IPv6 Operations Working Group S. Krishnan
Internet-Draft Ericsson
Intended status: Informational D. Thaler
Expires: April 28, 2011 Microsoft
J. Hoagland
Symantec
October 25, 2010
Security Concerns With IP Tunneling
draft-ietf-v6ops-tunnel-security-concerns-04
Abstract
A number of security concerns with IP tunnels are documented in this
memo. The intended audience of this document includes network
administrators and future protocol developers. The primary intent of
this document is to raise the awareness level regarding the security
issues with IP tunnels as deployed today.
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
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 28, 2011.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Tunnels May Bypass Security . . . . . . . . . . . . . . . . . 3
2.1. Network Security Bypass . . . . . . . . . . . . . . . . . 3
2.2. IP Ingress and Egress Filtering Bypass . . . . . . . . . . 5
2.3. Source Routing After the Tunnel Client . . . . . . . . . . 6
3. Challenges in Inspecting and Filtering Content of Tunneled
Data Packets . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Inefficiency of Selective Network Filtering of All
Tunneled Packets . . . . . . . . . . . . . . . . . . . . . 7
3.2. Problems with deep packet inspection of tunneled data
packets . . . . . . . . . . . . . . . . . . . . . . . . . 8
4. Increased Exposure Due to Tunneling . . . . . . . . . . . . . 9
4.1. NAT Holes Increase Attack Surface . . . . . . . . . . . . 9
4.2. Exposure of a NAT Hole . . . . . . . . . . . . . . . . . . 11
4.3. Public Tunnels Widen Holes in Restricted NATs . . . . . . 12
5. Tunnel Address Concerns . . . . . . . . . . . . . . . . . . . 12
5.1. Feasibility of Guessing Tunnel Addresses . . . . . . . . . 12
5.2. Profiling Targets Based on Tunnel Address . . . . . . . . 13
6. Additional Security Concerns . . . . . . . . . . . . . . . . . 14
6.1. Attacks Facilitated By Changing Tunnel Server Setting . . 14
7. Mechanisms to secure the use of tunnels . . . . . . . . . . . 16
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17
9. Security Considerations . . . . . . . . . . . . . . . . . . . 17
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
11. Informative References . . . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19
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1. Introduction
With NAT devices becoming increasingly more prevalent, there have
recently been many tunneling protocols developed that go through NAT
devices or firewalls by tunneling over UDP or TCP. For example,
Teredo [RFC4380], L2TPv2 [RFC2661], and L2TPv3 [RFC3931] all tunnel
IP packets over UDP. Similarly, many SSL VPN solutions that tunnel
IP packets over HTTP (and hence over TCP) are deployed today.
This document discusses security concerns with tunneling IP packets,
and includes recommendations where relevant.
The primary intent of this document is to help improve security
deployments using tunnel protocols. In addition, the document aims
to provide information that can be used in any new or updated tunnel
protocol specification. The intended audience of this document
includes network administrators and future protocol developers.
2. Tunnels May Bypass Security
2.1. Network Security Bypass
2.1.1. Problem
Tunneled IP traffic may not receive the intended level of inspection
or policy application by network-based security devices unless such
devices are specifically tunnel-aware. This reduces defense in depth
and may cause security gaps. This applies to all network-located
devices and to any end-host based firewalls whose existing hooking
mechanism(s) would not show them the IP packet stream after the
tunnel client does decapsulation or before it does encapsulation.
2.1.2. Discussion
Evasion by tunneling is often a problem for network-based security
devices such as network firewalls, intrusion detection and prevention
systems, and router controls. To provide such functionality in the
presence of tunnels, the developer of such devices must add support
for parsing each new protocol. There is typically a significant lag
between when the security developer recognizes that a tunnel will be
used (or will be remotely usable) to a significant degree and when
the parsing can be implemented in a product update, the update tested
and released, and customers begin using the update. Late changes in
the protocol specification or in the way it is implemented can cause
additional delays. This becomes a significant security concern when
a delay in applied coverage is occurring frequently. One way to cut
down on this lag is for security developers to follow the progress of
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new IETF protocols but this will still not account for any new
proprietary protocols.
