Updated Transport Layer Security (TLS) Server Identity Check Procedure for Email-Related Protocols
RFC 7817

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Internet Engineering Task Force (IETF)                       A. Melnikov
Request for Comments: 7817                                     Isode Ltd
Updates: 2595, 3207, 3501, 5804                               March 2016
Category: Standards Track
ISSN: 2070-1721

 Updated Transport Layer Security (TLS) Server Identity Check Procedure
                      for Email-Related Protocols

Abstract

   This document describes the Transport Layer Security (TLS) server
   identity verification procedure for SMTP Submission, IMAP, POP, and
   ManageSieve clients.  It replaces Section 2.4 (Server Identity Check)
   of RFC 2595 and updates Section 4.1 (Processing After the STARTTLS
   Command) of RFC 3207, Section 11.1 (STARTTLS Security Considerations)
   of RFC 3501, and Section 2.2.1 (Server Identity Check) of RFC 5804.

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/rfc7817.

Copyright Notice

   Copyright (c) 2016 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.

Melnikov                     Standards Track                    [Page 1]
RFC 7817           TLS Server Identity Check for Email        March 2016

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   3
   3.  Email Server Certificate Verification Rules . . . . . . . . .   3
   4.  Compliance Checklist for Certification Authorities  . . . . .   5
     4.1.  Notes on Handling of Delegated Email Services by
           Certification Authorities . . . . . . . . . . . . . . . .   5
   5.  Compliance Checklist for Mail Service Providers and
       Certificate Signing Request Generation Tools  . . . . . . . .   6
     5.1.  Notes on Hosting Multiple Domains . . . . . . . . . . . .   7
   6.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   7.  Operational Considerations  . . . . . . . . . . . . . . . . .   9
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Appendix A.  Changes to RFCs 2595, 3207, 3501, and 5804 . . . . .  12
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  13
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  13

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.  Section 3 of this
   document replaces Section 2.4 of [RFC2595].

   Note that this document doesn't apply to use of TLS in MTA-to-MTA
   SMTP.

   This document provides a consistent TLS server identity verification
   procedure across multiple email-related protocols.  This should make
   it easier for Certification Authorities (CAs) and ISPs to deploy TLS
   for email use and would enable email client developers to write more
   secure code.

Melnikov                     Standards Track                    [Page 2]
RFC 7817           TLS Server Identity Check for Email        March 2016

2.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",