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Melnikov 3 Internet-Draft Isode Ltd 4 Updates: 2595, 3207, 3501, 5804 (if March 23, 2015 5 approved) 6 Intended status: Standards Track 7 Expires: September 24, 2015 9 Updated TLS Server Identity Check Procedure for Email Related Protocols 10 draft-ietf-uta-email-tls-certs-02 12 Abstract 14 This document describes TLS server identity verification procedure 15 for SMTP Submission, IMAP, POP and ManageSieve clients. It replaces 16 Section 2.4 of RFC 2595. 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 September 24, 2015. 35 Copyright Notice 37 Copyright (c) 2015 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. Conventions Used in This Document . . . . . . . . . . . . . . 2 54 3. Email Server Certificate Verification Rules . . . . . . . . . 3 55 4. Compliance Checklist for Certificate Authorities . . . . . . 4 56 5. Compliance Checklist for Mail Service Providers and 57 Certificate Signing Request generation tools . . . . . . . . 4 58 6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 4 59 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 60 8. Security Considerations . . . . . . . . . . . . . . . . . . . 5 61 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 62 9.1. Normative References . . . . . . . . . . . . . . . . . . 6 63 9.2. Informative References . . . . . . . . . . . . . . . . . 6 64 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 7 65 Appendix B. Changes since draft-ietf-uta-email-tls-certs-00 . . 7 66 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7 68 1. Introduction 70 Use of TLS by SMTP Submission, IMAP, POP and ManageSieve clients is 71 described in [RFC3207], [RFC3501], [RFC2595] and [RFC5804] 72 respectively. Each of the documents describes slightly different 73 rules for server certificate identity verification (or doesn't define 74 any rules at all). In reality, email client and server developers 75 implement many of these protocols at the same time, so it would be 76 good to define modern and consistent rules for verifying email server 77 identities using TLS. 79 This document describes the updated TLS server identity verification 80 procedure for SMTP Submission [RFC4409] [RFC3207], IMAP [RFC3501], 81 POP [RFC1939] and ManageSieve [RFC5804] clients. It replaces 82 Section 2.4 of RFC 2595. 84 Note that this document doesn't apply to use of TLS in MTA-to-MTA 85 SMTP. 87 The main goal of the document is to provide consistent TLS server 88 identity verification procedure across multiple email related 89 protocols. This should make it easier for Certificate Authorities 90 and ISPs to deploy TLS for email use, and would enable email client 91 developers to write more secure code. 93 2. Conventions Used in This Document 95 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 96 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 97 document are to be interpreted as described in [RFC2119]. 99 3. Email Server Certificate Verification Rules 101 During a TLS negotiation, an email client (i.e., an SMTP, IMAP, POP3 102 or ManageSieve client) MUST check its understanding of the server 103 hostname against the server's identity as presented in the server 104 Certificate message, in order to prevent man-in-the-middle attacks. 105 Matching is performed according to the rules specified in Section 6 106 of [RFC6125], including "certificate pinning" and the procedure on 107 failure to match. The following inputs are used by the verification 108 procedure used in [RFC6125]: 110 1. The client MUST use the server hostname it used to open the 111 connection as the value to compare against the server name as 112 expressed in the server certificate (the reference identity). 113 The client MUST NOT use any form of the server hostname derived 114 from an insecure remote source (e.g., insecure DNS lookup). 115 CNAME canonicalization is not done. 117 The rules and guidelines defined in [RFC6125] apply to an email 118 server certificates, with the following supplemental rules: 120 1. Support for the DNS-ID identifier type (subjectAltName of dNSName 121 type [RFC5280]) is REQUIRED in Email client software 122 implementations. 124 2. Support for the SRV-ID identifier type (subjectAltName of SRVName 125 type [RFC4985]) is REQUIRED for email client software 126 implementations. List of SRV-ID types for email services is 127 specified in [RFC6186]. For ManageSieve the value "sieve" is 128 used. 130 3. URI-ID identifier type (subjectAltName of 131 uniformResourceIdentifier type [RFC5280]) MUST NOT be used by 132 clients for server verification, as URI-ID were not historically 133 used for email. 135 4. For backward compatibility with deployed software CN-ID 136 identifier type (CN attribute from the subject name, see 137 [RFC6125]) MAY be used for server identity verification. 139 5. Email protocols allow use of certain wilcards in identifiers 140 presented by email servers. The "*" wildcard character MAY be 141 used as the left-most name component of DNS-ID or CN-ID in the 142 certificate. For example, a DNS-ID of *.example.com would match 143 a.example.com, foo.example.com, etc. but would not match 144 example.com. Note that the wildcard character MUST NOT be used 145 as a fragment of the left-most name component (e.g., 146 *oo.example.com, f*o.example.com, or foo*.example.com). 148 4. Compliance Checklist for Certificate Authorities 150 1. CA MUST support issuance of server certificates with DNS-ID 151 identifier type (subjectAltName of dNSName type [RFC5280]). 153 2. CA MUST support issuance of server certificates with SRV-ID 154 identifier type (subjectAltName of SRVName type [RFC4985]) for 155 each type of email service. 157 3. For backward compatibility with deployed client base, CA MUST 158 support issuance of server certificates with CN-ID identifier 159 type (CN attribute from the subject name, see [RFC6125]). 161 4. CA MAY allow "*" (wildcard) as the left-most name component of 162 DNS-ID or CN-ID in server certificates it issues. 