Updated TLS Server Identity Check Procedure for Email Related Protocols
draft-ietf-uta-email-tls-certs-06

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, updates Section 4.1 of RFC 3207, updates Section 11.1 of RFC 3501, updates Section 2.2.1 of RFC 5804.

Status of This Memo

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This Internet-Draft will expire on June 6, 2016.

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Table of Contents

• 1. Introduction
• 2. Conventions Used in This Document
• 3. Email Server Certificate Verification Rules
• 4. Compliance Checklist for Certification Authorities
• 4.1. Notes on handling of SRV-ID by Certificate Authorities
• 5. Compliance Checklist for Mail Service Providers and Certificate Signing Request generation tools
• 5.1. Notes on hosted domains
• 6. Examples
• 7. IANA Considerations
• 8. Security Considerations
• 9. References
• 9.1. Normative References
• 9.2. Informative References
• Appendix A. Acknowledgements
• Appendix B. Changes since draft-ietf-uta-email-tls-certs-00
• Author's Address

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. [CREF1]Open Issue: This is covered by draft-friedl-uta-smtp-mta-certs, or should this be covered in this document instead, just in a separate section?

This document provides a 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].

The following terms or concepts are used through the document:

reference identifier:

(as defined in [RFC6125]) One of the domain names associated by the email (i.e., an SMTP, IMAP, POP3 or ManageSieve) client with the target email server and optionally an application service type for performing name checks on the server certificate. When name checks are applicable, at least one of the reference identifiers MUST match an [RFC6125] DNS-ID or SRV-ID (or if none are present the [RFC6125] CN-ID) of the server certificate.

CN-ID, DNS-ID, SRV-ID and URI-ID are identifier types (see [RFC6125] for details). For convenience, their short definitions from [RFC6125] are listed below:

• CN-ID = a Relative Distinguished Name (RDN) in the certificate subject field that contains one and only one attribute-type-and-value pair of type Common Name (CN), where the value matches the overall form of a domain name (informally, dot- separated letter-digit-hyphen labels).
• DNS-ID = a subjectAltName entry of type dNSName
• SRV-ID = a subjectAltName entry of type otherName whose name form is SRVName
• URI-ID = a subjectAltName entry of type uniformResourceIdentifier whose value includes both (i) a "scheme" and (ii) a "host" component (or its equivalent) that matches the "reg-name" rule (where the quoted terms represent the associated [RFC5234] productions from [RFC3986]).

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. This check is only performed after the server certificate passes certification path validation as described in Section 6 of [RFC5280]. 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. For DNS-ID and CN-ID identifier types the client MUST use one or more of the following as "reference identifiers": (a) the domain portion of the user's email address, (b) the hostname it used to open the connection (without CNAME canonicalization). The client MAY also use (c) a value securely derived from (a) or (b), such as using "secure" DNSSEC validated lookup.
2. When using email service discovery procedure specified in [RFC6186] the client MUST also use the domain portion of the user's email address as another "reference identifier" to compare against SRV-ID identifier in the server certificate.

The rules and guidelines defined in [RFC6125] apply to an email server certificate, 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 that support [RFC6186]. List of SRV-ID types for email services is specified in [RFC6186]. For the ManageSieve protocol the service name "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). [CREF2]Should the rules on "*" only matching one level be relaxed to cope with some existing deployments?

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. See Section 4.1 for more discussion on what this means for Certification Authorities.
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.

4.1. Notes on handling of SRV-ID by Certificate Authorities

TBD. List some possible recommendations and limitations of different approaches.

5. Compliance Checklist for Mail Service Providers and Certificate Signing Request generation tools

1. MUST include the DNS-ID identifier type in Certificate Signing Requests for the host name(s) where the email server(s) are running. SHOULD include the DNS-ID identifier type in Certificate Signing Requests for the domain portion of served email addresses.
2. If the email services provided are discoverable using DNS SRV as specified in [RFC6186], the Mail Service Provider MUST include the SRV-ID identifier type for each type of email service in Certificate Signing Requests.
3. SHOULD include CN-ID identifier type 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.

5.1. Notes on hosted domains

TBD. Compare one certificate that contains all hosted domains versa use of SNI or separate IPs for each hosted domain with its own certificate.

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". A certificate for this service needs to include DNS-IDs of "example.net" (because it is the domain portion of emails) and "mail.example.net" (this is what a user of this server enters manually, if not using [RFC6186]). It might also include CN-ID of "mail.example.net" for backward compatibility with deployed infrastructure.

Consider the IMAP-accessible email server from the previous paragraph which is additionally discoverable via DNS SRV lookups in domain "example.net" (DNS SRV records "_imap._tcp.example.net" and "_imaps._tcp.example.net"). In addition to DNS-ID/CN-ID identity types specified above, a certificate for this service also 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).

Consider the IMAP-accessible email server from the first paragraph which is running on a host also known as "mycompany.example.com". In addition to DNS-ID identity types specified above, a certificate for this service also needs to include DNS-ID of "mycompany.example.com" (this is what a user of this server enters manually, if not using [RFC6186]). It might also include CN-ID of "mycompany.example.com" instead of the CN-ID "mail.example.net" for backward compatibility with deployed infrastructure. (This is so, because a certificate can only include a single CN-ID)

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-ID 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-ID 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.

TLS Server Identity Check for Email relies on use of trustworthy DNS hostnames when constructing "reference identifiers" that are checked against an email server certificate. Such trustworthy names are either entered manually (for example if they are advertised on a Mail Service Provider's website), explicitly confirmed by the user (e.g. if they are a target of a DNS SRV lookup) or derived using a secure third party service (e.g. DNSSEC-protected SRV records which are verified by the client or trusted local resolver). Future work in this area might benefit from integration with DANE [RFC6698], but it is not covered by this document.

9. References

9.1. Normative References

9.2. Informative References

Appendix A. Acknowledgements

Thank you to Chris Newman, Viktor Dukhovni, Sean Turner, Russ Housley and Alessandro Vesely 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