Use of Transport Layer Security (TLS) in the Extensible Messaging and Presence Protocol (XMPP)
draft-ietf-uta-xmpp-06

Network Working Group                                     P. Saint-Andre
Internet-Draft                                                      &yet
Updates: 6120 (if approved)                                  T. Alkemade
Intended status: Standards Track
Expires: October 16, 2015                                 April 14, 2015

 Use of Transport Layer Security (TLS) in the Extensible Messaging and
                        Presence Protocol (XMPP)
                         draft-ietf-uta-xmpp-06

Abstract

   This document provides recommendations for the use of Transport Layer
   Security (TLS) in the Extensible Messaging and Presence Protocol
   (XMPP).  This document updates RFC 6120.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on October 16, 2015.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Recommendations . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Support for TLS . . . . . . . . . . . . . . . . . . . . .   3
     3.2.  Compression . . . . . . . . . . . . . . . . . . . . . . .   3
     3.3.  Session Resumption  . . . . . . . . . . . . . . . . . . .   3
     3.4.  Authenticated Connections . . . . . . . . . . . . . . . .   3
     3.5.  Server Name Indication  . . . . . . . . . . . . . . . . .   4
     3.6.  Human Factors . . . . . . . . . . . . . . . . . . . . . .   5
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Appendix A.  Implementation Notes . . . . . . . . . . . . . . . .   8
   Appendix B.  Acknowledgements . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   The Extensible Messaging and Presence Protocol (XMPP) [RFC6120]
   (along with its precursor, the so-called "Jabber protocol") has used
   Transport Layer Security (TLS) [RFC5246] (along with its precursor,
   Secure Sockets Layer or SSL) since 1999.  Both [RFC6120] and its
   predecessor [RFC3920] provided recommendations regarding the use of
   TLS in XMPP.  In order to address the evolving threat model on the
   Internet today, this document provides stronger recommendations.

   In particular, this document updates [RFC6120] by specifying that
   XMPP implementations and deployments MUST follow the best current
   practices documented in the "Recommendations for Secure Use of TLS
   and DTLS" [I-D.ietf-uta-tls-bcp].  This includes stronger
   recommendations regarding SSL/TLS protocol versions, fallback to
   lower versions, TLS-layer compression, TLS session resumption, cipher
   suites, public key lengths, forward secrecy, and other aspects of
   using TLS with XMPP.

2.  Terminology

   Various security-related terms are to be understood in the sense
   defined in [RFC4949].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

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3.  Recommendations

   The best current practices documented in the "Recommendations for
   Secure Use of TLS and DTLS" [I-D.ietf-uta-tls-bcp] are included here
   by reference.  Instead of repeating those recommendations here, this
   document mostly provides supplementary information regarding secure
   implementation and deployment of XMPP technologies.

3.1.  Support for TLS

   Support for TLS (specifically, the XMPP profile of STARTTLS) is
   mandatory for XMPP implementations, as already specified in [RFC6120]
   and its predecessor [RFC3920].

   The server (i.e., the XMPP receiving entity) to which a client or
   peer server (i.e., the XMPP initiating entity) connects might not
   offer a stream feature of <starttls xmlns='urn:ietf:params:xml:ns
   :xmpp-tls'/>.  Although in general this stream feature indicates that
   the server supports XMPP 1.0 and therefore supports TLS, that this
   stream feature might be stripped out by an attacker (see Section 2.1
   of [RFC7457]).  Similarly, the <required/> child element of the
   <starttls/> stream feature is used to indicate that negotiation of
   TLS is mandatory, but could also be stripped out by an attacker.
   Therefore, the initiating entity MUST NOT be deterred from attempting
   TLS negotiation even if the receiving entity does not advertise
   support for TLS.  Instead, the initiating entity SHOULD (based on
   local policy) proceed with the stream negotiation and attempt to
   negotiate TLS.

3.2.  Compression

   XMPP supports an application-layer compression technology [XEP-0138].
   Although this XMPP extension might have slightly stronger security
   properties than TLS-layer compression (since it is enabled after SASL
   authentication, as described in [XEP-0170]), this document neither
   encourages nor discourages use of XMPP-layer compression.

3.3.  Session Resumption

   In XMPP, TLS session resumption can be used in concert with the XMPP
   Stream Management extension; see [XEP-0198] for further details.