For example, for L2TP or Teredo, an unaware network security device
would inspect or regulate the outer IP and the IP-based UDP layer as
normal, but it would not recognize that there is an additional IP
layer contained inside the UDP payload to which it needs to apply the
same controls as it would to a native packet. (Of course, if the
device discards the packet due to something in the IP or UDP header,
such as referring to an unknown protocol, the embedded packet is no
longer a concern.) In addition, if the tunnel does encryption, the
network-based security device may not be able to do much, just as if
IPsec end-to-end encryption were used without tunneling.
Network security controls being not applied must be a concern to
those that set them up, since those controls are supposed to provide
an additional layer of defense against external attackers. If
network controls are being bypassed due to the use of tunneling, the
strength of the defense (i.e. the number of layers of defense) is
reduced. Since security administrators may have a significantly
reduced level of confidence without this layer, this becomes a
concern to them.
One implication of the security control bypass is that defense in
depth has been reduced, perhaps down to zero unless a local firewall
is in use as recommended in [RFC4380]. However, even if there are
host-based security controls that recognize tunnels, security
administrators may not have configured them with full security
control parity, even if all controls that were maintained by the
network are available on the host. Thus there may be gaps in desired
coverage.
Compounding this is that, unlike what would be the case for native
IP, some network administrators will not even be aware that their
hosts are globally reachable, if the tunnel provides connectivity to/
from the Internet; for example, they may not be expecting this for
hosts behind a stateful firewall. In addition, Section 3.2 discusses
how it may not be efficient to find all tunneled traffic for network
devices to examine.
2.1.3. Recommendations
Security administrators who do not consider tunneling an acceptable
risk should disable tunnel functionality either on the end-nodes
(hosts) or on the network nodes at the perimeter of their network.
However, there may be an awareness gap. Thus, due to the possible
negative security consequences, tunneling functionality should be
easy to disable on the host and through a central management facility
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if one is provided.
To minimize security exposure due to tunnels, we recommend that a
tunnel be an interface of last resort, independent of IP version.
Specifically, we suggest that when both native and tunneled access to
a remote host is available, that the native access be used in
preference to tunneled access except when the tunnel endpoint is
known to not bypass security (e.g., an IPsec tunnel to a gateway
provided by the security administrator of the network). This should
also promote greater efficiency and reliability.
Note that although Rule 7 of [RFC3484] section 6 will prefer native
connectivity over tunnels, this rule is only a tie-breaker when a
choice is not made by earlier rules; hence tunneling mechanisms that
are tied to a particular range of IP address space will be decided
based on the prefix precedence. For example, using the prefix policy
mechanism of [RFC3484] section 2.1, Teredo might have a precedence of
5 so that native IPv4 is preferred over Teredo.
2.2. IP Ingress and Egress Filtering Bypass
2.2.1. Problem
IP addresses inside tunnels are not subject to ingress and egress
filtering in the network they tunnel over, unless extraordinary
measures are taken. Only the tunnel endpoints can do such filtering.
2.2.2. Discussion
Ingress filtering (sanity-checking incoming destination addresses)
and egress filtering (sanity-checking outgoing source addresses) are
done to mitigate attacks and to make it easier to identify the source
of a packet and are considered to be a good practice. e.g. ingress
filtering at the network perimeter should not allow packets with a
source address that belongs to the network to enter the network from
the outside the network. This function is most naturally (and in the
general case, by requirement) done at network boundaries. Tunneled
IP traffic bypassing this network control is a specific case of
Section 2.1, but is illustrative.
2.2.3. Recommendations
Tunnel servers can apply ingress and egress controls to tunneled IP
addresses passing through them to and from tunnel clients.
Tunnel clients could make an effort to conduct ingress and egress
filtering.
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Implementations of protocols that embed an IPv4 address in a tunneled
IPv6 address directly between peers should perform filtering based on
checking the correspondence.
Implementations of protocols that accept tunneled packets directly
from a server, relay or protocol peer do filtering the same way as it
would be done on a native link with traffic from a router.
Some protocols such as 6to4 [RFC3056], Teredo, and ISATAP [RFC5214]
allow both other hosts and a router over a common tunnel. To perform
host-based filtering with such protocols a host would need to know
the outer IP address of each router from which it could receive
traffic, so that packets from hosts beyond the router will be
accepted even though the source address would not embed the router's
IP address. Router addresses might be learned via Secure Neighbor
Discovery (SEND) [RFC3971] or some other mechanism (e.g., [RFC5214]
section 8.3.2).
