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Versions: 00 01 02 03 04 05 06 07 08 09 10 RFC 5991

IPv6 Operations Working Group                                S. Krishnan
Internet-Draft                                                  Ericsson
Expires: May 13, 2008                                        J. Hoagland
                                                       November 10, 2007

                        Teredo Security Updates

Status of this Memo

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Copyright Notice

   Copyright (C) The IETF Trust (2007).


   Additional security concerns with Teredo are documented, beyond what
   is in RFC 4380.  This is based on an independent analysis of Teredo's
   security implications.  The primary intent of this document is to
   provide information and recommendations to the IETF that can be used
   in any updated Teredo specification.  The second intended audience is
   anyone that can help improve security in Teredo as deployed, so they
   will be aware of these concerns.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Teredo Bypasses Security . . . . . . . . . . . . . . . . . . .  3
     2.1.  Teredo Bypasses Network Security . . . . . . . . . . . . .  3
     2.2.  IPv6 Ingress and Egress Filtering Bypass . . . . . . . . .  5
     2.3.  Source Routing After the Teredo Client . . . . . . . . . .  6
   3.  Challenges in Inspecting and Filtering Content of Teredo
       Data Packets . . . . . . . . . . . . . . . . . . . . . . . . .  7
     3.1.  Inefficiency of Selective Network Filtering of All
           Teredo Packets . . . . . . . . . . . . . . . . . . . . . .  7
     3.2.  Problems with deep packet inspection of Teredo data
           packets  . . . . . . . . . . . . . . . . . . . . . . . . .  8
   4.  Increased Exposure Due to Teredo . . . . . . . . . . . . . . . 10
     4.1.  Teredo NAT Holes Increase Attack Surface . . . . . . . . . 10
     4.2.  Unusually High Exposure of a NAT Hole  . . . . . . . . . . 11
     4.3.  Teredo Bubble Facility Widens Hole in Restricted NAT . . . 12
   5.  Teredo Address Concerns  . . . . . . . . . . . . . . . . . . . 13
     5.1.  Feasibility of Guessing Teredo Addresses . . . . . . . . . 13
     5.2.  Profiling Targets Based on Teredo Address  . . . . . . . . 14
   6.  Additional Security Concerns . . . . . . . . . . . . . . . . . 16
     6.1.  Attacks Facilitated By Changing Teredo Server Setting  . . 16
     6.2.  RFC 4380 Implies That Teredo Improves Security . . . . . . 18
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 19
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 19
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 19
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 20
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
   Intellectual Property and Copyright Statements . . . . . . . . . . 21

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1.  Introduction

   An independent analysis of Teredo's security implications was
   conducted by Symantec[TTPTPNSOSI], based on the Teredo specification
   ([RFC4380]).  This analysis uncovered some security concerns
   associated with Teredo which are not documented in the Teredo
   specification.  This document discloses these additional concerns and
   includes any recommendations where relevant.  This Internet Draft is
   also influenced to an extent by an examination of the Teredo
   implementation on Microsoft Windows Vista [WVNASA].

   The primary intent of this document is to provide information so that
   can be used in any updated Teredo specification.  Secondarily, this
   document can help improve security in Teredo as deployed (including
   those that implement Teredo, security providers, and network security
   administrators) become aware of any valid security concerns.

2.  Teredo Bypasses Security

2.1.  Teredo Bypasses Network Security

2.1.1.  Problem

   IPv6 traffic tunneled with Teredo will not receive the intended level
   of inspection or policy application by network-based security
   devices, unless the devices are specifically Teredo aware and
   capable.  This reduces defense in depth and may cause security gaps.
   This applies to all network-located devices and to end-host based
   firewalls whose existing hooking mechanism(s) would not show them the
   IP packet stream after the Teredo client does decapsulation.

2.1.2.  Discussion

   Evasion by tunneling is often a problem for network-based security
   devices such as firewalls, intrusion detection and prevention
   systems, and router controls.  The vendor of such devices must add
   support for detunneling for each new protocol.  There is typically a
   significant lag between when the vendor recognizes that a tunnel will
   be used (or will be remotely usable) to a significant degree and when
   the detunneling can be implemented in a product update, the update
   tested and released, and the customer begins 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.

