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IETF RFC 7590
Last modified on Saturday, June 20th, 2015
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Internet Engineering Task Force (IETF) P. Saint-Andre
Request for Comments: 7590 &yet
Updates: 6120 T. Alkemade
Category: Standards Track June 2015
ISSN: 2070-1721
Use of Transport Layer Security (TLS) in the
Extensible Messaging and Presence Protocol (XMPP)
 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
This is an Internet Standards Track document.
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). Further information on
Internet Standards is available in 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
http://www.rfc-editor.org/info/RFC 7590.
Copyright Notice
Copyright (c) 2015 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.
Saint-Andre & Alkemade Standards Track PAGE 1 
RFC 7590 XMPP TLS June 2015
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 . . . . . . . . . . . . . . . . 4
3.5. Server Name Indication . . . . . . . . . . . . . . . . . 5
3.6. Human Factors . . . . . . . . . . . . . . . . . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . 5
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.1. Normative References . . . . . . . . . . . . . . . . . . 6
5.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. Implementation Notes . . . . . . . . . . . . . . . . 9
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
The Extensible Messaging and Presence Protocol (XMPP) [RFC 6120]
(along with its precursor, the so-called "Jabber protocol") has used
Transport Layer Security (TLS) [RFC 5246] (along with its precursor,
Secure Sockets Layer or SSL) since 1999. Both [RFC 6120] and its
predecessor [RFC 3920] 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 [RFC 6120] by specifying that
XMPP implementations and deployments MUST follow the best current
practices documented in the "Recommendations for Secure Use of TLS
and DTLS" [RFC 7525]. 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 [RFC 4949].
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
[RFC 2119].
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RFC 7590 XMPP TLS June 2015
3. Recommendations
The best current practices documented in the "Recommendations for
Secure Use of TLS and DTLS" [RFC 7525] 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 [RFC 6120]
and its predecessor [RFC 3920].
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 and offers TLS, this stream feature might be
stripped out by an attacker (see Section 2.1 of [RFC 7457]).
Similarly, the <required/> child element of the <starttls/> stream
feature is used to indicate that negotiation of TLS is mandatory;
however, this 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
Simple Authentication and Security Layer (SASL) authentication, as
described in [XEP-0170]), this document neither encourages nor
discourages use of XMPP-layer compression.
3.3. Session Resumption
To improve the reliability of communications over XMPP, it is common
practice for clients and servers to implement the stream management
extension [XEP-0198]. Although that specification includes a method
for resumption of XMPP streams at the application layer, also using
session resumption at the TLS layer further optimizes the overall
process of resuming an XMPP session (see [XEP-0198] for detailed
information). Whether or not XEP-0198 is used for application-layer
Saint-Andre & Alkemade Standards Track PAGE 3
RFC 7590 XMPP TLS June 2015
session resumption, implementations MUST follow the recommendations
provided in [RFC 7525] regarding TLS-layer session resumption.
3.4. Authenticated Connections
Both the core XMPP specification [RFC 6120] and the CertID
specification [RFC 6125] provide recommendations and requirements for
certificate validation in the context of authenticated connections.
This document does not supersede those specifications (e.g., it does
not modify the recommendations in [RFC 6120] 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 [RFC 4422]), as already stated in the core XMPP
specification [RFC 6120]. In particular:
o Clients MUST authenticate servers.
o Servers MUST authenticate clients.
o Servers SHOULD authenticate other servers.
This document does not mandate that servers need to authenticate peer
servers, although such authentication is strongly preferred.
Unfortunately, in multi-tenanted environments it can be extremely
difficult to obtain and deploy PKIX certificates with the proper
Subject Alternative Names (see [XMPP-DNA] and [PKIX-POSH] for
details). To overcome that difficulty, the Domain Name Associations
(DNAs) specification [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
[DANE-SRV] and PKIX over Secure HTTP (POSH) as defined in
[PKIX-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 [RFC 7258], even an encrypted
but unauthenticated connection might be better than an unencrypted
connection in these scenarios (this is similar to the "better-than-
nothing security" approach for IPsec [RFC 5386]). Encrypted but
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 encrypted but unauthenticated connections when Server Dialback
keys [XEP-0220] are used; such keys on their own provide only weak
Saint-Andre & Alkemade Standards Track PAGE 4
RFC 7590 XMPP TLS June 2015
identity verification (made stronger through the use of DNSSEC
[RFC 4033]), but this at least enables encryption of server-to-server
connections. The DNA prooftypes mentioned above are intended to
mitigate the residual need for encrypted but unauthenticated
connections in these scenarios.
3.5. Server Name Indication
Although there is no harm in supporting the TLS Server Name
Indication (SNI) extension [RFC 6066], 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 [RFC 6120].
