Operational Security Capabilities for                            F. Gont
IP Network Infrastructure (opsec)                    Huawei Technologies
Internet-Draft                                         December 12, 2012
Intended status: Informational
Expires: June 15, 2013


  Virtual Private Network (VPN) traffic leakages in dual-stack hosts/
                                networks
                    draft-ietf-opsec-vpn-leakages-00

Abstract

   The subtle way in which the IPv6 and IPv4 protocols co-exist in
   typical networks, together with the lack of proper IPv6 support in
   popular Virtual Private Network (VPN) products, may inadvertently
   result in VPN traffic leaks.  That is, traffic meant to be
   transferred over a VPN connection may leak out of such connection and
   be transferred in the clear on the local network.  This document
   discusses some scenarios in which such VPN leakages may occur, either
   as a side effect of enabling IPv6 on a local network, or as a result
   of a deliberate attack from a local attacker.  Additionally, it
   discusses possible mitigations for the aforementioned issue.

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 June 15, 2013.

Copyright Notice

   Copyright (c) 2012 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



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   (http://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  IPv4 and IPv6 co-existence . . . . . . . . . . . . . . . . . .  4
   3.  Virtual Private Networks in IPv4/IPv6 dual-stack
       hosts/networks . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  VPN traffic-leakages in legitimate scenarios . . . . . . . . .  6
   5.  VPN traffic-leakage attacks  . . . . . . . . . . . . . . . . .  7
   6.  Mitigations to VPN traffic-leakage vulnerabilities . . . . . .  8
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     10.1.  Normative References  . . . . . . . . . . . . . . . . . . 12
     10.2.  Informative References  . . . . . . . . . . . . . . . . . 12
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14


























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1.  Introduction

   It is a very common practice for employees working at remote
   locations to establish a VPN connection with their office or home
   office.  This is typically done to gain access to some resources only
   available within the company's network, but also to secure the host's
   traffic against attackers that might be connected to the same remote
   location.  In some scenarios, it is even assumed that employing a VPN
   connection makes the use of insecure protocols (e.g. that transfer
   sensitive information in the clear) acceptable, as the VPN provides
   security services (such as confidentiality) for all communications
   made over the VPN.

   Many VPN products that are typically employed for the aforementioned
   VPN connections only support the IPv4 protocol: that is, they perform
   the necessary actions such that IPv4 traffic is sent over the VPN
   connection, but they do nothing to secure IPv6 traffic originated
   from (or being received at) the host employing the VPN client.
   However, the hosts themselves are typically dual-stacked: they
   support (and enable by default) both IPv4 and IPv6 (even if such IPv6
   connectivity is simply "dormant" when they connect to IPv4-only
   networks).  When the IPv6 connectivity of such hosts is enabled, they
   may end up employing an IPv6-unaware VPN client in a dual-stack
   network.  This may have "unexpected" consequences, as explained
   below.

   The subtle way in which the IPv4 and IPv6 protocols interact and co-
   exist in dual-stacked networks might, either inadvertently or as a
   result of a deliberate attack, result in VPN traffic leakages -- that
   is, traffic meant to be transferred over a VPN connection could leak
   out of the VPN connection and be transmitted in the clear on the
   local network, without employing the VPN services at all.

   Section 2 provides some background about IPv6 and IPv4 co-existence,
   summarizing how IPv4 and IPv4 interact on a typical dual-stacked
   network.  Section 3 describes the underlying problem that leads to
   the aforementioned VPN traffic leakages.  Section 4 describes
   legitimate scenarios in which such traffic leakages might occur,
   while Section 5 describes how VPN traffic leakages can be triggered
   by deliberate attacks.











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2.  IPv4 and IPv6 co-existence

   The co-existence of the IPv4 and IPv6 protocols has a number of
   interesting and subtle aspects that may have "surprising"
   consequences.  While IPv6 is not backwards-compatible with IPv4, the
   two protocols are "glued" together by the Domain Name System (DNS).

   For example, consider a site (say, www.example.com) that has both
   IPv4 and IPv6 support.  The corresponding domain name
   (www.example.com, in our case) will contain both A and AAAA DNS
   resource records (RRs).  Each A record will contain one IPv4 address,
   while each AAAA record will contain one IPv6 address -- and there
   might be more than one instance of each of these record types.  Thus,
   when a dual-stacked client application means to communicate with the
   aforementioned site, it can request both A and AAAA records, and use
   any of the available addresses.  The preferred address family (IPv4
   or IPv6) and the specific address that will be used (assuming more
   than one address of each family is available) varies from one
   protocol implementation to another, with many host implementations
   preferring IPv6 addresses over IPv4 addresses.

