Network Working Group P. Saint-Andre
Internet-Draft &yet
Updates: 6120 (if approved) T. Alkemade
Intended status: Standards Track
Expires: May 15, 2015 November 11, 2014
Use of Transport Layer Security (TLS) in the Extensible Messaging and
Presence Protocol (XMPP)
draft-ietf-uta-xmpp-03
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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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. Unauthenticated Connections . . . . . . . . . . . . . . . 4
3.6. Server Name Indication . . . . . . . . . . . . . . . . . 4
3.7. Human Factors . . . . . . . . . . . . . . . . . . . . . . 4
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
6.1. Normative References . . . . . . . . . . . . . . . . . . 5
6.2. Informative References . . . . . . . . . . . . . . . . . 6
Appendix A. Implementation Notes . . . . . . . . . . . . . . . . 7
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
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.
NOTE: Unless explicitly noted otherwise, all of the
recommendations specified in [I-D.ietf-uta-tls-bcp] apply to XMPP.
In the main, this document merely provides supplementary
information; those who implement and deploy XMPP technologies are
expected to follow the recommendations of [I-D.ietf-uta-tls-bcp].
This document updates [RFC6120].
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
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 . Although in general this stream feature indicates that
the server supports XMPP 1.0 and therefore supports TLS, it is
possible that this stream feature might be stripped out by an
attacker (see Section 2.1 of [I-D.ietf-uta-tls-attacks]). Therefore,
the initiating entity SHOULD proceed with the stream negotiation even
if the receiving entity does not advertise support for TLS.
Similarly, although a receiving entity SHOULD include the
child element to indicate that negotiation of TLS is mandatory, an
initiating entity MUST NOT depend on receiving the flag
in determining whether TLS will be enforced for the stream.
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
Use of session IDs [RFC5246] is RECOMMENDED instead of session
tickets [RFC5077], since XMPP does not in general use state
management technologies such as tickets or "cookies" [RFC6265].
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. 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.
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3.5. Unauthenticated Connections
Given the pervasiveness of passive eavesdropping, even an
unauthenticated connection might be better than an unencrypted
connection (this is similar to the "better than nothing security"
approach for IPsec [RFC5386]). In particular, because of current
deployment challenges for authenticated connections between XMPP
servers (see [I-D.ietf-xmpp-dna] and [I-D.ietf-xmpp-posh] for
details), it might be reasonable for XMPP server implementations to
accept unauthenticated connections when the Server Dialback protocol
[XEP-0220] is used for weak identity verification; this will at least
enable encryption of server-to-server connections. Unauthenticated
connections include connections negotiated using anonymous Diffie-
Hellman algorithms or using self-signed certificates, among other
scenarios.
3.6. 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].
3.7. 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 client-to-server or server-to-server connection is
encrypted and authenticated.
o Determine the version of TLS used for a client-to-server or
server-to-server connection.
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.
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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 (sending 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
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.
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-07 (work in progress), November 2014.
[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.
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, January 2008.
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[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-uta-tls-attacks]
Sheffer, Y., Holz, R., and P. Saint-Andre, "Summarizing
Current Attacks on TLS and DTLS", draft-ietf-uta-tls-
attacks-05 (work in progress), October 2014.
[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-08 (work in progress),
October 2014.
[I-D.ietf-xmpp-posh]
Miller, M. and P. Saint-Andre, "PKIX over Secure HTTP
(POSH)", draft-ietf-xmpp-posh-02 (work in progress),
October 2014.
[RFC3920] Saint-Andre, P., Ed., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 3920, October 2004.
[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.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
April 2011.
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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.
Authors' Addresses
Peter Saint-Andre
&yet
Email: peter@andyet.com
URI: https://andyet.com/
Thijs Alkemade
Email: me@thijsalkema.de
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