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Network Working Group                                         M. Bagnulo
Internet-Draft                                                      UC3M
Intended status: Experimental                            A. Andersdotter
Expires: January 9, 2020                                      Article 19
                                                               C. Paasch
                                                            July 8, 2019

 Privacy threats and possible countermeasures for Multipath-TCP (MPTCP)


   This note performs a differential analysis of the threats regarding
   privacy of the Multipath TCP protocol compared to regular TCP and
   proposes a set of countermeasures for the threats identified.

Status of This Memo

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   This Internet-Draft will expire on January 9, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Threat Analysis . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Types of attackers  . . . . . . . . . . . . . . . . . . .   3
     2.2.  Detailed attack mechanics.  . . . . . . . . . . . . . . .   4
       2.2.1.  Attacks using MP_CAPABLE and MP_JOIN. . . . . . . . .   4
       2.2.2.  Attacks using ADD_ADDR. . . . . . . . . . . . . . . .   4
   3.  Countermeasures.  . . . . . . . . . . . . . . . . . . . . . .   4
   4.  MPTCP privacy features. . . . . . . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   8.  Informative References  . . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   Multipath-TCP (MPTCP) [RFC6824] [I-D.ietf-mptcp-rfc6824bis] is a set
   of extensions to TCP that enable the use of multiple IP addresses
   throughout the lifetime of a (MP)TCP connection.  The use of multiple
   addresses in a connection allows two main uses cases, namely mobility
   and multihoming.  In the case of multihoming, if an endpoint is
   connected to the Internet through multiple interfaces simultaneously
   (each ones having a different IP address), the use of MPTCP allow
   additional fault tolerance as the connection can be preserved by
   using an alternative IP address even if the IP address originally
   used to establish the connection is rendered unavailable.  In the
   case of mobility, as an endpoint changes is attachment to the
   Internet, it acquires a new IP address associated to its new
   attachment point.  By using MPTCP, connections can be preserved
   throughout the changes of attachment points and their respective IP
   addresses by adding the new IP addresses to the ongoing MPTCP

   Because of its very nature, the operation of MPTCP presents privacy
   implications, as other protocols that bind multiple IP addresses to a
   given endpoint [I-D.nordmark-id-loc-privacy].  Because MPTCP
   explicitly associated multiple IP addresses to a given connection and
   hence to a given endpoint, it discloses information about the node
   whereabouts to third parties.  In this note, we perform an analysis
   of the privacy implications of the operation of the MPTCP compared to
   regular TCP and we provide a set of countermeasures to address the
   identified threats.  It is out of the scope of this document to
   identify privacy threats that equally affect both MPTCP and TCP, such

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   as the ones resulting from exchanging unencrypted data that can be
   observed by eavesdroppers along the path.  As mentioned earlier, we
   only identify threats against privacy introduced by MPTCP which are
   not present in TCP.

2.  Threat Analysis

   The threats concerning privacy of the use of MPTCP are essentially

      Movement tracking: In the case that MPTCP is used for mobility,
      the use of multiple addresses in the same MPTCP connection can be
      used by an attacker to track the movement of the victim.  Since IP
      addresses can be related to location (in a more or less accurate
      manner), by observing the different addresses used in a MPTCP
      connection, the attacker can track the itinerary of the victim.

      More accurate positioning.  If multiple address are used
      simultaneously in a MPTCP connection, this implies that the
      endpoint is connected at the multiple attachment points at the
      same time, potentially providing the means for a more accurate
      positioning of the endpoint (e.g. if an endpoint is exposing the
      IP address of the cellular access it is providing less information
      than when it also exposes the IP address of an Internet coffee
      wifi access).

2.1.  Types of attackers

   We classify the types of attackers based on their topological
   location, which determines the amount of information they have access
   to. the different types of attackers are:

      Partially on-path attacker.  This attacker is located along one or
      more, but not all the paths used to exchange MPTCP signaling
      information.  As such, it is able to see some but not all the
      MPTCP packets.

      Full On-path attacker.  This attacker is able to eavesdrop all the
      MPTCP signaling message exchange, but it is not the other endpoint
      of the information.

      Endpoint: in this case, the other endpoint of the MPTCP connection
      is the attacker (in the sense that it will use the information
      revealed through MPTCP for other purposes beyond the MPTCP
      operation e.g. the endpoint may decide to sell the location and
      tracking information of the MPTCP endpoints to third parties).

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2.2.  Detailed attack mechanics.