164 5. Compliance Checklist for Mail Service Providers and Certificate 165 Signing Request generation tools 167 1. SHOULD include the DNS-ID identifier type (subjectAltName of 168 dNSName type [RFC5280]) in Certificate Signing Requests for both 169 the right hand side of served email addresses, as well as for the 170 host name where the email server(s) are running. 172 2. If the email services provided are discoverable using DNS SRV as 173 specified in [RFC6186], MSP MUST include the SRV-ID identifier 174 type (subjectAltName of SRVName type [RFC4985]) for each type of 175 email service in Certificate Signing Requests. 177 3. SHOULD include CN-ID identifier type (CN attribute from the 178 subject name, see [RFC6125]) for the host name where the email 179 server(s) is running in Certificate Signing Requests for backward 180 compatibility with deployed email clients. (Note, a certificate 181 can only include a single CN-ID, so if a mail service is running 182 on multiple hosts, either each host has to use different 183 certificate with its own CN-ID, a single certificate with 184 multiple DNS-IDs, or a single certificate with wildcard in CN-ID 185 can be used). 187 4. MAY include "*" (wildcard) as the left-most name component of 188 DNS-ID or CN-ID in Certificate Signing Requests. 190 6. Examples 192 Consider an IMAP-accessible email server which supports both IMAP and 193 IMAPS (IMAP-over-TLS) at the host "mail.example.net" servicing email 194 addresses of the form "user@example.net" and discoverable via DNS SRV 195 lookups in domain "example.net" (DNS SRV records 196 "_imap._tcp.example.net" and "_imaps._tcp.example.net"). A 197 certificate for this service needs to include SRV-IDs of 198 "_imap.example.net" and "_imaps.example.net" (see [RFC6186]. Note 199 that unlike DNS SRV there is no "_tcp" component in SRV-IDs) along 200 with DNS-IDs of "example.net" and "mail.example.net". It might also 201 include CN-IDs of "mail.example.net" for backward compatibility with 202 deployed infrastructure. 204 Consider an SMTP Submission server at the host "submit.example.net" 205 servicing email addresses of the form "user@example.net" and 206 discoverable via DNS SRV lookups in domain "example.net" (DNS SRV 207 records "_submission._tcp.example.net"). A certificate for this 208 service needs to include SRV-IDs of "_submission.example.net" (see 209 [RFC6186]) along with DNS-IDs of "example.net" and 210 "submit.example.net". It might also include CN-IDs of 211 "submit.example.net" for backward compatibility with deployed 212 infrastructure. 214 Consider a host "mail.example.net" servicing email addresses of the 215 form "user@example.net" and discoverable via DNS SRV lookups in 216 domain "example.net", which runs SMTP Submission, IMAPS and POP3S 217 (POP3-over-TLS) and ManageSieve services. Each of the servers can 218 use their own certificate specific to their service (see examples 219 above). Alternatively they can all share a single certificate that 220 would include SRV-IDs of "_submission.example.net", 221 "_imaps.example.net", "_pop3s.example.net" and "_sieve.example.net" 222 along with DNS-IDs of "example.net" and "mail.example.net". It might 223 also include CN-IDs of "mail.example.net" for backward compatibility 224 with deployed infrastructure. 226 7. IANA Considerations 228 This document doesn't require any action from IANA. 230 8. Security Considerations 232 The goal of this document is to improve interoperability and thus 233 security of email clients wishing to access email servers over TLS 234 protected email protocols, by specifying a consistent set of rules 235 that email service providers, email client writers and certificate 236 authorities can use when creating server certificates. 238 9. References 239 9.1. Normative References 241 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 242 Requirement Levels", BCP 14, RFC 2119, March 1997. 244 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 245 October 2008. 247 [RFC4409] Gellens, R. and J. Klensin, "Message Submission for Mail", 248 RFC 4409, April 2006. 250 [RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over 251 Transport Layer Security", RFC 3207, February 2002. 253 [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 254 4rev1", RFC 3501, March 2003. 256 [RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3", 257 STD 53, RFC 1939, May 1996. 259 [RFC5804] Melnikov, A. and T. Martin, "A Protocol for Remotely 260 Managing Sieve Scripts", RFC 5804, July 2010. 262 [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and 263 Verification of Domain-Based Application Service Identity 264 within Internet Public Key Infrastructure Using X.509 265 (PKIX) Certificates in the Context of Transport Layer 266 Security (TLS)", RFC 6125, March 2011. 268 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 269 Housley, R., and W. Polk, "Internet X.509 Public Key 270 Infrastructure Certificate and Certificate Revocation List 271 (CRL) Profile", RFC 5280, May 2008. 273 [RFC4985] Santesson, S., "Internet X.509 Public Key Infrastructure 274 Subject Alternative Name for Expression of Service Name", 275 RFC 4985, August 2007. 277 9.2. Informative References 279 [RFC2595] Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC 280 2595, June 1999. 282 [RFC6186] Daboo, C., "Use of SRV Records for Locating Email 283 Submission/Access Services", RFC 6186, March 2011. 285 Appendix A. Acknowledgements 287 Thank you to Chris Newman for comments on this document. 289 The editor of this document copied lots of text from RFC 2595 and RFC 290 6125, so the hard work of editors of these document is appreciated. 292 Appendix B. Changes since draft-ietf-uta-email-tls-certs-00 294 [[Note to RFC Editor: Please delete this section before publication]] 296 Added another example, clarified that subjectAltName and DNS SRV are 297 using slightly different syntax. 299 As any certificate can only include one CN-ID, corrected examples. 301 Split rules to talk seperately about requirements on MUAs, CAs and 302 MSPs/CSR generation tools. 304 Updated Introduction section. 306 Author's Address 308 Alexey Melnikov 309 Isode Ltd 310 14 Castle Mews 311 Hampton, Middlesex TW12 2NP 312 UK 314 EMail: Alexey.Melnikov@isode.com