Operational Security Capabilities for                            F. Gont
IP Network Infrastructure (opsec)                                UK CPNI
Internet-Draft                                            April 24, 2012
Intended status: BCP
Expires: October 26, 2012


             Security Implications of IPv6 on IPv4 networks
           draft-gont-opsec-ipv6-implications-on-ipv4-nets-00

Abstract

   This document discusses the security implications of native IPv6
   support and IPv6 transition/co-existence technologies on "IPv4-only"
   networks, and describes possible mitigations for the aforementioned
   issues.

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   This Internet-Draft will expire on October 26, 2012.

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   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   include Simplified BSD License text as described in Section 4.e of



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


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Security Implications of native IPv6 support . . . . . . . . .  4
   3.  Security Implications of tunneling Mechanisms  . . . . . . . .  5
     3.1.  Filtering 6to4 . . . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Filtering ISATAP . . . . . . . . . . . . . . . . . . . . .  6
     3.3.  Filtering Teredo . . . . . . . . . . . . . . . . . . . . .  7
     3.4.  Filtering 6over4 . . . . . . . . . . . . . . . . . . . . .  8
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   5.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 11
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13
































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1.  Introduction

   Most general-purpose operating systems implement and enable by
   default native IPv6 support and a number of transition-co-existence
   technologies.  In those cases in which such devices are deployed on
   networks that are assumed to be IPv4-only, the aforementioned
   technologies could be leveraged by local or remote attackers for a
   number of (illegitimate) purposes.

   For example, a Network Intrusion Detection System (NIDS) might be
   prepared to detect attack patterns for IPv4 traffic, but might be
   unable to detect the same attack patterns when a transition/
   co-existence technology is leveraged for that purpose.  Additionally,
   an IPv4 firewall might enforce a specific security policy in IPv4,
   but might be unable to enforce the same policy in IPv6.  Finally,
   some transition/co-existence mechanisms (notably Teredo) are designed
   to traverse Network Address Translators (NATs), which in many
   deployments provide a minimum level of protection by only allowing
   those instances of communication that have been initiated from the
   internal network.  Thus, these mechanisms might cause an internal
   host with otherwise limited IPv4 connectivity to become globally
   reachable over IPv6, therefore resulting in increased (and possibly
   unexpected) host exposure.  That is, the aforementioned technologies
   might inadvertently allow incoming IPv6 connections from the Internet
   to hosts behind the organizational firewall.

   In general, the aforementioned security implications can be mitigated
   by enforcing security controls on native IPv6 traffic and on IPv4-
   tunneled traffic.  Among such controls is the enforcement of
   filtering policies, such that undesirable traffic is blocked.

   This document discusses the security implications of IPv6 and IPv6
   transition/co-existence technologies on (allegedly) IPv4-only
   networks, and provides guidance on how to mitigate the aforementioned
   issues.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].












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2.  Security Implications of native IPv6 support

   Most popular operating systems include IPv6 support that is enabled
   by default.  This means that even if a network is expected to be
   IPv4-only, much of its infrastructure is nevertheless likely to be
   IPv6 enabled.  For example, hosts are likely to have at least link-
   local IPv6 connectivity which might be exploited by attackers with
   access to the local network.

      [CORE2007] is a security advisory about a buffer overflow which
      could be remotely-exploited by leveraging link-local IPv6
      connectivity that is enabled by default.

   Additionally, unless appropriate measures are taken, an attacker with
   access to an 'IPv4-only' local network could impersonate a local
   router and cause local hosts to enable their IPv6 connectivity (e.g.
   by sending Router Advertisement messages), possibly circumventing
   security controls that were are enforced only on IPv4 communications.

      [Waters2011] provides an example of how this could be achieved
      using publicly available tools.

   SLAAC-based attacks [RFC3756] can be mitigated with technologies such
   as RA-Guard [RFC6105] [I-D.ietf-v6ops-ra-guard-implementation].
   However, RA-Guard cannot mitigate attack vectors that employ IPv6
   link-local addresses, since configuration of such addresses does not
   rely on Router Advertisement messages.

   In order to mitigate attacks based on native IPv6 traffic, IPv6
   security controls should be enforced on both IPv4 and IPv6 networks.
   The aforementioned controls might include: deploying IPv6-enabled
   NIDS, implementing IPv6 firewalling, etc.

      In some very specific scenarios (e.g., military operations
      networks) in which only IPv4 service might be desired, a network
      administrator might disable IPv6 support in all the communicating
      devices.














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3.  Security Implications of tunneling Mechanisms

   Unless properly managed, tunneling mechanisms may result in negative
   security implication.  [RFC6169] describes the security implications
   of tunneling mechanisms in detail.  Therefore, tunneling mechanisms
   should be a concern not only to network administrators that have
   consciously deployed them, but also to network and security
   administrators whose security policies might be bypassed by
   exploiting these mechanisms.

      [CERT2009] contains some examples of how tunnels can be leveraged
      to bypass firewall rules.