2.3. Source Routing After the Tunnel Client
2.3.1. Problem
If the encapsulated IP packet specifies source routing beyond the
recipient tunnel client, the host may forward the IP packet to the
specified next hop. This may be unexpected and contrary to
administrator wishes and may have bypassed network-based source
routing controls.
2.3.2. Discussion
A detailed discussion of issues related to source routing can be
found in [RFC5095] and [SECA-IP].
2.3.3. Recommendations
Tunnel clients should by default discard tunneled IP packets that
specify additional routing, as recommended in [RFC5095] and
[SECA-IP], though they may also allow the user to configure what
source routing types are allowed. All pre-existing source routing
controls should be upgraded to apply these controls to tunneled IP
packets as well.
3. Challenges in Inspecting and Filtering Content of Tunneled Data
Packets
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3.1. Inefficiency of Selective Network Filtering of All Tunneled
Packets
3.1.1. Problem
There is no mechanism to both efficiently and immediately filter all
tunneled packets (other than the obviously faulty method of filtering
all packets). This limits the ability to prevent tunnel use on a
network.
3.1.2. Discussion
Given concerns about tunnel security or a network's lack of
preparedness for tunnels, a network administrator may wish to simply
block all use of tunnels that bypass security policies. He or she
may wish to do so using network controls; this could be either due to
not having the capability to disable tunneling on all hosts attached
to the network or due to wanting an extra layer of prevention.
One simple method of doing this easily for many tunnel protocols is
to block outbound packets to the UDP or TCP port used (e.g.,
destination UDP port is 3544 for Teredo, UDP port 1701 for L2TP,
etc.). This prevents a tunnel client from establishing a new tunnel.
However, existing tunnels will not necessarily be affected if the
blocked port is used only for initial setup. In addition, if the
blocking is applied on the outside of the client's NAT device, the
NAT device will retain the port mapping for the client. In some
cases, however, blocking all traffic to a given outbound port (e.g.,
port 80) may interfere with non-tunneled traffic so this should be
used with caution.
Another simple alternative, if the tunnel server addresses are well-
known, is to filter out all traffic to/from such addresses.
The other approach is to find all packets to block in the same way as
would be done for inspecting all packets (Section 3.2). However;
this faces the difficulties in terms of efficiency of filtering, as
is discussed there.
3.1.3. Recommendations
Developers of protocols that tunnel over UDP or TCP (including HTTP)
to reach the Internet should disable their protocols in networks that
wish to enforce security policies on the user traffic. (Windows, for
example, disables Teredo by default if it detects that it is within
an enterprise network that contains a Windows domain controller.)
Administrators of such networks may wish to filter all tunneled
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traffic at the boundaries of their networks. It is sufficient to
filter out the tunneled connection requests (if they can be
identified) to stop further tunneled traffic. The easiest mechanism
for this would be to filter out outgoing traffic sent to the
destination port defined by the tunneling protocol, and incoming
traffic with that source port. Similarly, in certain cases, it is
also possible to use the IP protocol field to identify and filter
tunneled packets. e.g. 6to4 [RFC3056] is a tunneling mechanism that
uses the IPv4 packets to carry encapsulated IPv6 packets, and can be
identified by the IPv4 protocol type 41.
3.2. Problems with deep packet inspection of tunneled data packets
3.2.1. Problem
There is no efficient mechanism for network-based devices, which are
not the tunnel endpoint, to inspect the contents of all tunneled data
packets, the way they can for native packets. This makes it
difficult to apply the same controls as they do to native IP.
3.2.2. Discussion
Some tunnel protocols are easy to identify, such as if all data
packets are encapsulated using a well-known UDP or TCP port that is
unique to the protocol.
Other protocols, however, either use dynamic ports for data traffic,
or else share ports with other protocols (e.g., tunnels over HTTP).
The implication of this is that network-based devices that wish to
passively inspect (and perhaps selectively apply policy to) all
encapsulated traffic must inspect all TCP or UDP packets (or at least
all packets not part of a session that is known not to be a tunnel).
This is imperfect since a heuristic must then be applied to determine
if a packet is indeed part of a tunnel. This may be too slow to make
use of in practice, especially if it means that all TCP or UDP
packets must be taken off of the device's "fast path".
One heuristic that can be used on packets to determine if they are
tunnel-related or not is as follows. For each known tunnel protocol,
attempt parsing the packet as if it were a packet of that protocol,
destined to the local host (i.e., where the local host has the
destination address in the inner IP header, if any). If all syntax
checks pass, up to and including the inner IP header (if the tunnel
doesn't use encryption), then treat the packet as if it is a tunneled
packet of that protocol.
