Updated TLS Server Identity Check Procedure for Email Related Protocols
draft-ietf-uta-email-tls-certs-03
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| Document | Type | Active Internet-Draft (uta WG) | |
|---|---|---|---|
| Author | Alexey Melnikov | ||
| Last updated | 2015-06-17 | ||
| Stream | Internet Engineering Task Force (IETF) | ||
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draft-ietf-uta-email-tls-certs-03
Network Working Group A. Melnikov
Internet-Draft Isode Ltd
Updates: 2595, 3207, 3501, 5804 (if June 17, 2015
approved)
Intended status: Standards Track
Expires: December 19, 2015
Updated TLS Server Identity Check Procedure for Email Related Protocols
draft-ietf-uta-email-tls-certs-03
Abstract
This document describes TLS server identity verification procedure
for SMTP Submission, IMAP, POP and ManageSieve clients. It replaces
Section 2.4 of RFC 2595.
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 December 19, 2015.
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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . 2
3. Email Server Certificate Verification Rules . . . . . . . . . 3
4. Compliance Checklist for Certification Authorities . . . . . 4
5. Compliance Checklist for Mail Service Providers and
Certificate Signing Request generation tools . . . . . . . . 4
6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 4
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
8. Security Considerations . . . . . . . . . . . . . . . . . . . 5
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
9.1. Normative References . . . . . . . . . . . . . . . . . . 6
9.2. Informative References . . . . . . . . . . . . . . . . . 6
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 7
Appendix B. Changes since draft-ietf-uta-email-tls-certs-00 . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
Use of TLS by SMTP Submission, IMAP, POP and ManageSieve clients is
described in [RFC3207], [RFC3501], [RFC2595] and [RFC5804]
respectively. Each of the documents describes slightly different
rules for server certificate identity verification (or doesn't define
any rules at all). In reality, email client and server developers
implement many of these protocols at the same time, so it would be
good to define modern and consistent rules for verifying email server
identities using TLS.
This document describes the updated TLS server identity verification
procedure for SMTP Submission [RFC6409] [RFC3207], IMAP [RFC3501],
POP [RFC1939] and ManageSieve [RFC5804] clients. It replaces
Section 2.4 of RFC 2595.
Note that this document doesn't apply to use of TLS in MTA-to-MTA
SMTP.
The main goal of the document is to provide consistent TLS server
identity verification procedure across multiple email related
protocols. This should make it easier for Certification Authorities
and ISPs to deploy TLS for email use, and would enable email client
developers to write more secure code.
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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3. Email Server Certificate Verification Rules
During a TLS negotiation, an email client (i.e., an SMTP, IMAP, POP3
or ManageSieve client) MUST check its understanding of the server
hostname against the server's identity as presented in the server
Certificate message, in order to prevent man-in-the-middle attacks.
Matching is performed according to the rules specified in Section 6
of [RFC6125], including "certificate pinning" and the procedure on
failure to match. The following inputs are used by the verification
procedure used in [RFC6125]:
1. The client MUST use the server hostname it used to open the
connection as the value to compare against the server name as
expressed in the server certificate (the reference identity).
The client MUST NOT use any form of the server hostname derived
from an insecure remote source (e.g., insecure DNS lookup).
CNAME canonicalization is not done.
The rules and guidelines defined in [RFC6125] apply to an email
server certificates, with the following supplemental rules:
1. Support for the DNS-ID identifier type (subjectAltName of dNSName
type [RFC5280]) is REQUIRED in Email client software
implementations.
2. Support for the SRV-ID identifier type (subjectAltName of SRVName
type [RFC4985]) is REQUIRED for email client software
implementations. List of SRV-ID types for email services is
specified in [RFC6186]. For ManageSieve the value "sieve" is
used.
3. URI-ID identifier type (subjectAltName of
uniformResourceIdentifier type [RFC5280]) MUST NOT be used by
clients for server verification, as URI-ID were not historically
used for email.
4. For backward compatibility with deployed software CN-ID
identifier type (CN attribute from the subject name, see
[RFC6125]) MAY be used for server identity verification.
5. Email protocols allow use of certain wilcards in identifiers
presented by email servers. The "*" wildcard character MAY be
used as the left-most name component of DNS-ID or CN-ID in the
certificate. For example, a DNS-ID of *.example.com would match
a.example.com, foo.example.com, etc. but would not match
example.com. Note that the wildcard character MUST NOT be used
as a fragment of the left-most name component (e.g.,
*oo.example.com, f*o.example.com, or foo*.example.com).
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4. Compliance Checklist for Certification Authorities
1. CA MUST support issuance of server certificates with DNS-ID
identifier type (subjectAltName of dNSName type [RFC5280]).
2. CA MUST support issuance of server certificates with SRV-ID
identifier type (subjectAltName of SRVName type [RFC4985]) for
each type of email service.
3. For backward compatibility with deployed client base, CA MUST
support issuance of server certificates with CN-ID identifier
type (CN attribute from the subject name, see [RFC6125]).