   To help mitigate these issues, a good security practice is to only
   allow traffic deemed as "necessary" (i.e., the so-called "default
   deny" policy).  Therefore, security administrators should only allow
   IPv6 transition co-existence traffic as a result of an explicit
   decision, rather than as a result of lack of awareness about such
   traffic.

      It should be noted that this recommendation is aimed at a network
      that is the target of such traffic (such as an enterprise
      network).  IPv6-transition traffic should not be filtered e.g. by
      an ISP when it is transit traffic.

   Additionally, it is highly recommended that in those networks where
   specific transition mechanisms are not explicitly deployed, not only
   the corresponding traffic should be filtered at the organizational
   perimeter, but also the corresponding mechanisms disabled on each
   node connected to the organizational network.  This not only prevents
   security breaches resulting from accidental use of these mechanisms,
   but also disables this functionality altogether, possibly mitigating
   vulnerabilities that might be present in the host implementation of
   this transition/co-existence mechanisms.

   IPv6-in-IPv4 tunnelling mechanisms (such as 6to4 or configured
   tunnels) can generally be blocked by dropping IPv4 packets that
   contain a Protocol field set to 41.  Security devices such as NIDS
   might also include signatures that detect such transition/
   co-existence traffic.

3.1.  Filtering 6to4

   6to4 [RFC3056] is an address assignment and router-to-router, host-
   to-router, and router-to-host automatic tunnelling mechanism that is
   meant to provide IPv6 connectivity between IPv6 sites and hosts
   across the IPv4 Internet.




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   As discussed in Section 3, all IPv6-in-IPv4 traffic, including 6to4,
   could be easily blocked by filtering IPv4 that contain their Protocol
   field set to 41.  This is the most effective way of filtering such
   traffic.

   Additional filtering rules that might be incorporated include:

   o  Filter outgoing IPv4 packets that have their Destination Address
      set to an address that belongs to the prefix 192.88.99.0/24.

   o  Filter incoming IPv4 packets that have their Source Address set to
      an address that belongs to the prefix 192.88.99.0/24.

         It has been suggested that 6to4 relays send their packets with
         their IPv4 Source Address set to 192.88.99.1.

   o  Filter outgoing IPv4 packets that have their Destination Address
      set to the IPv4 address of well-known 6to4 relays.

   o  Filter incoming IPv4 packets that have their Destination Address
      set to the IPv4 address of well-known 6to4 relays.

   These last two filtering policies will generally be unnecessary, and
   possibly unfeasible to enforce (given the number of potential 6to4
   relays, and the fact that many relays may remain unknown to the
   network administrator).  If anything, they should be applied with the
   additional requirement that such IPv4 packets have their Protocol
   field set to 41, to avoid the case where other services available at
   the same IPv4 address as a 6to4 relay are mistakenly made
   inaccessible.

   If 6to4 traffic is meant to be filtered while other IPv6-in-IPv4
   traffic is allowed, then the following filtering rules could be
   applied:

   o  Filter outgoing IPv4 packets that have their Protocol field set to
      41, and that have an IPv6 Source Address (embedded in the IPv4
      payload) that belongs to the prefix 2002::/16.

   o  Filter incoming IPv4 packets that have their Protocol field set to
      41, and that have an IPv6 Destination address (embedded in the
      IPv4 payload) that belongs to the prefix 2002::/16.

3.2.  Filtering ISATAP

   ISATAP [RFC5214] is an Intra-site tunnelling protocol, and thus it is
   generally expected that such traffic will not traverse the
   organizational firewall of an IPv4-only.  Nevertheless, ISATAP



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   traffic is easily filtered as described in Section 3 of this
   document.

3.3.  Filtering Teredo

   Teredo [RFC4380] is an address assignment and automatic tunnelling
   technology that provides IPv6 connectivity to dual-stack nodes that
   are behind one or more Network Address Translators (NATs), by
   encapsulating IPv6 packets in IPv4-based UDP datagrams.  Teredo is
   meant to be a 'last resort' IPv6 connectivity technology, to be used
   only when other technologies such as 6to4 cannot be deployed (e.g.,
   because the edge device has not been assigned a public IPv4 address).

   As noted in [RFC4380], in order for a Teredo client to configure its
   Teredo IPv6 address, it must contact a Teredo server, through the
   Teredo service port (UDP port number 3544).

   To prevent the Teredo initialization process from succeeding, and
   hence prevent the use of Teredo, an organizational firewall could
   filter outgoing UDP packets with a Destination Port of 3544.

   It is clear that such a filtering policy does not prevent an attacker
   from running its own Teredo server in the public Internet, using a
   non-standard UDP port for the Teredo service port (i.e., a port
   number other than 3544).

   The most popular operating system that includes an implementation of
   Teredo in the default installation is Microsoft Windows.  Microsoft
   Windows obtains the Teredo server addresses (primary and secondary)
   by resolving the domain name teredo.ipv6.microsoft.com into DNS A
   records.  A network administrator may want to prevent Microsoft
   Windows hosts from obtaining Teredo service by filtering at the
   organizational firewall outgoing UDP datagrams (i.e., IPv4 packets
   with the Protocol field set to 17) that contain in the IPv4
   Destination Address any of the IPv4 addresses that the domain name
   teredo.ipv6.microsoft.com maps to.  Additionally, the firewall would
   filter incoming UDP datagrams from any of the IPv4 addresses to which
   the domain names of well-known Teredo servers (such as
   teredo.ipv6.microsoft.com) resolve.

