Internet DRAFT - draft-ietf-uta-xmpp
draft-ietf-uta-xmpp
Network Working Group P. Saint-Andre
Internet-Draft &yet
Updates: 6120 (if approved) T. Alkemade
Intended status: Standards Track
Expires: October 25, 2015 April 23, 2015
Use of Transport Layer Security (TLS) in the Extensible Messaging and
Presence Protocol (XMPP)
draft-ietf-uta-xmpp-07
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 Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on October 25, 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 . . . . . . . . . . . . . . . . 4
3.5. Server Name Indication . . . . . . . . . . . . . . . . . 5
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 . . . . . . . . . . . . . . . . . 7
Appendix A. Implementation Notes . . . . . . . . . . . . . . . . 8
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
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
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
session resumption, implementations MUST follow the recommendations
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provided in [I-D.ietf-uta-tls-bcp] regarding TLS-layer session
resumption.
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
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:
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 [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 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 [RFC5386]). 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
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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 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 [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].
3.6. 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 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,
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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.
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.
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[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-13 (work in
progress), April 2015.
[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.
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[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.
[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.
Ben Campbell, Spencer Dawkins, and Barry Leiba provided helpful input
during IESG review.
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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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