   Specifically for Teredo, a Teredo-unaware network security device
   would inspect or regulate the IPv4 and the IPv4-based UDP layer as

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   normal for IPv4, but it would not recognize that there is an
   additional IP layer contained inside the UDP payload that it needs to
   apply the same controls as it would to a native packet.  (Of course,
   if device discards the packet due to something in the IPv4 or UDP
   header, such as referring to an unknown protocol, the Teredo packet
   is no longer a concern.)  Teredo also only recently reached RFC
   status (February 2006), is widely applicable, requires no support
   from the local or organizational network, and looks ready to be
   widely used.  Furthermore the tunnel created by the Teredo client is
   open-ended and allows bidirectional traffic.

   Network security controls being not applied must be a concern to
   those that set them up, since those controls are supposed to
   adequately regulate all traffic.  If network controls are being
   bypassed due to the use of IPv6 via Teredo, the burden of controls
   shifts to the Teredo client host.  Since security administrators may
   not have full control over all the nodes on their network, they
   sometimes prefer to implement security controls on the network.

   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 as a mitigation in RFC 4380.
   However, even if there are host-based security controls that
   recognize Teredo, 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
   IPv6, some network administrators will not even be aware that their
   hosts are globally addressable; for example, they may not be
   expecting this for hosts with RFC-1918 [RFC1918] addresses behind a
   NAT.  In addition, Section 3.2 discusses how it may not be efficient
   to find all Teredo traffic for network devices to examine.

2.1.3.  Recommendations

   Of course security administrators should disable Teredo functionality
   unless their network-based security controls adequately recognize the
   tunneled traffic (unless they consider it an acceptable risk).
   However, there may be an awareness gap.  Thus, due to the possible
   negative security consequences, we recommend that explicit user
   action be required to enable a Teredo client for the first time, at
   least for the time being.  When Teredo is being enabled or when it is
   going to be used for the first time, perhaps there should be a
   descriptive warning about the possible evasion that will occur.  In
   addition, Teredo client functionality should be easy to disable on
   the host and through a central management facility if one is

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   RFC 4380 requires that Teredo be an IPv6 provider of last resort.  To
   minimize security exposure due to Teredo, we recommend that Teredo
   also be an IP provider of last resort.  Specifically, we suggest that
   when both IPv4- and IPv6-based access to a remote host is available,
   that the IPv4-based access be used in preference to IPv6 access that
   needs to use Teredo.  This should also promote greater efficiency and

   We specifically note that we could find no pre-existing mechanism for
   Teredo to use that could automate its functionality being disabled
   unless all network-based security controls were aware of it.  A
   separate type of consent request packet would be needed.  (Such a
   consent request service could have application beyond Teredo.)

2.2.  IPv6 Ingress and Egress Filtering Bypass

2.2.1.  Problem

   IPv6 addresses inside Teredo tunnels are not subject ingress and
   egress filtering, unless extraordinary measures are taken.

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.  This is most
   naturally (and in the general case, by requirement) done at network
   boundaries.  Teredo-tunneled IPv6 traffic bypassing this network
   control is a specific case of Section Section 2.1, but is

2.2.3.  Recommendations

   The recommendations in Section 2.1.3 can help here.  For this problem
   specifically, there are two locations in which ingress and egress
   filtering could be restored.

   Network based:  network-based devices (e.g. routers) could be updated
      to find all Teredo packets and to apply ingress and egress
      controls equally to Teredo tunneled IPv6-addresses.

   Teredo client based:  Teredo clients could make an effort to conduct
      ingress and egress filtering.  However, there are at least two
      problems inherent in attempting to do address filtering from this
      vantage point: knowing the network addresses to filter (drop the

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      packets of) and knowing whether a peer is from the same network.

