Internet DRAFT - draft-gont-opsec-vpn-leakages
draft-gont-opsec-vpn-leakages
Operational Security Capabilities for F. Gont
IP Network Infrastructure (opsec) Huawei Technologies
Internet-Draft October 15, 2012
Intended status: Informational
Expires: April 18, 2013
Virtual Private Network (VPN) traffic leakages in dual-stack hosts/
networks
draft-gont-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
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This Internet-Draft will expire on April 18, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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.
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 RFC 2119 [RFC2119].
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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.
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 that include the corresponding RDNSS option, 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, 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
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
[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-04 (work in
progress), May 2012.
[I-D.gont-opsec-dhcpv6-shield]
Gont, F., "DHCPv6-Shield: Protecting Against Rogue DHCPv6
Servers", draft-gont-opsec-dhcpv6-shield-00 (work in
progress), May 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]
"SI6 Networks' IPv6 toolkit",
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<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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