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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group P. Saint-Andre 3 Internet-Draft &yet 4 Updates: 6120 (if approved) T. Alkemade 5 Intended status: Standards Track 6 Expires: May 30, 2015 November 26, 2014 8 Use of Transport Layer Security (TLS) in the Extensible Messaging and 9 Presence Protocol (XMPP) 10 draft-ietf-uta-xmpp-04 12 Abstract 14 This document provides recommendations for the use of Transport Layer 15 Security (TLS) in the Extensible Messaging and Presence Protocol 16 (XMPP). This document updates RFC 6120. 18 Status of This Memo 20 This Internet-Draft is submitted in full conformance with the 21 provisions of BCP 78 and BCP 79. 23 Internet-Drafts are working documents of the Internet Engineering 24 Task Force (IETF). Note that other groups may also distribute 25 working documents as Internet-Drafts. The list of current Internet- 26 Drafts is at http://datatracker.ietf.org/drafts/current/. 28 Internet-Drafts are draft documents valid for a maximum of six months 29 and may be updated, replaced, or obsoleted by other documents at any 30 time. It is inappropriate to use Internet-Drafts as reference 31 material or to cite them other than as "work in progress." 33 This Internet-Draft will expire on May 30, 2015. 35 Copyright Notice 37 Copyright (c) 2014 IETF Trust and the persons identified as the 38 document authors. All rights reserved. 40 This document is subject to BCP 78 and the IETF Trust's Legal 41 Provisions Relating to IETF Documents 42 (http://trustee.ietf.org/license-info) in effect on the date of 43 publication of this document. Please review these documents 44 carefully, as they describe your rights and restrictions with respect 45 to this document. Code Components extracted from this document must 46 include Simplified BSD License text as described in Section 4.e of 47 the Trust Legal Provisions and are provided without warranty as 48 described in the Simplified BSD License. 50 Table of Contents 52 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 53 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2 54 3. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 3 55 3.1. Support for TLS . . . . . . . . . . . . . . . . . . . . . 3 56 3.2. Compression . . . . . . . . . . . . . . . . . . . . . . . 3 57 3.3. Session Resumption . . . . . . . . . . . . . . . . . . . 3 58 3.4. Authenticated Connections . . . . . . . . . . . . . . . . 3 59 3.5. Unauthenticated Connections . . . . . . . . . . . . . . . 4 60 3.6. Server Name Indication . . . . . . . . . . . . . . . . . 4 61 3.7. Human Factors . . . . . . . . . . . . . . . . . . . . . . 4 62 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 63 5. Security Considerations . . . . . . . . . . . . . . . . . . . 5 64 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 65 6.1. Normative References . . . . . . . . . . . . . . . . . . 5 66 6.2. Informative References . . . . . . . . . . . . . . . . . 6 67 Appendix A. Implementation Notes . . . . . . . . . . . . . . . . 7 68 Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 7 69 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7 71 1. Introduction 73 The Extensible Messaging and Presence Protocol (XMPP) [RFC6120] 74 (along with its precursor, the so-called "Jabber protocol") has used 75 Transport Layer Security (TLS) [RFC5246] (along with its precursor, 76 Secure Sockets Layer or SSL) since 1999. Both [RFC6120] and its 77 predecessor [RFC3920] provided recommendations regarding the use of 78 TLS in XMPP. In order to address the evolving threat model on the 79 Internet today, this document provides stronger recommendations. 81 NOTE: Unless explicitly noted otherwise, all of the 82 recommendations specified in [I-D.ietf-uta-tls-bcp] apply to XMPP. 83 In the main, this document merely provides supplementary 84 information; those who implement and deploy XMPP technologies are 85 expected to follow the recommendations of [I-D.ietf-uta-tls-bcp]. 87 This document updates [RFC6120]. 89 2. Terminology 91 Various security-related terms are to be understood in the sense 92 defined in [RFC4949]. 94 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 95 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 96 "OPTIONAL" in this document are to be interpreted as described in 97 [RFC2119]. 99 3. Recommendations 101 3.1. Support for TLS 103 Support for TLS (specifically, the XMPP profile of STARTTLS) is 104 mandatory for XMPP implementations, as already specified in [RFC6120] 105 and its predecessor [RFC3920]. 