      The network addresses to filter could be approximated from
      enumerating the addresses on the network interface the Teredo
      client is using; at least the /64 of global unicast addresses can
      be assumed to be in use on the network.  Router Solicitations
      [RFC2461] could also be made.

      Peers known to be local due to the Teredo local discovery
      procedure can be excluded from filtering, but the scope of that
      knowledge is limited to a broadcast domain, whereas ingress and
      egress filtering generally applies to a larger scope.

2.3.  Source Routing After the Teredo Client

2.3.1.  Problem

   If the encapsulated IPv6 packet specifies source routing beyond the
   recipient Teredo client, the host may forward the IPv6 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

   IPv6 source routing, while provided for in RFC 2460 [RFC2460] and
   required in some cases such as mobile IPv6, is often not needed or
   desired in a given network.  Thus it is often blocked, at least for
   certain types of source routing.  The danger is that source routing,
   by providing a reflection point, violates assumptions made in network
   security decisions.  In addition there is often no compelling case
   for why it would be needed.

   Consider the case where a Teredo packet reaches a Teredo client (and
   is accepted) and the encapsulated IPv6 packet contains a Routing
   header.  The Routing header indicates that this is not the intended
   destination (just a hop in a source route).  Unless the Teredo client
   has source routing disabled, it would pass the IPv6 packet on to the
   next hop.  One way to use this for an attack is to have the next hop
   be a node internal to the network the client is on.  Another is to
   pass the packet back outside, using the Teredo node as a reflection

   This behavior is not specific to Teredo packets; it works in the same
   way for all IPv6 packets.  However, with native IPv6 packets, a
   gateway prohibition of source routed packets would have prevented the
   packet from even reaching the internal host.  With Teredo active, the
   burden is placed upon the end hosts, at least those running Teredo.

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   Source routing post-Teredo may also be a surprising possibility
   (packets on an end-to-end tunnel not stopping at the end) that might
   not have been anticipated in network controls, especially given that
   a NAT was traversed in the process.  RFC 4380 made no mention of
   source routing.

2.3.3.  Recommendations

   Teredo clients should by default discard tunneled IPv6 packets that
   specify additional routing, 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
   Teredo tunneled IPv6 packets as well.

3.  Challenges in Inspecting and Filtering Content of Teredo Data

3.1.  Inefficiency of Selective Network Filtering of All Teredo Packets

3.1.1.  Problem

   There is no mechanism to both efficiently and immediately filter all
   Teredo packets.  This limits the ability to prevent Teredo use on a

3.1.2.  Discussion

   Given concerns about Teredo security or a network's lack of
   preparedness for Teredo, a network administrator may wish to simply
   block all Teredo use.  He or she may wish to do so using network
   controls; this could be either due to not having confidence in the
   ability to disable it on all hosts attached to the network or due to
   wanting an extra layer of prevention.

   One simple method to do that is easy to employ is to block outbound
   packets to UDP port 3544.  This prevents a Teredo client from
   connecting to its server and completing qualification.  Thus it can
   be assured that a host trying to establish a new Teredo address will
   be prevented from using Teredo tunneling.  However, existing Teredo
   clients will not be affected, at least not immediately.  In addition,
   if the blocking is applied on the outside of client's NAT, the NAT
   will retain the port mapping for the client and the client may or may
   not continue to use its Teredo address.  It is not known if blocking
   all outbound port 3544 will interfere with non-Teredo traffic.

   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,

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   this faces the difficulties in terms of efficient as was present

3.1.3.  Recommendations

   Teredo is NOT RECOMMENDED as a solution for managed networks.
   Administrators of such networks may wish to filter all Teredo traffic
   at the boundaries of their networks.  It is sufficient to filter out
   the Teredo connection requests to stop further Teredo traffic.  The
   easiest mechanism for this would be to filter out incoming traffic
   with source port 3544 and outgoing traffic with destination port

3.2.  Problems with deep packet inspection of Teredo data packets

3.2.1.  Problem

   There is no efficient mechanism for network-based devices to inspect
   the contents of Teredo data packets, the way they can for native IPv6
   packets.  This makes it difficult to apply the same controls as they
   do to native IPv6.

