KARP                                                          S. Hartman
Internet-Draft                                         Painless Security
Intended status: Informational                                  D. Zhang
Expires: May 30, 2013                        Huawei Technologies co. ltd
                                                       November 26, 2012

        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 RFC6518.  Key components of solutions to
   gaps identified in this draft are already underway.

Requirements Language

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

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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   This Internet-Draft will expire on May 30, 2013.

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   publication of this document.  Please review these documents

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   carefully, as they describe your rights and restrictions with respect
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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Requirements to Meet . . . . . . . . . . . . . . . . . . .  3
     1.2.  Requirements notation  . . . . . . . . . . . . . . . . . .  4
   2.  Current State  . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  OSPFv2 . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.2.  OSPFv3 . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  Impacts of OSPF Replays  . . . . . . . . . . . . . . . . . . .  6
   4.  Gap Analysis and Specific Requirements . . . . . . . . . . . .  8
   5.  Solution Work  . . . . . . . . . . . . . . . . . . . . . . . .  9
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12

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1.  Introduction

   This document analyzes the current state of OSPFv2 and OSPFv3
   according to the requirements of [RFC6518].  This draft builds on
   several previous analysis efforts into routing security.  The OPSEC
   working group put together [RFC6039] an analysis of cryptographic
   issues with routing protocols.  Earlier, the RPSEC working group put
   together [I-D.ietf-rpsec-ospf-vuln] a detailed analysis of OSPF
   vulnerabilities.  Solution work to address gaps identified in this
   analysis is underway [I-D.ietf-ospf-security-extension-manual-keying]

   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

   There are a number of requirements described in section 3 of
   [I-D.ietf-karp-threats-reqs] that OSPF does not currently meet.  The
   gaps are as follows:

   o  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.ietf-karp-ops-model] are possible.

   o  Replay Protection: The requirements document addresses
      requirements for both inter-connection replay protection and
      intra-connection replay protection.  OSPFv3 has no replay
      protection at all.  OSPFv2 has most of the mechanisms necessary
      for intra-connection replay protection.  Unfortunately, OSPFv2
      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.  OSPFv2 has no inter-connection replay
      protection; this creates significant denial-of-service

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   o  Packet Prioritization: OSPFv3 uses IPsec [RFC4301]to process
      packets.  This complicates implementations that wish to process
      some packets such as hellos and acknowledgements above others.  In
      addition, if IPsec replay mechanisms were used, packets would need
      to be processed at least by IPsec even if they were low priority.

   o  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].

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 of choice for new MACs.

   However, RFC 5709 [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

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   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 support for SHA-2
   rather than SHA-1.

   These security services provide integrity protection on each packet.
   In addition, limited replay detection is provided.  The sequence
   number is non-decreasing.  So, once a router has increased its
   sequence number, an attacker cannot replay an old packet.
   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
   implementations use, e.g., "seconds since reboot" as their sequence
   numbers.  The sequence numbers are thus only increased by every
   second, permitting an opportunity for intra-connection replay.  Also,
   no mechanism is provided to deal with the loss of anti-replay state;
   if sequence numbers are reused when a router reboots, then inter-
   connection replays are straight forward.  In
   [I-D.ietf-ospf-security-extension-manual-keying], the OSPFv2 sequence
   number is expanded to 64-bits with the least significant 32-bit value
   containing a strictly increasing sequence number and the most
   significant 32-bit value containing the boot count.  The boot count
   is retained in non-volatile storage for the deployment life of a OSPF
   router.  Therefore, the sequence number will never decrease even
   after a cold reboot.

   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 perform replay attacks.  See Section 3
   for analysis of these attacks.  In
   [I-D.ietf-ospf-security-extension-manual-keying], this issue is
   addressed by changing the definition of Apad from a constant defined
   in [RFC5709] to the source address from the IP header of the OSPFv2
   protocol packet.  In this way, the source address from the IP header
   is incorporated in the cryptographic authentication computation, and
   any change of the IP source address will be detected.

   The mechanism 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

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   [I-D.ietf-karp-crypto-key-table] regarding lifetime.

   The OSPFv2 replay mechanism does not handle prioritized transmission
   of OSPF Hello and Link State Acknowledgement packets as recommended
   in [RFC4222].  When OSPF packets are transmitted with varied
   prioritization, they can arrive out-of-order resulting in packets
   with lower prioritization being discarded.

2.2.  OSPFv3

   RFC 4552 describes how the IPsec 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) [RFC4301] 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
   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.