It is possible that non-tunnel packets will match as tunneled using
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this heuristic, but tunneled packets (of the known types of tunnels)
should not escape inspection, absent implementation bugs.
For some protocols, it may be possible to monitor setup exchanges to
know to expect that data will be exchanged on certain ports later.
(Note that this does not necessarily apply to Teredo, for example,
since communicating with another Teredo client behind a cone NAT
[RFC5389] device does not require such signaling. In such cases this
control will not work. However, deprecation of the cone bit as
discussed in [RFC5991] means this technique may indeed work with
updated Teredo implementations.)
3.2.3. Recommendations
As illustrated above, it should be clear that inspecting the contents
of tunneled data packets is highly complex and often impractical.
For this reason, if a network wishes to monitor IP traffic, tunneling
across, as opposed to tunneling to, the security boundary is not
recommended. For example, to provide an IPv6 transition solution,
the network should provide native IPv6 connectivity or a tunnel
solution (e.g., ISATAP or 6over4) that encapsulates data packets
between hosts and a router within the network.
4. Increased Exposure Due to Tunneling
4.1. NAT Holes Increase Attack Surface
4.1.1. Problem
If the tunnel allows inbound access from the public Internet, the
opening created in a NAT device due to a tunnel client increases its
Internet attack surface area. If vulnerabilities are present, this
increased exposure can be used by attackers and their programs.
If the tunnel allows inbound access only from a private network
(e.g., a remote network to which one has VPN'ed), the opening created
in the NAT device still increases its attack surface area, although
not as much as in the public Internet case.
4.1.2. Discussion
When a tunnel is active, a mapped port is maintained on the NAT
device through which remote hosts can send packets and perhaps
establish connections. The following sequence is intended to sketch
out the processing on the tunnel client host that can be reached
through this mapped port; the actual processing for a given host may
be somewhat different.
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1. Link-layer protocol processing
2. (Outer) IP host firewall processing
3. (Outer) IP processing by stack
4. UDP/TCP processing by stack
5. Tunnel client processing
6. (Inner) IP host firewall processing
7. (Inner) IP processing by stack
8. Various upper layer processing may follow
The inner firewall (and other security) processing may or may not be
present, but if it is, some of the inner IP processing may be
filtered. (For example, [RFC4380] section 7.1 recommends that an
IPv6 host firewall be used on all Teredo clients.)
(By the virtue of the tunnel being active, we can infer that the
inner host firewall is unlikely to do any filtering based on the
outer IP.) Any of this processing may expose vulnerabilities an
attacker can exploit; similarly these may expose information to an
attacker. Thus, even if firewall filtering is in place (as is
prudent) and filters all incoming packets, the exposed area is larger
than if a native IP Internet connection were in place, due to the
processing that takes place before the inner IP is reached
(specifically, the UDP/TCP processing, the tunnel client processing,
and additional IP processing, especially if one is IPv4 and the other
is IPv6).
One possibility is that a layer 3 targeted worm makes use of a
vulnerability in the exposed processing. The main benefit tunneling
provides to worms is enabling L3 reachability to the end host. Even
a thoroughly firewalled host could be subject to a worm that spreads
with a single UDP packet if the right remote code vulnerability is
present.
4.1.3. Recommendations
This problem seems inherent in tunneling being active on a host, so
the solution seems to be to minimize tunneling use.
For example, it can be active only when it is really needed and only
for as long as needed. So, the tunnel interface can be initially not
configured and only used when it is entirely the last resort. The
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interface should then be deactivated (ideally, automatically) again
as soon as possible. Note however that the hole will remain in the
NAT device for some amount of time after this, so some processing of
incoming packets is inevitable unless the client's native IP address
behind the NAT device is changed.
4.2. Exposure of a NAT Hole
4.2.1. Problem
Attackers are more likely to know about a tunnel client's NAT hole
than a typical hole in the NAT device. If they know about the hole,
they could try to use it.
4.2.2. Discussion
There are at least three reasons why an attacker may be more likely
to learn of the tunnel client's exposed port than a typical NAT
exposed port:
1. The NAT mapping for a tunnel is typically held open for a
significant period of time, and kept stable. This increases the
chance of it being discovered.