3.4.  Authenticated Connections

   Both the core XMPP specification [RFC6120] and the "CertID"
   specification [RFC6125] provide recommendations and requirements for
   certificate validation in the context of authenticated connections.
   This document does not supersede those specifications (e.g., it does

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   not modify the recommendations in [RFC6120] regarding the Subject
   Alternative Names or other certificate details that need to be
   supported for authentication of XMPP connections using PKIX
   certificates).

   Wherever possible, it is best to prefer authenticated connections
   (along with SASL [RFC4422]), as already stated in the core XMPP
   specification [RFC6120].  In particular, clients MUST authenticate
   servers and servers MUST authenticate clients.

   This document does not mandate that servers need to authenticate peer
   servers, although such authentication is strongly preferred and
   servers SHOULD authenticate each other.  Unfortunately, in multi-
   tenanted environments it can be extremely difficult to obtain and
   deploy PKIX certificates with the proper Subject Alternative Names
   (see [I-D.ietf-xmpp-dna] and [I-D.ietf-xmpp-posh] for details).  To
   overcome that difficulty, the Domain Name Associations (DNA)
   specification [I-D.ietf-xmpp-dna] describes a framework for XMPP
   server authentication methods, which include not only PKIX but also
   DNS-Based Authentication of Named Entities (DANE) as defined in
   [I-D.ietf-dane-srv] and PKIX over Secure HTTP (POSH) as defined in
   [I-D.ietf-xmpp-posh].  These methods can provide a basis for server
   identity verification when appropriate PKIX certificates cannot be
   obtained and deployed.

   Given the pervasiveness of eavesdropping [RFC7258], even an
   unauthenticated connection might be better than an unencrypted
   connection in these scenarios (this is similar to the "better than
   nothing security" approach for IPsec [RFC5386]).  Unauthenticated
   connections include connections negotiated using anonymous Diffie-
   Hellman mechanisms or using self-signed certificates, among others.
   In particular for XMPP server-to-server interactions, it can be
   reasonable for XMPP server implementations to accept unauthenticated
   but encrypted connections when Server Dialback keys [XEP-0220] are
   used; such keys on their own provide only weak identity verification
   (made stronger through the use of DNSSEC [RFC4033]), but this at
   least enables encryption of server-to-server connections.  The DNA
   prooftypes described above are intended to mitigate the residual need
   for unauthenticated connections in these scenarios.

3.5.  Server Name Indication

   Although there is no harm in supporting the TLS Server Name
   Indication (SNI) extension [RFC6066], this is not necessary since the
   same function is served in XMPP by the 'to' address of the initial
   stream header as explained in Section 4.7.2 of [RFC6120].

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3.6.  Human Factors

   It is strongly encouraged that XMPP clients provide ways for end
   users (and that XMPP servers provide ways for administrators) to
   complete the following tasks:

   o  Determine if a given incoming or outgoing XML stream is encrypted
      using TLS.

   o  Determine the version of TLS used for encryption of a given
      stream.

   o  If authenticated encryption is used, determine how the connection
      was authenticated or verified (e.g., via PKI, DANE, POSH, or
      Server Dialback).

   o  Inspect the certificate offered by an XMPP server.

   o  Determine the cipher suite used to encrypt a connection.

   o  Be warned if the certificate changes for a given server.

4.  IANA Considerations

   This document requests no actions of the IANA.

5.  Security Considerations

   The use of TLS can help limit the information available for
   correlation to the network and transport layer headers as opposed to
   the application layer.  As typically deployed, XMPP technologies do
   not leave application-layer routing data (such as XMPP 'to' and
   'from' addresses) at rest on intermediate systems, since there is
   only one hop between any two given XMPP servers.  As a result,
   encrypting all hops (sender's client to sender's server, sender's
   server to recipient's server, recipient's server to recipient's
   client) can help to limit the amount of "metadata" that might leak.

   It is possible that XMPP servers themselves might be compromised.  In
   that case, per-hop encryption would not protect XMPP communications,
   and even end-to-end encryption of (parts of) XMPP stanza payloads
   would leave addressing information and XMPP roster data in the clear.
   By the same token, it is possible that XMPP clients (or the end-user
   devices on which such clients are installed) could also be
   compromised, leaving users utterly at the mercy of an adversary.

   This document and related actions to strengthen the security of the
   XMPP network are based on the assumption that XMPP servers and

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   clients have not been subject to widespread compromise.  If this
   assumption is valid, then ubiquitous use of per-hop TLS channel
   encryption and more significant deployment of end-to-end object
   encryption technologies will serve to protect XMPP communications to
   a measurable degree, compared to the alternatives.

