requirements outlined in the threats and requirements
   document.  This section explains how some of these general
   requirements map specifically onto the OSPF protocol and enumerates
   the specific gaps that need to be addressed.

   There is a general requirement for inter-connection replay
   protection.  In the context of OSPF, this means that if an adjacency
   goes down between two neighbors and later is re-established,
   replaying packets from before the adjacency went down cannot disrupt
   the adjacency.  In the context of OSPF, intra-connection replay
   protection means that replaying a packet cannot prevent an adjacency
   from forming or disrupt an adjacency.  Meeting the requirements for
   intra-connection and inter-connection replay protection is a
   significant gap between the optimal state and where OSPF is today.

   Since OSPF uses fields in the IP header, the general requirement to
   protect the IP header and handle neighbor identification applies.
   This is another gap that needs to be addressed.  Because the replay
   protection will depend on neighbor identification, the replay
   protection cannot be adequately addressed without handling this issue
   as well.

   In order to encourage deployment of OSPFv3 security, an
   authentication option is required that does not have the deployment
   challenges of IPsec.

   In order to support the requirement for simple preshared keys, OSPF
   needs to make sure that when the same key is used for two different
   purposes, no problems result.

   In order to support packet prioritization, the information needed to
   prioritize OSPF packets (the packet type) MUST be at a constant
   location in the packet.

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5.  Solution Work

   A security solution will be developed for OSPFv2 and OSPFv3 based on
   the OSPFv2 cryptographic authentication option.  This solution will
   have the following improvements over the existing OSPFv2 option:

      Detect liveness of neighbors by adding additional information to
      the Hello exchanges in order to detect inter-connection replay

      Add a form of simple key derivation so that if the same preshared
      key is used for OSPF and other purposes, related key attacks do
      not result

      Support OSPFv3 authentication without use of IPsec

      Specify processing rules sufficient to permit replay detection and
      packet prioritization

      Emphasize requirements already present in the OSPF specification
      sufficient to permit key migration without disrupting adjacencies

      Specify the proper use of the key table for OSPF

      Protect the source IP address

      Require that sequence numbers be incremented on each packet

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6.  Security Considerations

   This memo discusses and compiles vulnerabilities in the existing OSPF
   cryptographic handling.

   In analyzing proposed improvements to OSPF per-packet security, it is
   desirable to consider how these improvements interact with potential
   improvements in overall routing security.  For example, the impact of
   replay attacks currently depends on the LSA sequence number
   mechanism.  If cryptographic protections against insider attackers
   are considered by future work, that work will need to provide a
   solution that meets the needs of the per-packet replay defense as
   well as protection of routing data from insider attack.  RFC 2154
   [RFC2154] provides an experimental solution for end-to-end protection
   of routing data in OSPF.  It may be beneficial to consider how
   improvements to the per-packet protections would interact with such a
   mechanism to future-proof these mechanisms.

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7.  Acknowledgments

   Funding for Sam Hartman's work on this memo is provided by Huawei.

   The authors would like to thank Ran Atkinson and Manav Bhatia for
   valuable comments.

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8.  References

8.1.  Normative References

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

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

   [RFC4552]  Gupta, M. and N. Melam, "Authentication/Confidentiality
              for OSPFv3", RFC 4552, June 2006.

   [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.

8.2.  Informative References

   [FIPS180]  US National Institute of Standards and Technology, "Secure
              Hash Standard (SHS)", August 2002.

              Hartman, S. and D. Zhang, "Operations Model for Router
              Keying", draft-hartman-karp-ops-model-01 (work in
              progress), October 2010.

              Housley, R. and T. Polk, "Database of Long-Lived Symmetric
              Cryptographic Keys", draft-ietf-karp-crypto-key-table-00
              (work in progress), November 2010.

              Lebovitz, G. and M. Bhatia, "Keying and Authentication for
              Routing Protocols (KARP) Design Guidelines",
              draft-ietf-karp-design-guide-00 (work in progress),
              February 2010.

              Lebovitz, G., Bhatia, M., and R. White, "The Threat
              Analysis and Requirements for Cryptographic Authentication
              of Routing Protocols' Transports",
              draft-ietf-karp-threats-reqs-01 (work in progress),
              October 2010.

              Jaeggli, J., Hares, S., Bhatia, M., Manral, V., and R.
              White, "Issues with existing Cryptographic Protection
              Methods for Routing Protocols",

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              draft-ietf-opsec-routing-protocols-crypto-issues-06 (work
              in progress), June 2010.

              Jones, E. and O. Moigne, "OSPF Security Vulnerabilities
              Analysis", draft-ietf-rpsec-ospf-vuln-02 (work in
              progress), June 2006.

   [RFC2154]  Murphy, S., Badger, M., and B. Wellington, "OSPF with
              Digital Signatures", RFC 2154, June 1997.

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

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Authors' Addresses

   Sam Hartman
   Painless Security

   Email: hartmans-ietf@mit.edu

   Dacheng Zhang

   Email: zhangdacheng@huawei.com

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