      [RFC6724] specifies an algorithm for selecting a destination
      address from a list of IPv6 and IPv4 addresses.  [RFC6555]
      discusses the challenge of selecting the most appropriate
      destination address, along with a proposed implementation approach
      that mitigates connection-establishment delays.

   This "co-existence" between IPv6 and IPv4 means that, when a dual-
   stacked client means to communicate with some other system, the
   availability of A and AAAA DNS resource records will typically affect
   which protocol is employed to communicate with that system.




















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3.  Virtual Private Networks in IPv4/IPv6 dual-stack hosts/networks

   Many Virtual Private Network (VPN) implementations do not support the
   IPv6 protocol -- or, what is worse, they completely ignore IPv6.
   This typically means that, when establishing a VPN connection, the
   VPN software takes care of the IPv4 connectivity by, e.g. inserting
   an IPv4 default route that causes all IPv4 traffic to be sent over
   the VPN connection (as opposed to sending the traffic in the clear,
   employing the local router).  However, if IPv6 is not supported (or
   completely ignored), any packets destined to an IPv6 address will be
   sent in the clear using the local IPv6 router.  That is, the VPN
   software will do nothing about the IPv6 traffic.

   The underlying problem here is that while IPv4 and IPv6 are two
   different protocols incompatible with each other, the two protocols
   are glued together by the Domain Name System.  Therefore, for dual-
   stacked systems, it is not possible to secure secure the
   communication with another system without securing both protocols
   (IPv6 and IPv4).
































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4.  VPN traffic-leakages in legitimate scenarios

   Consider a dual-stacked host that employs IPv4-only VPN software to
   establish a VPN connection with a VPN server, and that the host now
   attaches to a dual-stacked network (that provides both IPv6 and IPv4
   connectivity).  If some application on the client needs to
   communicate with a dual-stacked destination, the client will
   typically query both A and AAAA DNS resource records.  Since the host
   will have both IPv4 and IPv6 connectivity, and the intended
   destination will have both A and AAAA DNS resource records, one of
   the possible outcomes is that the host will employ IPv6 to
   communicate with the aforementioned system.  Since the VPN software
   does not support IPv6, the IPv6 traffic will not employ the VPN
   connection, and will be sent in the clear on the local network.

   This could inadvertently expose sensitive traffic that was assumed to
   be secured by the VPN software.  In this particular scenario, the
   resulting VPN traffic leakage is a side-effect of employing IPv6-
   unaware software in a dual-stacked host/network.
































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5.  VPN traffic-leakage attacks

   A local attacker could deliberately trigger IPv6 connectivity on the
   victim host by sending forged ICMPv6 Router Advertisement messages
   [RFC4861].  Such packets could be sent by employing standard software
   such as rtadvd [RTADVD], or by employing packet-crafting tools such
   as the [SI6-Toolkit] or THC-IPv6 [THC-IPv6].  Once IPv6 connectivity
   has been enabled, communications with dual-stacked systems could
   result in VPN traffic leakages, as previously mentioned.

   While this attack may be useful enough (due to the increasing number
   of IPv6-enabled sites), it will only lead to traffic leakages when
   the destination system is dual-stacked.  However, it is usually
   trivial for an attacker to trigger such VPN leakages for any
   destination systems: an attacker could simply advertise himself as
   the local recursive DNS server by sending forged Router Advertisement
   messages [RFC4861] that include the corresponding RDNSS option
   [RFC6106], and then perform a DNS spoofing attack such that he can
   become a "Man in the Middle" and intercept the corresponding traffic.
   As with the previous attack scenario, packet-crafting tools such as
   [SI6-Toolkit] and [THC-IPv6] can readily perform this attack.

      Some systems are known to prefer IPv6-based recursive DNS servers
      over IPv4-based ones, and hence the "malicious" recursive DNS
      servers would be preferred over the legitimate ones advertised by
      the VPN server.

























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6.  Mitigations to VPN traffic-leakage vulnerabilities

   There are a number of possible mitigations for the VPN traffic-
   leakage vulnerability discussed in this document.

   If the VPN client is configured by administrative decision to
   redirect all traffic for IPv4 to the VPN, it should:

   1.  If IPv6 is not supported, disable IPv6 support in all network
       interfaces

          For IPv6-unaware VPN clients, the most simple mitigation
          (although not necessarily the most desirable one) would be to
          disable IPv6 support in all network interface cards when a VPN
          connection is meant to be employed.  Thus, applications on the
          host running the VPN client software will have no other option
          than to employ IPv4, and hence they will simply not even try
          to send/process IPv6 traffic.