2. In some protocols (e.g., Teredo), the external IP address and
port are contained in the client's address that is used end-to-
end and possibly even advertised in a name resolution system.
While the tunnel protocol itself might only distribute this
address in IP headers, peers, routers, and other on-path nodes
still see the client's IP address. Although this point does not
apply directly to protocols (e.g., L2TP) that do not construct
the inner IP address based on the outer IP address, the inner IP
address is still known to peers, routers, etc. and can still be
reached by attackers without knowing the external IP address or
port.
3. The tunnel protocol often contains more messages that are
exchanged and with more parties (e.g., due to a longer path
length) than without using the tunnel, offering more chance for
visibility into the port and address in use.
4.2.3. Recommendations
The recommendations from Section 4.1 seem to apply here as well:
minimize tunnel use.
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4.3. Public Tunnels Widen Holes in Restricted NATs
4.3.1. Problem
Tunnels that allow inbound connectivity from the Internet (e.g.,
Teredo, tunnel brokers, etc) essentially disable the filtering
behavior of the NAT for all tunnel client ports. This eliminates NAT
devices filtering for such ports and may eliminate the need for an
attacker to spoof an address.
4.3.2. Discussion
NATs that implement Address-Dependent or Address and Port-Dependent
Filtering [RFC4787] limit the source of incoming packets to just
those that are a previous destination. This poses a problem for
tunnels that intend to allow inbound connectivity from the Internet.
Various protocols (e.g., Teredo, STUN [RFC5389], etc.) provide a
facility for peers, upon request, to become a previous destination.
This works by sending a "bubble" packet via a server, which is passed
to the client, and then sent by the client (through the NAT) to the
originator.
This removes any NAT-based barrier to attackers sending packets in
through the client's service port. In particular, an attacker would
no longer need to either be an actual previous destination or to
forge its addresses as a previous destination. When forging, the
attacker would have had to learn of a previous destination and then
would face more challenges in seeing any returned traffic.
4.3.3. Recommendations
If the tunnel can provide connectivity to the Internet, the tunnel
client should run a host firewall on the tunnel interface. Also,
minimizing public tunnel use (see Section 4.1.3) would lower the
attack opportunity related to this exposure.
5. Tunnel Address Concerns
5.1. Feasibility of Guessing Tunnel Addresses
5.1.1. Problem
For some types of tunneling protocols, it may be feasible to guess IP
addresses assigned to tunnels, either when looking for a specific
client or when looking for an arbitrary client. This is in contrast
to native IPv6 addresses in general, but is no worse than for native
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IPv4 addresses today.
For example, some protocols (e.g., 6to4 and Teredo) use well-defined
address ranges. As another example, using well-known public servers
for Teredo or tunnel brokers also implies using a well known address
range.
5.2. Profiling Targets Based on Tunnel Address
5.2.1. Problem
An attacker encountering an address associated with a particular
tunneling protocol or well-known tunnel server has the opportunity to
infer certain relevant pieces of information that can be used to
profile the host before sending any packets. This can reduce the
attacker's footprint and increase the attacker's efficiency.
5.2.2. Discussion
The tunnel address reveals some information about the nature of the
client.
o That a host has a tunnel address associated with a given protocol
means that the client is running on some platform for which there
exists a tunnel client implementation of that protocol. In
addition, if some platforms have that protocol installed by
default and where the host's default rules for using it make it
susceptible to being in use, then it is more likely to be running
on such a platform than on one where it is not used by default.
For example, as of this writing, seeing a Teredo address suggests
that the host it is on is probably running Windows.
o Similarly, the use of an address associated with a particular
tunnel server also suggests some information. Tunnel client
software is often deployed, installed, and/or configured using
some degree of automation. It seems likely that the majority of
the time the tunnel server that results from the initial
configuration will go unchanged from the initial setting.
Moreover, the server that is configured for use may be associated
with a particular means of installation, which often suggests the
platform. For example, if the server field in a Teredo address is
one of the IPv4 addressees to which teredo.ipv6.microsoft.com
resolves, it suggests that the host is running Windows.
o The external IPv4 address of a NAT device can of course be readily
associated with a particular organization or at least an ISP, and
hence putting this address in an IPv6 address reveals this
information. However, this is no different than using a native IP
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address, and hence is not new with tunneling.
o It is also possible that external client port numbers may be more
often associated with particular client software or the platform
on which it is running. The usefulness of this for platform
determination is, however, reduced by the different NAT port
number assignment behaviors. In addition, the same observations
would apply to use of UDP or TCP over native IP as well, and hence
this is not new with tunneling.