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. Security Considerations
The use of TLS can help to limit the information available for
correlation between the XMPP application layer and the underlying
network and transport layers. 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, and recipient's server to
recipient's client) can help to limit the amount of metadata that
might leak.
Saint-Andre & Alkemade Standards Track PAGE 5
RFC 7590 XMPP TLS June 2015
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.
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 [RFC 7247] and
[RFC 7572].
5. References
5.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<http://www.rfc-editor.org/info/RFC 2119>.
[RFC 4949] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC 4949, August 2007,
<http://www.rfc-editor.org/info/RFC 4949>.
[RFC 5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC 5246, August 2008,
<http://www.rfc-editor.org/info/RFC 5246>.
[RFC 6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC 6120,
March 2011, <http://www.rfc-editor.org/info/RFC 6120>.
Saint-Andre & Alkemade Standards Track PAGE 6
RFC 7590 XMPP TLS June 2015
[RFC 6125] 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, DOI 10.17487/RFC 6125, March
2011, <http://www.rfc-editor.org/info/RFC 6125>.
[RFC 7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC 7525, May
2015, <http://www.rfc-editor.org/info/RFC 7525>.
5.2. Informative References
[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.", Work in Progress,
draft-ietf-dane-srv-14, April 2015.
[PKIX-POSH] Miller, M. and P. Saint-Andre, "PKIX over Secure HTTP
(POSH)", Work in Progress, draft-ietf-xmpp-posh-04,
February 2015.
[RFC 3920] Saint-Andre, P., Ed., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 3920, DOI 10.17487/RFC 3920,
October 2004, <http://www.rfc-editor.org/info/RFC 3920>.
[RFC 4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC 4033, March 2005,
<http://www.rfc-editor.org/info/RFC 4033>.
[RFC 4422] Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
Authentication and Security Layer (SASL)", RFC 4422,
DOI 10.17487/RFC 4422, June 2006,
<http://www.rfc-editor.org/info/RFC 4422>.
[RFC 5386] Williams, N. and M. Richardson, "Better-Than-Nothing
Security: An Unauthenticated Mode of IPsec", RFC 5386,
DOI 10.17487/RFC 5386, November 2008,
<http://www.rfc-editor.org/info/RFC 5386>.
[RFC 6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC 6066, January 2011,
<http://www.rfc-editor.org/info/RFC 6066>.
Saint-Andre & Alkemade Standards Track PAGE 7
RFC 7590 XMPP TLS June 2015
[RFC 7247] 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, DOI 10.17487/RFC 7247, May 2014,
<http://www.rfc-editor.org/info/RFC 7247>.
[RFC 7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is
an Attack", BCP 188, RFC 7258, DOI 10.17487/RFC 7258, May
2014, <http://www.rfc-editor.org/info/RFC 7258>.
[RFC 7457] Sheffer, Y., Holz, R., and P. Saint-Andre, "Summarizing
Known Attacks on Transport Layer Security (TLS) and
Datagram TLS (DTLS)", RFC 7457, DOI 10.17487/RFC 7457,
February 2015, <http://www.rfc-editor.org/info/RFC 7457>.
[RFC 7572] 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", RFC 7572,
DOI 10.17487/RFC 7572, June 2015,
<http://www.rfc-editor.org/info/RFC 7572>.
[XEP-0138] Hildebrand, J. and P. Saint-Andre, "Stream Compression",
XSF XEP 0138, May 2009,
<http://xmpp.org/extensions/xep-0138.html>.
[XEP-0170] Saint-Andre, P., "Recommended Order of Stream Feature
Negotiation", XSF XEP 0170, January 2007,
<http://xmpp.org/extensions/xep-0170.html>.
[XEP-0198] Karneges, J., Saint-Andre, P., Hildebrand, J., Forno, F.,
Cridland, D., and M. Wild, "Stream Management", XSF XEP
0198, June 2011,
<http://xmpp.org/extensions/xep-0198.html>.
[XEP-0220] Miller, J., Saint-Andre, P., and P. Hancke, "Server
Dialback", XSF XEP 0220, August 2014,
<http://xmpp.org/extensions/xep-0220.html>.
[XMPP-DNA] Saint-Andre, P. and M. Miller, "Domain Name Associations
(DNA) in the Extensible Messaging and Presence Protocol
(XMPP)", Work in Progress, draft-ietf-xmpp-dna-10, March
2015.
Saint-Andre & Alkemade Standards Track PAGE 8
RFC 7590 XMPP TLS June 2015
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.
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.
Ben Campbell, Spencer Dawkins, and Barry Leiba provided helpful input
during IESG review.
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
Saint-Andre & Alkemade Standards Track PAGE 9
RFC TOTAL SIZE: 20393 bytes
PUBLICATION DATE: Saturday, June 20th, 2015
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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