3.2.2.  Discussion

   The only well known port that Teredo traffic uses is UDP 3544 and RFC
   4380 only requires that to be used for the Teredo server service
   port.  The client and relay components can use any port they wish.

   The implication of this is that network-based devices that wish to
   passively inspect (and perhaps selectively apply policy to) all
   encapsulated Teredo-based traffic must inspect all UDP packets (or at
   least all UDP packets not part of session that is known not to be
   Teredo).  This is inefficient (more so that say 6to4), especially
   considering that a heuristic must then be applied to determine if a
   packet is indeed Teredo.  This may be too slow to make use of in
   practice, especially if it means that all UDP packets must be taken
   off of the device's "fast path".

   One heuristic that can be used on UDP packets to determine if they
   are Teredo-related or not is as follows:

   1.   The packet is not Teredo if it is not UDP over IPv4.

   2.   Set T to the UDP payload offset.

   3.   Set E to the end of the packet plus one.

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   4.   If E-T < 40 (the length of an IPv6 base header), the packet is
        not Teredo.

   5.   If the octets starting with T are 0x0001 (an indication of
        authentication data), T= T+13 plus the lengths of the client
        identifier and the authentication value, assuming T is the start
        of authentication data.

   6.   If E-T < 40, the packet is not Teredo.

   7.   If the octets starting with T are 0x0000 (an indication of
        origin encapsulation), T= T+8.

   8.   If E-T < 40, the packet is not Teredo.

   9.   If the octets starting with T is 0x0000 or 0x0001, loop back to
        step 5.

   10.  If the most significant nibble of the octet at T is not 6, the
        packet is not Teredo.

   11.  Assuming T is the start of an IPv6 header, set L to value of the
        payload length field, S to the start of the source address, and
        D to the start of the destination address.

   12.  If E-T != L+40, the packet is not Teredo.

   13.  If neither S nor D start with 0x20010000 (the Teredo prefix),
        the packet is not Teredo.

   14.  The packet is assumed to be Teredo, with the IPv6 header
        starting at T.

   This is similar to the packet reception checks in [RFC4380].  The
   loop is present due to the possibility that some Teredo component
   will accept a Teredo packet even if the authentication and origin
   encapsulation are reversed or repeated and that either an attacker or
   an evasive user will use that to evade inspection.  It is possible
   that non-Teredo packets will match as Teredo using this heuristic (in
   which case additional heuristics can be added), but Teredo packets
   should not escape inspection, absent implementation bugs.

   It is not possible to monitor Teredo setup on specific ports to know
   to expect that Teredo traffic will appear on certain ports later
   since in some cases there are no Teredo setup packets (e.g., when a
   Teredo client is sending a packet to another Teredo client that is
   not behind a restricted NAT).

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3.2.3.  Recommendations

   As illustrated above, it is very clear that inspecting the contents
   of Teredo data packets is highly complex and impractical.  For this
   reason, if a network wishes to monitor IPv6 traffic, Teredo is NOT
   RECOMMENDED as a transition solution.  As an alternative, the network
   may provide native IPv6 connectivity or a managed network solution
   like ISATAP [RFC4214]

4.  Increased Exposure Due to Teredo

4.1.  Teredo NAT Holes Increase Attack Surface

4.1.1.  Problem

   The opening created in a NAT due to a Teredo client increases its
   Internet attack surface area.  If vulnerabilities are present, this
   increased exposure can be used by attackers and their programs.

4.1.2.  Discussion

   When a Teredo client is active, a mapped port is maintained on the
   NAT through which Internet hosts can send packets and perhaps
   establish connections.  The following sequence is intended to sketch
   out the processing on the Teredo client host that can be reached
   through this; the actual processing for a given host may be somewhat

   1.  IPv4 host firewall processing

   2.  IPv4 processing by stack

   3.  UDP processing by stack

   4.  Teredo client processing

   5.  IPv6 host firewall processing

   6.  IPv6 processing by stack

   7.  various upper layer processing may follow

   The firewall (and other security) processing may or may not be
   present, but if it is, some of the IPv6 processing may be filtered.
   (By the virtue of the Teredo client being active, we can infer that
   the IPv4 firewall is unlikely to do any filtering for this.)  Any of
   this processing may expose vulnerabilities an attacker can exploit;

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   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 non-trivial.

