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RFC 7376 - Problems with Session Traversal Utilities for NAT (ST

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Internet Engineering Task Force (IETF)                          T. Reddy
Request for Comments: 7376                               R. Ravindranath
Category: Informational                                            Cisco
ISSN: 2070-1721                                               M. Perumal
                                                                A. Yegin
                                                          September 2014

        Problems with Session Traversal Utilities for NAT (STUN)
 Long-Term Authentication for Traversal Using Relays around NAT (TURN)


   This document discusses some of the security problems and practical
   problems with the current Session Traversal Utilities for NAT (STUN)
   authentication for Traversal Using Relays around NAT (TURN) messages.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Notational Conventions  . . . . . . . . . . . . . . . . . . .   4
   3.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Problems with STUN Long-Term Authentication for TURN  . . . .   4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   Traversal Using Relays around NAT (TURN) [RFC5766] is a protocol that
   is often used to improve the connectivity of Peer-to-Peer (P2P)
   applications (as defined in Section 2.7 of [RFC5128]).  TURN allows a
   connection to be established when one or both sides are incapable of
   a direct P2P connection.  The TURN server is also a building block to
   support interactive, real-time communication using audio, video,
   collaboration, games, etc., between two peer web browsers using the
   Web Real-Time Communication (WebRTC) [WebRTC-Overview] framework.

   A TURN server is also used in the following scenarios:

   o  For privacy, users of WebRTC-based web applications may use a TURN
      server to hide host candidate addresses from the remote peer.

   o  Enterprise networks deploy firewalls that typically block UDP
      traffic.  When SIP user agents or WebRTC endpoints are deployed
      behind such firewalls, media cannot be sent over UDP across the
      firewall but must instead be sent using TCP (which causes a

      different user experience).  In such cases, a TURN server deployed
      in the DeMilitarized Zone (DMZ) might be used to traverse

   o  The use case explained in Section 3.3.5 of [WebRTC-Use-Cases]
      ("Simple Video Communication Service, enterprise aspects") refers
      to deploying a TURN server in the DMZ to audit all media sessions
      from inside an Enterprise premises to any external peer.

   o  A TURN server could also be deployed for RTP Mobility
      [TURN-Mobility], etc.

   o  A TURN server may be used for IPv4-to-IPv6, IPv6-to-IPv6, and
      IPv6-to-IPv4 relaying [RFC6156].

   o  Interactive Connectivity Establishment (ICE) [RFC5245]
      connectivity checks using server reflexive candidates could fail
      when the endpoint is behind a NAT [RFC3235] that performs address-
      dependent mapping as described in Section 4.1 of [RFC4787].  In
      such cases, a relayed candidate allocated from the TURN server is
      used for media.

   Session Traversal Utilities for NAT (STUN) [RFC5389] specifies an
   authentication mechanism called the long-term credential mechanism.
   Section 4 of TURN [RFC5766] specifies that TURN servers and clients
   must implement this mechanism; Section 4 of TURN [RFC5766] also
   specifies that the TURN server must demand that all requests from the
   client be authenticated using this mechanism or that an equally
   strong or stronger mechanism for client authentication be used.

   In the above scenarios, applications would use the ICE protocol for
   gathering candidates.  An ICE agent can use TURN to learn server
   reflexive and relayed candidates.  If the TURN server requires that
   the TURN request be authenticated, then the ICE agent will use the
   long-term credential mechanism explained in Section 10 of [RFC5389]
   for authentication and message integrity.  Section 10 of the TURN
   specification [RFC5766] explains the importance of the long-term
   credential mechanism to mitigate various attacks.  Client
   authentication is essential to prevent unauthorized users from
   accessing the TURN server, and misuse of credentials could impose
   significant cost on the victim TURN server.

   This document focuses on listing security problems and practical
   problems with current STUN authentication for TURN so that it can
   serve as the basis for stronger authentication mechanisms.

   An Allocate request is more likely than a Binding request to be
   identified by a server administrator as needing client authentication
   and integrity protection of messages exchanged.  Hence, the issues
   discussed here regarding STUN authentication are applicable mainly in
   the context of TURN messages.

2.  Notational Conventions

   This document uses terminology defined in [RFC5389] and [RFC5766].

3.  Scope

   This document can be used as input for designing solution(s) to
   address problems with the current STUN authentication for TURN

4.  Problems with STUN Long-Term Authentication for TURN

   1.  As described in [RFC5389], the long-term credential mechanism
       could provide to users a long-term credential in the form of a
       traditional "log-in" username and password; this credential would
       not change for extended periods of time.  The key derived from
       the user credentials would be used to provide message integrity
       for every TURN request/response.  However, an attacker that is
       capable of eavesdropping on a message exchange between a client
       and server can determine the password by trying a number of
       candidate passwords and checking to see if one of them is correct
       by calculating the message integrity using these candidate
       passwords and comparing them with the message integrity value in
       the MESSAGE-INTEGRITY attribute.