107 The server (i.e., the XMPP receiving entity) to which a client or 108 peer server (i.e., the XMPP initiating entity) connects might not 109 offer a stream feature of . Although in general this stream feature indicates that 111 the server supports XMPP 1.0 and therefore supports TLS, it is 112 possible that this stream feature might be stripped out by an 113 attacker (see Section 2.1 of [I-D.ietf-uta-tls-attacks]). Therefore, 114 the initiating entity SHOULD proceed with the stream negotiation even 115 if the receiving entity does not advertise support for TLS. 116 Similarly, although a receiving entity SHOULD include the 117 child element to indicate that negotiation of TLS is mandatory, an 118 initiating entity MUST NOT depend on receiving the flag 119 in determining whether TLS will be enforced for the stream. 121 3.2. Compression 123 XMPP supports an application-layer compression technology [XEP-0138]. 124 Although this XMPP extension might have slightly stronger security 125 properties than TLS-layer compression (since it is enabled after SASL 126 authentication, as described in [XEP-0170]), this document neither 127 encourages nor discourages use of XMPP-layer compression. 129 3.3. Session Resumption 131 In XMPP, TLS session resumption can be used in concert with the XMPP 132 Stream Management extension; see [XEP-0198] for further details. 134 3.4. Authenticated Connections 136 Both the core XMPP specification [RFC6120] and the "CertID" 137 specification [RFC6125] provide recommendations and requirements for 138 certificate validation in the context of authenticated connections. 139 This document does not supersede those specifications. Wherever 140 possible, it is best to prefer authenticated connections (along with 141 SASL [RFC4422]), as already stated in the core XMPP specification 142 [RFC6120]. In particular, clients MUST authenticate servers. 143 Because this document does not mandate that servers need to 144 authenticate peer servers, unauthenticated server-to-server 145 connections are allowed (consistent with current practice on the XMPP 146 network). 148 This document does not modify the recommendations in [RFC6120] 149 regarding the Subject Alternative Names (or other certificate 150 details) that need to be supported for authentication of XMPP 151 connections. 153 3.5. Unauthenticated Connections 155 Given the pervasiveness of passive eavesdropping, even an 156 unauthenticated connection might be better than an unencrypted 157 connection (this is similar to the "better than nothing security" 158 approach for IPsec [RFC5386]). In particular, because of current 159 deployment challenges for authenticated connections between XMPP 160 servers (see [I-D.ietf-xmpp-dna] and [I-D.ietf-xmpp-posh] for 161 details), it can be reasonable for XMPP server implementations to 162 accept unauthenticated connections when the Server Dialback protocol 163 [XEP-0220] is used for weak identity verification; this will at least 164 enable encryption of server-to-server connections. Unauthenticated 165 connections include connections negotiated using anonymous Diffie- 166 Hellman algorithms or using self-signed certificates, among other 167 scenarios. 169 3.6. Server Name Indication 171 Although there is no harm in supporting the TLS Server Name 172 Indication (SNI) extension [RFC6066], this is not necessary since the 173 same function is served in XMPP by the 'to' address of the initial 174 stream header as explained in Section 4.7.2 of [RFC6120]. 176 3.7. Human Factors 178 It is strongly encouraged that XMPP clients provide ways for end 179 users (and that XMPP servers provide ways for administrators) to 180 complete the following tasks: 182 o Determine if a client-to-server or server-to-server connection is 183 encrypted and authenticated. 185 o Determine the version of TLS used for a client-to-server or 186 server-to-server connection. 188 o Inspect the certificate offered by an XMPP server. 190 o Determine the cipher suite used to encrypt a connection. 192 o Be warned if the certificate changes for a given server. 