4. CA MAY allow "*" (wildcard) as the left-most name component of
DNS-ID or CN-ID in server certificates it issues.
5. Compliance Checklist for Mail Service Providers and Certificate
Signing Request generation tools
1. SHOULD include the DNS-ID identifier type (subjectAltName of
dNSName type [RFC5280]) in Certificate Signing Requests for both
the right hand side of served email addresses, as well as for the
host name where the email server(s) are running.
2. If the email services provided are discoverable using DNS SRV as
specified in [RFC6186], MSP MUST include the SRV-ID identifier
type (subjectAltName of SRVName type [RFC4985]) for each type of
email service in Certificate Signing Requests.
3. SHOULD include CN-ID identifier type (CN attribute from the
subject name, see [RFC6125]) for the host name where the email
server(s) is running in Certificate Signing Requests for backward
compatibility with deployed email clients. (Note, a certificate
can only include a single CN-ID, so if a mail service is running
on multiple hosts, either each host has to use different
certificate with its own CN-ID, a single certificate with
multiple DNS-IDs, or a single certificate with wildcard in CN-ID
can be used).
4. MAY include "*" (wildcard) as the left-most name component of
DNS-ID or CN-ID in Certificate Signing Requests.
6. Examples
Consider an IMAP-accessible email server which supports both IMAP and
IMAPS (IMAP-over-TLS) at the host "mail.example.net" servicing email
addresses of the form "user@example.net" and discoverable via DNS SRV
lookups in domain "example.net" (DNS SRV records
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"_imap._tcp.example.net" and "_imaps._tcp.example.net"). A
certificate for this service needs to include SRV-IDs of
"_imap.example.net" (when STARTTLS is used on the IMAP port) and
"_imaps.example.net" (when TLS is used on IMAPS port). See [RFC6186]
for more details. (Note that unlike DNS SRV there is no "_tcp"
component in SRV-IDs) along with DNS-IDs of "example.net" and
"mail.example.net". It might also include CN-IDs of
"mail.example.net" for backward compatibility with deployed
infrastructure.
Consider an SMTP Submission server at the host "submit.example.net"
servicing email addresses of the form "user@example.net" and
discoverable via DNS SRV lookups in domain "example.net" (DNS SRV
records "_submission._tcp.example.net"). A certificate for this
service needs to include SRV-IDs of "_submission.example.net" (see
[RFC6186]) along with DNS-IDs of "example.net" and
"submit.example.net". It might also include CN-IDs of
"submit.example.net" for backward compatibility with deployed
infrastructure.
Consider a host "mail.example.net" servicing email addresses of the
form "user@example.net" and discoverable via DNS SRV lookups in
domain "example.net", which runs SMTP Submission, IMAPS and POP3S
(POP3-over-TLS) and ManageSieve services. Each of the servers can
use their own certificate specific to their service (see examples
above). Alternatively they can all share a single certificate that
would include SRV-IDs of "_submission.example.net",
"_imaps.example.net", "_pop3s.example.net" and "_sieve.example.net"
along with DNS-IDs of "example.net" and "mail.example.net". It might
also include CN-IDs of "mail.example.net" for backward compatibility
with deployed infrastructure.
7. IANA Considerations
This document doesn't require any action from IANA.
8. Security Considerations
The goal of this document is to improve interoperability and thus
security of email clients wishing to access email servers over TLS
protected email protocols, by specifying a consistent set of rules
that email service providers, email client writers and Certification
Authorities can use when creating server certificates.
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9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
[RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail",
STD 72, RFC 6409, November 2011.
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, February 2002.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, March 2003.
[RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3",
STD 53, RFC 1939, May 1996.
[RFC5804] Melnikov, A. and T. Martin, "A Protocol for Remotely
Managing Sieve Scripts", RFC 5804, July 2010.
[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.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
[RFC4985] Santesson, S., "Internet X.509 Public Key Infrastructure
Subject Alternative Name for Expression of Service Name",
RFC 4985, August 2007.
9.2. Informative References
[RFC2595] Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC
2595, June 1999.
[RFC6186] Daboo, C., "Use of SRV Records for Locating Email
Submission/Access Services", RFC 6186, March 2011.
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Appendix A. Acknowledgements
Thank you to Chris Newman and Sean Turner for comments on this
document.
The editor of this document copied lots of text from RFC 2595 and RFC
6125, so the hard work of editors of these document is appreciated.
Appendix B. Changes since draft-ietf-uta-email-tls-certs-00
[[Note to RFC Editor: Please delete this section before publication]]
Added another example, clarified that subjectAltName and DNS SRV are
using slightly different syntax.
As any certificate can only include one CN-ID, corrected examples.
Split rules to talk seperately about requirements on MUAs, CAs and
MSPs/CSR generation tools.
Updated Introduction section.
Author's Address
Alexey Melnikov
Isode Ltd
14 Castle Mews
Hampton, Middlesex TW12 2NP
UK
EMail: Alexey.Melnikov@isode.com
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