   2.  When a TURN server is deployed in the DMZ and requires that
       requests be authenticated using the long-term credential
       mechanism as described in [RFC5389], the TURN server needs to be
       aware of the username and password to validate the message
       integrity of the requests and to provide message integrity for
       responses.  This results in management overhead on the TURN
       server.  Long-term credentials (username, realm, and password)
       need to be stored on the server side, using an MD5 hash over the
       credentials, which is not considered best current practice.
       [RFC6151] discusses security vulnerabilities of MD5 and
       encourages implementers not to use it.  It is not possible to use
       STUN long-term credentials in implementations that are compliant
       with US FIPS 140-2 [FIPS-140-2], since MD5 isn't an approved

   3.  The long-term credential mechanism discussed in [RFC5389]
       specifies that the TURN client must include a username value in
       the USERNAME STUN attribute.  An adversary snooping the TURN
       messages between the TURN client and server can identify the
       users involved in the call, resulting in privacy leakage.  If
       TURN usernames are linked to real usernames, then privacy leakage
       will result, but in certain scenarios TURN usernames need not be
       linked to any real usernames given to users, as the usernames are
       just provisioned on a per-company basis.

   4.  STUN authentication relies on HMAC-SHA1 [RFC2104].  There is no
       mechanism for hash agility in the protocol itself, although
       Section 16.3 of [RFC5389] does discuss a plan for migrating to a
       more secure algorithm in case HMAC-SHA1 is found to be

   5.  A man-in-the middle attacker posing as a TURN server challenges
       the client to authenticate, learns the USERNAME of the client,
       and later snoops the traffic from the client, thereby identifying
       user activity; this results in privacy leakage.

   6.  Hosting multiple realms on a single IP address is challenging
       with TURN.  When a TURN server needs to send the REALM attribute
       in response to an unauthenticated request, it has no useful
       information for determining which realm it should send in the
       response, except the source transport address of the TURN
       request.  Note that this is a problem with multi-tenant scenarios
       only; this may not be a problem when the TURN server is located
       in enterprise premises.

   7.  In WebRTC, the JavaScript code needs to know the username and
       password to use in the W3C RTCPeerConnection API to access the
       TURN server.  This exposes user credentials to the JavaScript
       code, which could be malicious; the malicious JavaScript code
       could then misuse or leak the credentials.  If the credentials
       happen to be used for accessing services other than TURN, then
       the security implications are much larger.

5.  Security Considerations

   This document lists problems with current STUN authentication for
   TURN so that it can serve as the basis for stronger authentication

6.  References

6.1.  Normative References

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              October 2008, <http://www.rfc-editor.org/info/rfc5389>.

   [RFC5766]  Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
              Relays around NAT (TURN): Relay Extensions to Session
              Traversal Utilities for NAT (STUN)", RFC 5766, April 2010,

   [RFC6156]  Camarillo, G., Novo, O., and S. Perreault, "Traversal
              Using Relays around NAT (TURN) Extension for IPv6", RFC
              6156, April 2011,

6.2.  Informative References

              NIST, "Security Requirements for Cryptographic Modules",
              FIPS PUB 140-2, May 2001, <http://csrc.nist.gov/

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104, February
              1997, <http://www.rfc-editor.org/info/rfc2104>.

   [RFC3235]  Senie, D., "Network Address Translator (NAT)-Friendly
              Application Design Guidelines", RFC 3235, January 2002,

   [RFC4787]  Audet, F. and C. Jennings, "Network Address Translation
              (NAT) Behavioral Requirements for Unicast UDP", BCP 127,
              RFC 4787, January 2007,

   [RFC5128]  Srisuresh, P., Ford, B., and D. Kegel, "State of Peer-to-
              Peer (P2P) Communication across Network Address
              Translators (NATs)", RFC 5128, March 2008,

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245, April
              2010, <http://www.rfc-editor.org/info/rfc5245>.

   [RFC6151]  Turner, S. and L. Chen, "Updated Security Considerations
              for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
              RFC 6151, March 2011,

              Wing, D., Patil, P., Reddy, T., and P. Martinsen,
              "Mobility with TURN", Work in Progress, draft-wing-tram-
              turn-mobility-02, September 2014.

              Alvestrand, H., "Overview: Real Time Protocols for
              Browser-based Applications", Work in Progress, draft-ietf-
              rtcweb-overview-11, August 2014.

              Holmberg, C., Hakansson, S., and G. Eriksson, "Web Real-
              Time Communication Use-cases and Requirements", Work in
              Progress, draft-ietf-rtcweb-use-cases-and-requirements-14,
              February 2014.


   The authors would like to thank Dan Wing, Harald Alvestrand, Sandeep
   Rao, Prashanth Patil, Pal Martinsen, Marc Petit-Huguenin, Gonzalo
   Camarillo, Brian E.  Carpenter, Spencer Dawkins, Adrian Farrel, and
   Simon Perreault for their comments and reviews.

Authors' Addresses

   Tirumaleswar Reddy
   Cisco Systems, Inc.
   Cessna Business Park, Varthur Hobli
   Sarjapur Marathalli Outer Ring Road
   Bangalore, Karnataka  560103

   EMail: tireddy@cisco.com

   Ram Mohan Ravindranath
   Cisco Systems, Inc.
   Cessna Business Park, Varthur Hobli
   Sarjapur Marathalli Outer Ring Road
   Bangalore, Karnataka  560103

   EMail: rmohanr@cisco.com

   Muthu Arul Mozhi Perumal
   Ferns Icon
   Doddanekundi, Mahadevapura
   Bangalore, Karnataka  560037

   EMail: muthu.arul@gmail.com

   Alper Yegin

   EMail: alper.yegin@yegin.org


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