      As these IPv4 addresses might change over time, an administrator
      should obtain these addresses himself when implementing the
      filtering policy, and should also be prepared to maintain this
      list updated over time.







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      The corresponding addresses can be easily obtained from a UNIX
      host by issuing the command 'dig teredo.ipv6.microsoft.com a'
      (without quotes).

   It should be noted that even with all these filtering policies in
   place, a node in the internal network might still be able to
   communicate with some Teredo clients.  That is, it could configure an
   IPv6 address itself (without even contacting a Teredo server), and
   might send Teredo traffic to those peers for which intervention of
   the host's Teredo server is not required (e.g., Teredo clients behind
   a cone NAT).

3.4.  Filtering 6over4

   [RFC2529] specifies a mechanism known as 6over4 or 'IPv6 over IPv4'
   (or colloquially as 'virtual Ethernet'), which comprises a set of
   mechanisms and policies to allow isolated IPv6 hosts located on
   physical links with no directly-connected IPv6 router, to become
   fully functional IPv6 hosts by using an IPv4 domain that supports
   IPv4 multicast as their virtual local link.

      This transition technology has never been widely deployed, because
      of the low level of deployment of multicast in most networks.

   6over4 encapsulates IPv6 packets in IPv4 packets with their Protocol
   field set to 41.  As a result, simply filtering all IPv4 packets that
   have a Protocol field equal to 41 will filter 6over4 (along with many
   other transition technologies).

   A more selective filtering could be enforced such that 6over4 traffic
   is filtered while other transition traffic is still allowed.  Such a
   filtering policy would block all IPv4 packets that have their
   Protocol field set to 41, and that have a Destination Address that
   belongs to the prefix 239.0.0.0/8.

   This filtering policy basically blocks 6over4 Neighbor Discovery
   traffic directed to multicast addresses, thus preventing Stateless
   Address Auto-configuration (SLAAC), address resolution, etc.
   Additionally, it would prevent the 6over multicast addresses from
   being leveraged for the purpose of network reconnaissance.











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4.  Security Considerations

   This document discusses the security implications of IPv6 on IPv4
   networks, and describes a number of techniques to mitigate the
   aforementioned issues.














































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5.  Acknowledgements

   This document resulted from the project "Security Assessment of the
   Internet Protocol version 6 (IPv6)" [CPNI-IPv6], carried out by
   Fernando Gont on behalf of the UK Centre for the Protection of
   National Infrastructure (CPNI).

   Fernando Gont would like to thank the UK CPNI for their continued
   support.










































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6.  References

6.1.  Normative References

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

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC4380]  Huitema, C., "Teredo: Tunneling IPv6 over UDP through
              Network Address Translations (NATs)", RFC 4380,
              February 2006.

   [RFC5214]  Templin, F., Gleeson, T., and D. Thaler, "Intra-Site
              Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214,
              March 2008.

   [RFC3056]  Carpenter, B. and K. Moore, "Connection of IPv6 Domains
              via IPv4 Clouds", RFC 3056, February 2001.

   [RFC2529]  Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4
              Domains without Explicit Tunnels", RFC 2529, March 1999.

6.2.  Informative References

   [RFC3756]  Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor
              Discovery (ND) Trust Models and Threats", RFC 3756,
              May 2004.

   [RFC6105]  Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J.
              Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105,
              February 2011.

   [RFC6169]  Krishnan, S., Thaler, D., and J. Hoagland, "Security
              Concerns with IP Tunneling", RFC 6169, April 2011.

   [I-D.ietf-v6ops-ra-guard-implementation]
              Gont, F., "Implementation Advice for IPv6 Router
              Advertisement Guard (RA-Guard)",
              draft-ietf-v6ops-ra-guard-implementation-02 (work in
              progress), March 2012.

   [CERT2009]
              CERT, "Bypassing firewalls with IPv6 tunnels", 2009, <http
              ://www.cert.org/blogs/vuls/2009/04/
              bypassing_firewalls_with_ipv6.html>.




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   [CORE2007]
              CORE, "OpenBSD's IPv6 mbufs remote kernel buffer
              overflow", 2007,
              <http://www.coresecurity.com/content/open-bsd-advisorie>.

   [CPNI-IPv6]
              Gont, F., "Security Assessment of the Internet Protocol
              version 6 (IPv6)",  UK Centre for the Protection of
              National Infrastructure, (available on request).

   [Waters2011]
              Waters, A., "SLAAC Attack - 0day Windows Network
              Interception Configuration Vulnerability", 2011,
              <http://resources.infosecinstitute.com/slaac-attack/>.





































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Author's Address

   Fernando Gont
   UK Centre for the Protection of National Infrastructure

   Email: fernando@gont.com.ar
   URI:   http://www.cpni.gov.uk












































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