   This document covers only communication over the XMPP network and
   does not take into account gateways to non-XMPP networks.  As an
   example, for security considerations related to gateways between XMPP
   and the Session Initiation Protocol (SIP) see [RFC7247] and
   [I-D.ietf-stox-im].

6.  References

6.1.  Normative References

   [I-D.ietf-uta-tls-bcp]
              Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of TLS and DTLS", draft-
              ietf-uta-tls-bcp-11 (work in progress), February 2015.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2", RFC
              4949, August 2007.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 6120, March 2011.

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, March 2011.

6.2.  Informative References

   [I-D.ietf-dane-srv]
              Finch, T., Miller, M., and P. Saint-Andre, "Using DNS-
              Based Authentication of Named Entities (DANE) TLSA records
              with SRV and MX records.", draft-ietf-dane-srv-12 (work in
              progress), March 2015.

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   [I-D.ietf-stox-im]
              Saint-Andre, P., Houri, A., and J. Hildebrand,
              "Interworking between the Session Initiation Protocol
              (SIP) and the Extensible Messaging and Presence Protocol
              (XMPP): Instant Messaging", draft-ietf-stox-im-13 (work in
              progress), March 2015.

   [I-D.ietf-xmpp-dna]
              Saint-Andre, P. and M. Miller, "Domain Name Associations
              (DNA) in the Extensible Messaging and Presence Protocol
              (XMPP)", draft-ietf-xmpp-dna-10 (work in progress), March
              2015.

   [I-D.ietf-xmpp-posh]
              Miller, M. and P. Saint-Andre, "PKIX over Secure HTTP
              (POSH)", draft-ietf-xmpp-posh-04 (work in progress),
              February 2015.

   [RFC3920]  Saint-Andre, P., Ed., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 3920, October 2004.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements", RFC
              4033, March 2005.

   [RFC4422]  Melnikov, A. and K. Zeilenga, "Simple Authentication and
              Security Layer (SASL)", RFC 4422, June 2006.

   [RFC5386]  Williams, N. and M. Richardson, "Better-Than-Nothing
              Security: An Unauthenticated Mode of IPsec", RFC 5386,
              November 2008.

   [RFC6066]  Eastlake, D., "Transport Layer Security (TLS) Extensions:
              Extension Definitions", RFC 6066, January 2011.

   [RFC7247]  Saint-Andre, P., Houri, A., and J. Hildebrand,
              "Interworking between the Session Initiation Protocol
              (SIP) and the Extensible Messaging and Presence Protocol
              (XMPP): Architecture, Addresses, and Error Handling", RFC
              7247, May 2014.

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, May 2014.

   [RFC7457]  Sheffer, Y., Holz, R., and P. Saint-Andre, "Summarizing
              Known Attacks on Transport Layer Security (TLS) and
              Datagram TLS (DTLS)", RFC 7457, February 2015.

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   [XEP-0138]
              Hildebrand, J. and P. Saint-Andre, "Stream Compression",
              XSF XEP 0138, May 2009.

   [XEP-0170]
              Saint-Andre, P., "Recommended Order of Stream Feature
              Negotiation", XSF XEP 0170, January 2007.

   [XEP-0198]
              Karneges, J., Saint-Andre, P., Hildebrand, J., Forno, F.,
              Cridland, D., and M. Wild, "Stream Management", XSF XEP
              0198, June 2011.

   [XEP-0220]
              Miller, J., Saint-Andre, P., and P. Hancke, "Server
              Dialback", XSF XEP 0220, September 2013.

Appendix A.  Implementation Notes

   Some governments enforce legislation prohibiting the export of strong
   cryptographic technologies.  Nothing in this document ought to be
   taken as advice to violate such prohibitions.

Appendix B.  Acknowledgements

   The authors would like to thank the following individuals for their
   input: Dave Cridland, Philipp Hancke, Olle Johansson, Steve Kille,
   Tobias Markmann, Matt Miller, and Rene Treffer.

   Roni Even caught several important issues in his review on behalf of
   the General Area Review Team.

   Thanks to Leif Johansson and Orit Levin as chairs of the UTA WG, Ben
   Campbell and Joe Hildebrand as chairs of the XMPP WG, and Stephen
   Farrell as the sponsoring Area Director.

Authors' Addresses

   Peter Saint-Andre
   &yet

   Email: peter@andyet.com
   URI:   https://andyet.com/

   Thijs Alkemade

   Email: me@thijsalkema.de

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