3.  Impacts of OSPF Replays

   As discussed, neither version of OSPF meets the requirements of
   inter-connection or intra-connection replay protection.  In order to
   mount a replay, an attacker needs some mechanism to inject a packet;
   physical security can limit a particular deployment's vulnerability
   to replay attacks.  This section discusses the impacts of OSPF

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   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: if an attacker can cause an adjacency to go down, then
   replay state is lost.  Database Description packets also include a
   per-LSA sequence number that is part of the information that is
   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, the RPSEC
   analysis [I-D.ietf-rpsec-ospf-vuln] describes a number of attacks
   that are possible because of per-packet replays.  The most serious
   appear to be attacks against Hello packets, 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, then 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, then
   replay attacks should not disrupt OSPFv2.  Unfortunately, OSPFv2 does
   not have a procedure for dealing with sequence numbers reaching the
   maximum value.  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, replay state
   is lost.  So, after rebooting or when all adjacencies are lost, a
   router may allow its sequence number to decrease.  An attacker can
   cause significant damage by replaying a packet captured before the
   sequence number decrease at a time after the sequence number
   decrease.  If this happens, then the replayed packet will be accepted
   and the sequence number will be updated.  However, the legitimate
   sender will be using a lower sequence number, so legitimate packets

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   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, then
   connectivity with that neighbor will be disrupted until the adjacency

   OSPFv3 lacks the per-packet sequence number but has the per-LSA
   sequence number.  As such, OSPFv3 has no defense against denial of
   service attacks that exploit replay.

4.  Gap Analysis and Specific Requirements

   The 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

   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.

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

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:

      Address most inter-connection replay attacks by splitting the
      sequence number and requiring preservation of state so that the
      sequence number increases on every packet.

      Add a form of simple key derivation so that if the same preshared
      key is used for OSPF and other purposes, cross-protocol 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

   The key components of this solution work are already underway.
   OSPFv3 now supports an authentication option [RFC6506] that meets the
   requirements of this section, except that it does not describe how
   the key tables are used for OSPF.  OSPFv2 is being enhanced
   [I-D.ietf-ospf-security-extension-manual-keying] to protect the
   source address, provide inter-connection replay and describe how to
   use the key table.

6.  IANA Considerations

   This document makes no request of IANA.

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7.  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, then 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.  An
   experimental solution is discussed in [RFC2154] that explores 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.

   Implementations have a number of options in minimizing the potential
   denial of service impact of OSPF cryptographic authentication.  The
   Generalized TTL Security Mechanism (GTSM) [RFC5082] might be
   appropriate for OSPF packets other than those traversing virtual
   links.  Using this mechanism requires support of the sender; new OSPF
   cryptographic authentication could specify this behavior if desired.
   Alternatively implementations can limit the source addresses from
   which they accept packets.  Non-hello packets need only be accepted
   from existing neighbors.  If a system is under attack hello packets
   from existing neighbors could be prioritized over hellos from new
   neighbors.  These mechanisms can be considered to limit the potential
   impact of denial of service attacks on the cryptographic
   authentication mechanism itself.

8.  Acknowledgements

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

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

9.  References

9.1.  Normative References

              Lebovitz, G. and M. Bhatia, "Keying and Authentication for
              Routing Protocols (KARP) Overview, Threats, and

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              Requirements", draft-ietf-karp-threats-reqs-06 (work in
              progress), September 2012.

   [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.

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

9.2.  Informative References

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

              Housley, R., Polk, T., Hartman, S., and D. Zhang,
              "Database of Long-Lived Symmetric Cryptographic Keys",
              draft-ietf-karp-crypto-key-table-04 (work in progress),
              October 2012.

              Hartman, S. and D. Zhang, "Operations Model for Router
              Keying", draft-ietf-karp-ops-model-04 (work in progress),
              October 2012.

              Jaeggli, J., Hares, S., Bhatia, M., Manral, V., and R.
              White, "Issues with existing Cryptographic Protection
              Methods for Routing Protocols",
              draft-ietf-opsec-routing-protocols-crypto-issues-07 (work
              in progress), August 2010.

              Bhatia, M., Hartman, S., Zhang, D., and A. Lindem,
              "Security Extension for OSPFv2 when using Manual Key
              draft-ietf-ospf-security-extension-manual-keying-03 (work
              in progress), October 2012.

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

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              progress), June 2006.

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

   [RFC4222]  Choudhury, G., "Prioritized Treatment of Specific OSPF
              Version 2 Packets and Congestion Avoidance", BCP 112,
              RFC 4222, October 2005.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [RFC5082]  Gill, V., Heasley, J., Meyer, D., Savola, P., and C.
              Pignataro, "The Generalized TTL Security Mechanism
              (GTSM)", RFC 5082, October 2007.

   [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.

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

   [RFC6506]  Bhatia, M., Manral, V., and A. Lindem, "Supporting
              Authentication Trailer for OSPFv3", RFC 6506,
              February 2012.

Authors' Addresses

   Sam Hartman
   Painless Security

   Email: hartmans-ietf@mit.edu
   URI:   http://www.painless-security.com/

   Dacheng Zhang
   Huawei Technologies co. ltd
   Huawei Building No.3 Xinxi Rd., Shang-Di Information Industrial Base Hai-Dian District, Beijing

   Email: zhangdacheng@huawei.com

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