The platform, tunnel client software, or organization information can
be used by an attacker to target attacks more carefully. For
example, an attacker may decide to attack an address only if it is
likely to be associated with a particular platform or tunnel client
software with a known vulnerability. (This is similar to the ability
to guess some platforms based on the OUI in the EUI-64 portion of an
IPv6 address generated from a MAC address, since some platforms are
commonly used with interface cards from particular vendors.)
5.2.3. Recommendations
If installation programs randomized the server setting, that would
reduce the extent to which they can be profiled. Similarly,
administrators can choose to change the default setting to reduce the
degree to which they can be profiled ahead of time.
Randomizing the tunnel client port in use would mitigate any
profiling that can be done based on the external port, especially if
multiple different tunnel clients did this. Further discussion on
randomizing ports can be found at [TSV-PORT].
It is recommended that tunnel protocols minimize the propagation of
knowledge about whether the NAT is a cone NAT.
6. Additional Security Concerns
6.1. Attacks Facilitated By Changing Tunnel Server Setting
6.1.1. Problem
If an attacker could either change a tunnel client's server setting
or change the IP addresses to which a configured host name resolves
(e.g., by intercepting DNS queries) AND the tunnel is not
authenticated, it would let the attacker become a man in the middle.
This would allow them to at least monitor peer communication and at
worst to impersonate the remote peer.
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6.1.2. Discussion
A client's server has good visibility into the client's communication
with IP peers. If the server were switched to one that records this
information and makes it available to third parties (e.g.,
advertisers, competitors, spouses, etc.) then sensitive information
would be disclosed, especially if the client's host prefers the
tunnel over native IP. Assuming the server provides good service,
the user would not have reason to suspect the change.
Full interception of IP traffic could also be arranged (including
pharming) which would allow any number of deception or monitoring
attacks including phishing. We illustrate this with an example
phishing attack scenario.
It is often assumed that the tunnel server is a trusted entity. It
may be possible for malware or a malicious user to quietly change the
client's tunnel server setting and have the user be unaware their
trust has been misplaced for an indefinite period of time. However,
malware or a malicious user can do much worse than this, so this is
not a significant concern. Hence it is only important that an
attacker on the network cannot change the client's server setting.
1. A phisher sets up a malicious tunnel server (or tampers with a
legitimate one). This server, for the most part, provides
correct service.
2. An attacker, by some means, switches the host's tunnel server
setting, or spoofs a DNS reply, to point to the above server. If
neither DNS nor the tunnel setup is secured (i.e., if the client
does not authenticate the information), then the attacker's
tunnel server is seen as legitimate.
3. A user on the victim host types their bank's URL into his/her
browser.
4. The bank's hostname resolves to one or more IP addresses and the
tunnel is selected for socket connection for whatever reason
(e.g., the tunnel provides IPv6 connectivity and the bank has an
IPv6 address).
5. The tunnel client uses the server for help in connecting to the
bank's IP address. Some tunneling protocols use a separate
channel for signaling vs data, but this still allows the server
to place itself in the data path by an appropriate signal to the
client. For example, in Teredo, the client sends a ping request
through a server which is expected to come back through a data
relay, and a malicious server can simply send it back itself to
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indicate that is a data relay for the communication.
6. The rest works pretty much like any normal phishing transaction,
except that the attacker acts as a tunnel server (or data relay,
for protocols such as Teredo) and a host with the bank's IP
address.
This pharming type attack is not unique to tunneling. Switching DNS
server settings to a malicious DNS server or DNS cache poisoning in a
recursive DNS resolver could have a similar effect.
6.1.3. Recommendations
In general, anti-phishing and anti-fraud provisions should help with
aspects of this, as well as software that specifically monitors for
tunnel server changes.
Perhaps the best way to mitigate tunnel-specific attacks is to have
the client either authenticate the tunnel server, or at least the
means by which the tunnel server's IP address is determined. For
example, SSL VPNs use https URLs and hence authenticate the server as
being the expected one. Another mechanism, when IPv6 Router
Advertisements are sent over the tunnel is to use SEcure Neighbor
Discovery (SEND) [RFC3971] to verify that the client trusts the
server.