2.2.1.  Attacks using MP_CAPABLE and MP_JOIN.

   A MPTCP endpoint initiates a MPTCP connection by including the
   MP_CAPABLE option in the SYN message.  After that, it uses the
   MP_JOIN option to add subsequent subflows using other IP address to
   the existent connection.  The MP_JOIN message include a token that is
   used by the MPTCP receiver to identify which of the ongoing MPTCP
   connections this particular subflow is being added to.  In order for
   an attacker to bind the different address together, it must be able
   to observe at least two messages carrying two different addresses.
   In particular, by observing the initial MP_CAPABLE SYN and a
   following MP_JOIN message, the attacker can relate these two IP
   addresses.  Also, by observing two MP_JOIN messages carrying
   different IP addresses but the same toke, the attacker can also
   relate the two IP addresses together.

   This attack can be executed by any attacker that is capable of
   observing the different MP_CAPABLE and MP_JOIN messages.  So, for a
   partially on-path attacker, the attack will be as effective as the
   number of used path the attacker has access to.  If it only has
   access to one path, the attacker would not gather any information.
   Both full on-path attackers and the endpoint would have access to all
   the information, so the attack effectiveness is complete.

   Both versions of MPTCP, i.e. [RFC6824] [I-D.ietf-mptcp-rfc6824bis]
   are equally affected by this attack.

2.2.2.  Attacks using ADD_ADDR.

   The ADD_ADDR option allows the sender of the message to add an IP
   address to the existing connection.  From a privacy perspective, the
   packet containing the ADD_ADDR information already discloses a
   binding between two addresses, the address used a source address of
   the packet and the address included in the ADD_ADDR option.  This
   attack can be performed by any attacker who is able to observe the
   message, including partial and full on-path attackers and the
   endpoint itself.  This attack can be combined with the attack done
   using MP_CAPABLE AND MP_JOIN messages described in the previous
   section, to retrieve a larger set of addresses.  This attack affects
   both version [RFC6824] [I-D.ietf-mptcp-rfc6824bis] of MPTCP.

3.  Countermeasures.

   It is possible to design countermeasures to prevent the described

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   ADD_ADDR attack.

   In order to prevent the ADD_ADDR based attack, it would be possible
   to encrypt the address carried in the ADD_ADDR message, for example
   with the key exchanged in the MP_CAPABLE exchange.  By doing this,
   only the attackers who have observed the initial MP_CAPABLE message
   would be able to decrypt the content of the ADD_ADDR message,
   significantly limiting the attack surface.


   In order to prevent the MP_CAPABLE/MP_JOIN attack, it would necessary
   to change the token in every MP_JOIN message.  the difficulty with
   this of course is that the token is used a key to identify which
   MPTCP connection this new subflow belongs to.  Using different tokens
   would be possible as long as the receiver would be able to decrypt it
   and find the ongoing connection that this new subflow belongs to.

   For instance, the token could be the hash of the concatenation of a
   trail of n zeros, the key and the new IP address of the flow.  This
   token would change with every new subflow, since the IP address would
   change (we could also add the source port, to support the case of
   multiple subflows with the same source IP address).  Upon the
   reception of an MP_JOIN message, the receiver would need to try with
   all the keys of ongoing connections.  It will know it has succeeded,
   because the correct one will result in a trail of n zeros.  The
   problem with this mechanism is that it imposes an additional cost in
   terms of computation upon the establishment of a new subflow.

   Additional countermeasures could be int he form of a recommendation
   about when to establish a new subflow or when to announce new
   addresses using ADD_ADDR.  Generating awareness that doing so
   discloses private information of the endpoint would make
   implementations more conservative when advertising IP addresses.

4.  MPTCP privacy features.

   MPTCP also provides some positive side effects with regard to
   privacy.  In particular, because the information is spread across
   multiple paths, in order to be able to eavesdrop all the content of a
   MPTCP connection, the attacker needs to be present in all used paths,
   making more challenging for the attacker to fulfill its goal.

5.  Security Considerations

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6.  IANA Considerations

7.  Acknowledgements

   This work was supported by the Spanish Ministry of Economy and
   Competitiveness through the 5G-City project (TEC2016-76795-C6-3-R).

8.  Informative References

              Ford, A., Raiciu, C., Handley, M., Bonaventure, O., and C.
              Paasch, "TCP Extensions for Multipath Operation with
              Multiple Addresses", draft-ietf-mptcp-rfc6824bis-18 (work
              in progress), June 2019.

              Nordmark, E., "Privacy issues in ID/locator separation
              systems", draft-nordmark-id-loc-privacy-00 (work in
              progress), July 2018.

   [RFC6824]  Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
              "TCP Extensions for Multipath Operation with Multiple
              Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013,

Authors' Addresses

   Marcelo Bagnulo

   Email: marcelo@it.uc3m.es

   Amelia Andersdotter
   Article 19

   Email: amelia@article19.org

   Christoph Paasch

   Email: cpaasch@apple.com

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