   The exposed area is even larger than if a native IPv6 Internet
   connection was in place, due to the processing that takes place
   before IPv6 is reached.  It is also larger than for a native IPv4
   connection due to the UDP, Teredo, and IPv6 processing.

   One possibility is that a layer 3 targeted worm makes use of a
   vulnerability in the exposed processing.  While the main benefit to
   worms from Teredo is targeting at layer 3 reaching the end host, even
   a throughly firewalled host could be subject to a worm that spreads
   with a single UDP packet if the right remote code vulnerability is
   present; such worms can spread quickly as evidenced by Slammer.

4.1.3.  Recommendations

   This problem seems inherent in Teredo being active on a host, so the
   solution seems to be to minimize Teredo use.

   For example, it can be active only when it is really needed and only
   for as long as needed.  So, the Teredo interface can be initially not
   configured and only used when it is entirely the last resort.  The
   interface should then be deactivated again as soon as possible.  Note
   however that the hole will remain in the NAT for some amount of time
   after this, so some processing of incoming packets is inevitable
   (unless the client's IPv4 address is changed).

4.2.  Unusually High Exposure of a NAT Hole

4.2.1.  Problem

   Attackers are more likely to know about a Teredo client's NAT hole
   than a typical hole in the NAT.  If they know about the hole, they
   could try to use it.

4.2.2.  Discussion

   There are at least three reasons whey an attacker is more likely to
   learn of the Teredo client's exposed port than a typical NAT exposed

   1.  The NAT mapping is typically held open longer and kept more
       stable than would otherwise be the case.  This increases the
       chance of it being discovered.

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   2.  The external IP address and port is contained in the client's
       Teredo address.  While the Teredo protocol itself only
       distributes this address on packets, peers and even network
       components such as Teredo relays may record the Teredo address
       in, for example, log files; the address may even make its way
       onto, for example, peer-to-peer host advertisements.

   3.  The Teredo protocol contains more messages that are exchanged and
       with more parties than is typical, offering more chance for
       visibility into the port and address in use.  All Teredo protocol
       packets contain the client's external address and port.

4.2.3.  Recommendations

   The recommendations from Section 4.1 seem to apply here as well:
   minimize Teredo use.

4.3.  Teredo Bubble Facility Widens Hole in Restricted NAT

4.3.1.  Problem

   The bubble facility offered by clients and their servers to relays
   essentially turns a restricted NAT into an unrestricted one, for all
   Teredo client service ports.  This eliminates NAT filtering for such
   ports and may eliminate the need for an attacker to spoof an address.

4.3.2.  Discussion

   Restricted NATs and port restricted NATs [RFC3489] limit the source
   of incoming packets to just those that are a previous destination.
   This poses a problem for Teredo, so [RFC4380] provides a facility for
   relays, upon request, to become a previous destination.  This works
   by a "bubble" packet sent to the server, passed to the client, and
   then sent by the client (through the NAT) to the originator.
   However, any host on the Internet can use this facility, not just
   relays, since any host can serve as a host-only relay.

   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.

   There may be equivalent functionality in other protocols to provide
   this service.

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4.3.3.  Recommendations

   This facility is necessary for Teredo to operate, at least in its
   current form.  Minimizing Teredo use (see Section 4.1.3) would lower
   the attacker opportunity related to this exposure.

5.  Teredo Address Concerns

5.1.  Feasibility of Guessing Teredo Addresses

5.1.1.  Problem

   It may be feasible guess Teredo addresses, either when looking for a
   specific Teredo client or when looking for an arbitrary Teredo
   client.  This is in contrast to native IPv6 address in general.