On the host, it should require an appropriate level of privilege in
order to change the tunnel server setting (as well as other non-
tunnel-specific settings such as the DNS server setting, etc.).
Making it easy to see the current tunnel server setting (e.g., not
requiring privilege for this) should help detection of changes.
The scope of the attack can also be reduced by limiting tunneling use
in general but especially in preferring native IPv4 to tunneled IPv6;
this is because it is reasonable to expect that banks and similar web
sites will continue to be accessible over IPv4 for as long as a
significant fraction of their customers are still IPv4-only. Please
refer to Section 3 of [TUNNEL-LOOPS] for a detailed description and
mitigation measures for a class of attacks based on IPv6 automatic
tunnels.
7. Mechanisms to secure the use of tunnels
This document described several security issues with tunnels. This
does not mean that tunnels need to be avoided at any cost. On the
contrary, tunnels can be very useful if deployed, operated and used
properly. The threats against IP tunnels are documented here. If
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the threats can be mitigated, network administrators can efficiently
and securely use tunnels in their network. Several measures can be
taken in order to secure the operation of IPv6 tunnels:
o Operating on-premise tunnel servers/relays so that the tunneled
traffic does not cross border routers.
o Setting up internal routing to steer traffic to these servers/
relays
o Setting up of firewalls [RFC2979] to allow known and controllable
tunneling mechanisms and disallow unknown tunnels.
8. Acknowledgments
The authors would like to thank Remi Denis-Courmont, Fred Templin,
Jordi Palet Martinez, James Woodyatt, Christian Huitema, Brian
Carpenter, Nathan Ward, Kurt Zeilenga, Joel Halpern, Erik Kline,
Alfred Hoenes and Fernando Gont for reviewing earlier versions of the
document and providing comments to make this document better.
9. Security Considerations
This entire document discusses security concerns with tunnels.
10. IANA Considerations
This document does not require any IANA action.
11. Informative References
[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[RFC2529] Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4
Domains without Explicit Tunnels", RFC 2529, March 1999.
[RFC2661] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn,
G., and B. Palter, "Layer Two Tunneling Protocol "L2TP"",
RFC 2661, August 1999.
[RFC2979] Freed, N., "Behavior of and Requirements for Internet
Firewalls", RFC 2979, October 2000.
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Internet-Draft Tunneling Security Concerns October 2010
[RFC3056] Carpenter, B. and K. Moore, "Connection of IPv6 Domains
via IPv4 Clouds", RFC 3056, February 2001.
[RFC3484] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February 2003.
[RFC3931] Lau, J., Townsley, M., and I. Goyret, "Layer Two Tunneling
Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005.
[RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
Neighbor Discovery (SEND)", RFC 3971, March 2005.
[RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through
Network Address Translations (NATs)", RFC 4380,
February 2006.
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007.
[RFC5095] Abley, J., Savola, P., and G. Neville-Neil, "Deprecation
of Type 0 Routing Headers in IPv6", RFC 5095,
December 2007.
[RFC5214] Templin, F., Gleeson, T., and D. Thaler, "Intra-Site
Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214,
March 2008.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
October 2008.
[RFC5991] Thaler, D., Krishnan, S., and J. Hoagland, "Teredo
Security Updates", RFC 5991, September 2010.
[SECA-IP] Gont, F., "Security Assessment of the Internet Protocol
version 4", draft-ietf-opsec-ip-security-03 (work in
progress), April 2010.
[TSV-PORT]
Larsen, M. and F. Gont, "Transport Protocol Port
Randomization Recommendations",
draft-ietf-tsvwg-port-randomization-09 (work in progress),
August 2010.
[TUNNEL-LOOPS]
Nakibly, G. and F. Templin, "Routing Loop Attack using
IPv6 Automatic Tunnels: Problem Statement and Proposed
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Mitigations", draft-ietf-v6ops-tunnel-loops-00 (work in
progress), September 2010.
Authors' Addresses
Suresh Krishnan
Ericsson
8400 Decarie Blvd.
Town of Mount Royal, QC
Canada
Phone: +1 514 345 7900 x42871
Email: suresh.krishnan@ericsson.com
Dave Thaler
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
USA
Phone: +1 425 703 8835
Email: dthaler@microsoft.com
James Hoagland
Symantec Corporation
350 Ellis St.
Mountain View, CA 94043
US
Email: Jim_Hoagland@symantec.com
URI: http://symantec.com/
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