   2.  If IPv6 is supported, ensure that all IPv6 traffic is also sent
       via the VPN

   If the VPN client is configured to only send a subset of IPv4
   networks to the VPN tunnel (split-tunnel mode), and the VPN client
   does not support IPv6, it should disable IPv6 as well.  If it
   supports IPv6, it is the administrators responsibility to ensure that
   the correct corresponding sets of IPv4 and IPv6 networks get routed
   into the VPN tunnel.

   Additionally, VPN clients that support IPv6 should mitigate all ND-
   based attacks that may introduce new entries in the routing table,
   such attacks based on forged RA messages containing more specific
   routes [RFC4191], forged ICMPv6 Redirect messages, etc.

   A network may prevent local attackers from successfully performing
   the aforementioned attacks against other local hosts by implementing
   First-Hop Security solutions such as Router Advertisement Guard (RA-
   Guard) [RFC6105] and DHCPv6-Shield [I-D.gont-opsec-dhcpv6-shield].
   However, for obvious reasons, a host cannot and should not rely on
   this type of mitigations when connecting to an open network
   (cybercafe, etc.).

      Besides, popular implementations of RA-Guard are known to be
      vulnerable to evasion attacks
      [I-D.ietf-v6ops-ra-guard-implementation].






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7.  IANA Considerations

   This document has no actions for IANA.
















































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8.  Security Considerations

   This document discusses how traffic meant to be transferred over a
   VPN connection can leak out of the VPN, and hence appear in the clear
   on the local network.  This is the result of employing IPv6-unaware
   VPN client software on dual-stacked hosts.

   Possible ways to mitigate this problem include fixing the VPN client
   software, or disabling IPv6 connectivity on all network interfaces
   when the previous option is not feasible.









































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9.  Acknowledgements

   The author would like to thank (in alphabetical order) Gert Doering
   and Tor Houghton, who providing comments on earlier versions of this
   document.

   This documents has benefited from the input of Cameron Byrne, Gert
   Doering, Seth Hall, Tor Houghton, Alastair Johnson, Henrik Lund
   Kramshoj, and Jim Small, while discussing this topic on the
   ipv6hackers mailing-list [IPv6-Hackers].  It has also benefited from
   discussions with Andrew Yourtchenko on the opsec wg mailing-list
   [OPSEC-LIST].







































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10.  References

10.1.  Normative References

   [RFC4191]  Draves, R. and D. Thaler, "Default Router Preferences and
              More-Specific Routes", RFC 4191, November 2005.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

   [RFC6106]  Jeong, J., Park, S., Beloeil, L., and S. Madanapalli,
              "IPv6 Router Advertisement Options for DNS Configuration",
              RFC 6106, November 2010.

   [RFC6724]  Thaler, D., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol Version 6
              (IPv6)", RFC 6724, September 2012.

   [RFC6555]  Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
              Dual-Stack Hosts", RFC 6555, April 2012.

10.2.  Informative References

   [RFC6105]  Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J.
              Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105,
              February 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-07 (work in
              progress), November 2012.

   [I-D.gont-opsec-dhcpv6-shield]
              Gont, F. and W. Liu, "DHCPv6-Shield: Protecting Against
              Rogue DHCPv6 Servers", draft-gont-opsec-dhcpv6-shield-01
              (work in progress), October 2012.

   [IPv6-Hackers]
              "IPv6 Hackers mailing-list",
               http://lists.si6networks.com/listinfo/ipv6hackers/.

   [OPSEC-LIST]
              "OPSEC WG mailing-list",
               https://www.ietf.org/mailman/listinfo/opsec.

   [SI6-Toolkit]



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              "SI6 Networks' IPv6 toolkit",
              <http://www.si6networks.com/tools/ipv6toolkit>.

   [THC-IPv6]
              "The Hacker's Choice IPv6 Attack Toolkit",
              <http://www.thc.org/thc-ipv6/>.

   [RTADVD]   "rtadvd(8) manual page", <http://www.freebsd.org/cgi/
              man.cgi?query=rtadvd&sektion=8>.










































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Author's Address

   Fernando Gont
   Huawei Technologies
   Evaristo Carriego 2644
   Haedo, Provincia de Buenos Aires  1706
   Argentina

   Phone: +54 11 4650 8472
   Email: fgont@si6networks.com
   URI:   http://www.si6networks.com








































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