5.1.2.  Discussion

   Teredo addresses are structured and some of the fields contained them
   are fairly predictable.  This can be used to better predict the

   Teredo prefix:  This field is 32 bits and has a single IANA assigned

   Server:  This field is 32 bits and is set to the server in use.  The
      server to use is usually statically configured on the client; this
      may resolve to one or a small number of IPv4 addresses for the
      server.  Certain static configurations can be reasonably expected
      to be common (e.g., those that are the default with a Teredo
      client implementation).  This suggests that overall entropy of the
      server field will be low, i.e., that the server will not be hard
      to predict.  Attackers could confine their guessing to the most
      popular server IP addresses.

   Flags:  The flags field is 16 bits in length, but RFC 4380 provides
      for only one of these bits (the cone bit) to vary.

   Client port:  This 16 bit field corresponds to the external port
      number assigned to the client's Teredo service port.  Thus the
      value of this field depends on two factors (the chosen Teredo
      service port and the NAT port assignment behavior) and therefore
      it is harder to predict the entropy this field will have.  If
      clients tend to use a predictable port number and NATs are often
      port-preserving ([RFC4787]), then the port number can be rather

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   Client IPv4 address:  This 32 bit field corresponds to the external
      IPv4 address the NAT has assigned for the client port.  In
      principle, this can be any address in the assigned part of the
      IPv4 unicast address space.  However, if an attacker is looking
      for the address of a specific Teredo client, they will have to
      have the external IPv4 address pretty well narrowed down.  Certain
      IPv4 address ranges could also become well known for having a
      higher concentration of Teredo clients, making it easier to find
      an arbitrary Teredo client.  These addresses could correspond to
      large organizations that allows Teredo such as a university or
      enterprise or to Internet Service Providers that only provide
      their customers with RFC 1918 addresses.

   Optimizations in scanning can also reduce the number addresses that
   need to be checked.  For example, for addresses behind a cone NAT, it
   would likely be easy to probe if a specific port number is open on a
   IPv4 address, prior to trying to form a Teredo address for that
   address and port.

   Most of this is elaborated on more in [TTPTPNSOSI].

5.1.3.  Recommendations

   The Microsoft web site [MSTO] indicates that Windows Vista and
   Longhorn make additional use of the flags field, beyond what the RFC
   specifies. 12 bits (the "A" bits in "CRAAAAUG AAAAAAAA") are chosen
   at random by the client at the end of qualification.  Assuming there
   is no bias in those bit settings, then this adds 12 additional bits
   of entropy (4096 times as many addresses).  We recommend this be
   formally added to the next version of the Teredo specification.

   The other thing we can recommend is that the client chose the Teredo
   service port in as random manner as feasible, in case the NAT port
   assignment behavior is based on the internal port number.

5.2.  Profiling Targets Based on Teredo Address

5.2.1.  Problem

   An attacker encountering a Teredo address 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 Teredo address reveals some information about the nature of the
   client.  The information is reasonably reliable, even if some of it

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   is not tied to the Teredo protocol specification.

   o  That a host has a Teredo address at all means that there is a
      Teredo client implementation available for that platform.  It
      probably also means that it was installed by default and also that
      the hosts default rules for using it made it susceptible to being
      in use.  For example, as of this writing, seeing a Teredo address
      strongly suggests that the host it is on is running Windows Vista.

   o  The server field in the Teredo address also suggests some
      information.  Teredo client software most often get to the end
      user, installed, and configured using some degree of automation.
      It seems likely that the majority of the time the Teredo 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 particular means of installation, which
      often suggests the platform.  For example, if the server field in
      the Teredo address is one of the IPv4 addressees that
      teredo.ipv6.microsoft.com resolves to, that suggests that the host
      is running Windows.

   o  The external IPv4 address in a Teredo address can of course be
      readily associated with a particular organization or at least an

   o  It is also possible that external client port numbers may be more
      often associated with particular client software or the operating
      system it is running on.  The usefulness of this is reduced by the
      different NAT port number assignment behaviors, though the net
      result of this composition can not be determined without study.