194 4. IANA Considerations 196 This document requests no actions of the IANA. 198 5. Security Considerations 200 The use of TLS can help limit the information available for 201 correlation to the network and transport layer headers as opposed to 202 the application layer. As typically deployed, XMPP technologies do 203 not leave application-layer routing data (such as XMPP 'to' and 204 'from' addresses) at rest on intermediate systems, since there is 205 only one hop between any two given XMPP servers. As a result, 206 encrypting all hops (sending client to sender's server, sender's 207 server to recipient's server, recipient's server to recipient's 208 client) can help to limit the amount of "metadata" that might leak. 210 It is possible that XMPP servers themselves might be compromised. In 211 that case, per-hop encryption would not protect XMPP communications, 212 and even end-to-end encryption of (parts of) XMPP stanza payloads 213 would leave addressing information and XMPP roster data in the clear. 214 By the same token, it is possible that XMPP clients (or the end-user 215 devices on which such clients are installed) could also be 216 compromised, leaving users utterly at the mercy of an adversary. 218 This document and related actions to strengthen the security of the 219 XMPP network are based on the assumption that XMPP servers and 220 clients have not been subject to widespread compromise. If this 221 assumption is valid, then ubiquitous use of per-hop TLS channel 222 encryption and more significant deployment of end-to-end object 223 encryption technologies will serve to protect XMPP communications to 224 a measurable degree, compared to the alternatives. 226 6. References 228 6.1. Normative References 230 [I-D.ietf-uta-tls-bcp] 231 Sheffer, Y., Holz, R., and P. Saint-Andre, 232 "Recommendations for Secure Use of TLS and DTLS", draft- 233 ietf-uta-tls-bcp-07 (work in progress), November 2014. 235 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 236 Requirement Levels", BCP 14, RFC 2119, March 1997. 238 [RFC4949] Shirey, R., "Internet Security Glossary, Version 2", RFC 239 4949, August 2007. 241 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 242 (TLS) Protocol Version 1.2", RFC 5246, August 2008. 244 [RFC6120] Saint-Andre, P., "Extensible Messaging and Presence 245 Protocol (XMPP): Core", RFC 6120, March 2011. 247 [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and 248 Verification of Domain-Based Application Service Identity 249 within Internet Public Key Infrastructure Using X.509 250 (PKIX) Certificates in the Context of Transport Layer 251 Security (TLS)", RFC 6125, March 2011. 253 6.2. Informative References 255 [I-D.ietf-uta-tls-attacks] 256 Sheffer, Y., Holz, R., and P. Saint-Andre, "Summarizing 257 Current Attacks on TLS and DTLS", draft-ietf-uta-tls- 258 attacks-05 (work in progress), October 2014. 260 [I-D.ietf-xmpp-dna] 261 Saint-Andre, P. and M. Miller, "Domain Name Associations 262 (DNA) in the Extensible Messaging and Presence Protocol 263 (XMPP)", draft-ietf-xmpp-dna-08 (work in progress), 264 October 2014. 266 [I-D.ietf-xmpp-posh] 267 Miller, M. and P. Saint-Andre, "PKIX over Secure HTTP 268 (POSH)", draft-ietf-xmpp-posh-02 (work in progress), 269 October 2014. 271 [RFC3920] Saint-Andre, P., Ed., "Extensible Messaging and Presence 272 Protocol (XMPP): Core", RFC 3920, October 2004. 274 [RFC4422] Melnikov, A. and K. Zeilenga, "Simple Authentication and 275 Security Layer (SASL)", RFC 4422, June 2006. 277 [RFC5386] Williams, N. and M. Richardson, "Better-Than-Nothing 278 Security: An Unauthenticated Mode of IPsec", RFC 5386, 279 November 2008. 281 [RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions: 282 Extension Definitions", RFC 6066, January 2011. 284 [XEP-0138] 285 Hildebrand, J. and P. Saint-Andre, "Stream Compression", 286 XSF XEP 0138, May 2009. 288 [XEP-0170] 289 Saint-Andre, P., "Recommended Order of Stream Feature 290 Negotiation", XSF XEP 0170, January 2007. 292 [XEP-0198] 293 Karneges, J., Saint-Andre, P., Hildebrand, J., Forno, F., 294 Cridland, D., and M. Wild, "Stream Management", XSF XEP 295 0198, June 2011. 297 [XEP-0220] 298 Miller, J., Saint-Andre, P., and P. Hancke, "Server 299 Dialback", XSF XEP 0220, September 2013. 301 Appendix A. Implementation Notes 303 Some governments enforce legislation prohibiting the export of strong 304 cryptographic technologies. Nothing in this document ought to be 305 taken as advice to violate such prohibitions. 307 Appendix B. Acknowledgements 309 The authors would like to thank the following individuals for their 310 input: Dave Cridland, Philipp Hancke, Olle Johansson, Steve Kille, 311 Tobias Markmann, Matt Miller, and Rene Treffer. 313 Authors' Addresses 315 Peter Saint-Andre 316 &yet 318 Email: peter@andyet.com 319 URI: https://andyet.com/ 321 Thijs Alkemade 323 Email: me@thijsalkema.de