   The platform, Teredo client software, or organization information can
   be used by an attacker to target attacks more carefully.  For
   example, an attacker may decide to use an address if it corresponds
   to an organization they want to penetrate.  (That example would not
   be unique to Teredo addresses, but shows that Teredo reveals the same
   information.)  An attacker or worm might also decide to use a Teredo
   address only if it looks to be associated with Windows or a certain
   version of Windows.  (This does not seem to have a strong analogue in
   native IPv4 or IPv6 addresses.)

   The cone bit tells the attacker whether a bubble is needed to proceed
   a connection.  It may also have some value in terms of profiling to
   the extent that it reveals the security posture of the network.  If
   the cone bit is set, the attacker may decide it is fruitful to port
   scan the embedded external IPv4 address and others associated with
   the same organization, looking for open ports.

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5.2.3.  Recommendations

   Deprecating the cone bit would prevent the a priori revelation of the
   security posture of the NAT and would not reduce the functionality of
   the Teredo protocol.

   If installation programs randomized the server setting, that would
   reduce the extent to which they can be profiled.  Similarly,
   administrators can chose to change the default setting to reduce the
   degree to which they can be profiled ahead of time.

   Randomizing the Teredo client port in use would mitigate any
   profiling that can be done based on the external port, especially if
   multiple different Teredo clients did this.

6.  Additional Security Concerns

6.1.  Attacks Facilitated By Changing Teredo Server Setting

6.1.1.  Problem

   Malware or a malicious user could change a Teredo client's server
   setting.  This would allow them to at least monitor peer IPv6
   addresses and at worst pretend to represent the remote peer.

6.1.2.  Discussion

   [RFC4380] documents that the Teredo server must be a trusted entity.
   However, it may be possible for malware or a malicious user to
   quietly change the Teredo client's server setting and have the user
   be unaware their trust has been misplaced for an indefinite period of

   A client's server is involved in the Direct IPv6 Connectivity Test
   and in the bubble procedure, so it has good visibility into the
   client's IPv6 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
   is being disclosed, especially if the client's host prefers Teredo
   over native IPv4.  This is not technically difficult to set up,
   especially given the availability of open source Teredo server
   implementations.  Assuming the server provides good service, the user
   would not have reason to suspect the change.

   Full interception of IPv6 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

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   phishing attack scenario.

   1.  A phisher stands up a malicious Teredo server (or tampers with a
       legitimate one).  This server, for the most part, provides
       correct service.

   2.  Some malware reaches a victim host by some means and switches the
       host's Teredo server setting to reference the above server
       (either by IPv4 address or by hostname).

   3.  A user on the victim host types their bank's URL into his/her

   4.  The bank's hostname resolves to both IPv6 and IPv4 addresses and
       the IPv6 address is selected for the socket connection.
       (Alternately, it just resolves to IPv6.)

   5.  The host is behind an IPv4 NAT so no native IPv6 or ISTAP
       connection is possible, so the Teredo interface is used.

   6.  The Teredo client uses the server for help in connecting to the
       the bank's IPv6 address.  It asks the server to pass along an
       IPv6 ping so it can determine what Teredo relay to use in sending
       packets to the bank's IPv6 address and so it knows what relay to
       trust packets from for the peer.

   7.  The malicious server recognizes the IPv6 address as belonging to
       a bank that it wants to phish against, so it sends an
       encapsulated ping reply to the client.  This is made to look like
       a legitimate reply sent via a Teredo relay; however the relay it
       is supposedly returned from is actually a phishing site.  This
       site could even be on the same host as the malicious Teredo

   8.  The rest works pretty much like any normal phishing transaction,
       except that the phishing host acts as local Teredo relay, since
       the victim host thinks it is communicating via a Teredo relay
       with the bank's IPv6 address.

   This pharming type attack is not entirely novel, switching DNS server
   settings to a malicious DNS server could have similar effect.

6.1.3.  Recommendations

   The scope of the attack can be reduced by limiting Teredo use in
   general but especially in preferring native IPv4 to Teredo-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

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   long as a significant fraction of their customers are still behind
   IPv4 NATs.

   In general, anti-phishing and anti-fraud provisions should help with
   aspects of this, as well as software that specifically monitors for
   Teredo server changes.

   On the host, it should require an appropriate level of privilege in
   order to change the Teredo server setting and we recommend that the
   user be prompted when the Teredo server setting has been changed.
   Making it easy to see the current Teredo server setting (e.g., not
   requiring privilege for this) should help detection of changes.

6.2.  RFC 4380 Implies That Teredo Improves Security

6.2.1.  Problem

   The Security Considerations section of RFC 4380 states that it can
   argued that Teredo improves security.  The above sections argue to
   the contrary.  This misleading or inaccurate claim can be taken out
   of context and used to downplay Teredo security implications.

6.2.2.  Discussion

   The "Security Considerations" section of [RFC4380] begins with:

      "The main objective of Teredo is to provide nodes located behind a
      NAT with a globally routable IPv6 address.  The Teredo nodes can
      use IP security (IPsec) services ... without the configuration
      restrictions still present in 'Negotiation of NAT-Traversal in the
      IKE' [RFC3947].  As such, we can argue that the service has a
      positive effect on network security.  However, the security
      analysis must also envisage the negative effects of the Teredo

   We agree that Teredo improves the ability to use IPsec in traversing
   a NAT and the security properties that it provides are a benefit in
   certain cases, specifically when the alternate session directly
   involves NAT translation, IPsec is desired to be used, and
   circumstances allow IPsec to be used.  In this case the nice security
   properties IPsec can provide have been allowed by Teredo.  However,
   IPsec does not solve all security problems.

   It is hoped that by this point the reader will agree that Teredo
   introduces security risk and does not improve security overall.
   Hence we feel the sentence that "the service has a positive effect on
   network security" goes to far in stating its point, even considering
   the following sentence which may somewhat reduce the pointedness of

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   the claim.  Someone may not recognize the full security impact of
   Teredo after reading the sentence.

6.2.3.  Recommendations

   We recommend that no claims regarding a positive security impact from
   Teredo be made, unless the scope of such a claim is immediately
   clear.  We also recommend that the security concerns identified in
   this document be included in an updated Teredo standard document,
   except to the extent that the Teredo protocol has been improved to
   mitigate them.

7.  Security Considerations

   This document identified security concerns with Teredo that were not
   included in RFC 4380.

8.  IANA Considerations

   There are no IANA considerations from this document.

9.  References

9.1.  Normative References

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

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

   [RFC2461]  Narten, T., Nordmark, E., and W. Simpson, "Neighbor
              Discovery for IP Version 6 (IPv6)", RFC 2461,
              December 1998.

   [RFC3489]  Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy,
              "STUN - Simple Traversal of User Datagram Protocol (UDP)
              Through Network Address Translators (NATs)", RFC 3489,
              March 2003.

   [RFC4214]  Templin, F., Gleeson, T., Talwar, M., and D. Thaler,
              "Intra-Site Automatic Tunnel Addressing Protocol
              (ISATAP)", RFC 4214, October 2005.

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   [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.

9.2.  Informative References

   [MSTO]     Microsoft, "Teredo Overview", <http://www.microsoft.com/

              Hoagland, J., "The Teredo Protocol: Tunneling Past Network
              Security and Other Security Implications", November 2006,

   [WVNASA]   Hoagland, J., Conover, M., Newsham, T., and O. Whitehouse,
              "Windows Vista Network Surface Analysis", March 2007, <htt

Authors' Addresses

   Suresh Krishnan
   8400 Decarie Blvd.
   Town of Mount Royal, QC

   Phone: +1 514 345 7900 x42871
   Email: suresh.krishnan@ericsson.com

   James Hoagland
   Symantec Corporation
   350 Ellis St.
   Mountain View, CA  94043

   Email: Jim_Hoagland@symantec.com
   URI:   http://symantec.com/

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