Internet DRAFT - draft-ietf-uta-email-tls-certs
draft-ietf-uta-email-tls-certs
Network Working Group A. Melnikov
Internet-Draft Isode Ltd
Updates: 2595, 3207, 3501, 5804 (if December 29, 2015
approved)
Intended status: Standards Track
Expires: July 1, 2016
Updated TLS Server Identity Check Procedure for Email Related Protocols
draft-ietf-uta-email-tls-certs-09
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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 1, 2016.
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
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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
9. Security Considerations . . . . . . . . . . . . . . . . . . . 9
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
10.1. Normative References . . . . . . . . . . . . . . . . . . 9
10.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 12
Appendix B. Changes to RFC 2595, RFC 3207, RFC 3501, RFC 5804 . 12
Appendix C. Changes since draft-ietf-uta-email-tls-certs-00 . . 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 and ISPs to deploy TLS for
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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: (formally defined in [RFC6125]) One of the
domain names that the email client (an SMTP, IMAP, POP3 or
ManageSieve client) associates with the target email server. For
some identifier types, the identifier also includes an application
service type. Reference identifiers are used for performing name
checks on server certificates.
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
identity (client's reference identifiers) 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
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to the rules specified in Section 6 of [RFC6125], including the
relative order of matching of different identifier types,
"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 [RFC4033][RFC4034][RFC4035] 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 wildcards 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
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as a fragment of the left-most name component (e.g.,
*oo.example.com, f*o.example.com, or foo*.example.com).
4. Compliance Checklist for Certification Authorities
1. CA MUST support issuance of server certificates with DNS-ID
identifier type (subjectAltName of dNSName type [RFC5280]).
(Note that some DNS-IDs may refer to domain portions of email
addresses, so they might not have corresponding A/AAAA DNS
records.)
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 delegated email services by Certification
Authorities
[RFC6186] provides an easy way for organizations to autoconfigure
email clients. It also allows for delegation of email services to an
email hosting provider. When connecting to such delegated hosting
service an email client that attempts to verify TLS server identity
needs to know that if it connects to imap.hosting.example.net that
such server is authorized to provide email access for an email such
as alice@example.org. In absence of SRV-IDs, users of compliant
email clients would be forced to manually confirm exception, because
the TLS server certificate verification procedures specified in this
document would result in failure to match the TLS server certificate
against the expected domain(s). One way to provide such
authorization is for the TLS certificate for imap.hosting.example.net
to include SRV-ID(s) (or DNS-ID) for the example.org domain.
(Another way is for DNS SRV lookups to be protected by DNSSEC, but
this solution depends on ubiquitous use of DNSSEC and availability of
DNSSEC-aware APIs and thus is not discussed in this document. A
future update to this document might rectify this.)
A certification authority that receives a Certificate Signing Request
containing multiple unrelated DNS-IDs and/or SRV-IDs (e.g. DNS-ID of
example.org and DNS-ID of example.com) needs to verify that the
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entity that supplied such Certificate Signing Request is authorized
to provide email service for all requested domains.
The ability to issue certificates that contain an SRV-ID (or a DNS-ID
for the domain part of email addresses) implies the ability to verify
that entities requesting them are authorized to run email service for
these SRV-IDs/DNS-IDs. In particular, certification authorities that
can't verify such authorization (whether for a particular domain or
in general) MUST NOT include such email SRV-IDs/DNS-IDs in
certificates they issue. This document doesn't specify exact
mechanism(s) that can be used to achieve this. However, a few
special case recommendations are listed below.
A certification authority willing to sign a certificate containing a
particular DNS-ID SHOULD also support signing a certificate
containing one or more of email SRV-IDs for the same domain, because
the SRV-ID effectively provides more restricted access to an email
service for the domain (as opposed to unrestricted use of any
services for the same domain, as specified by DNS-ID).
A certification authority which also provides DNS service for a
domain can use DNS information to validate SRV-IDs/DNS-IDs for the
domain.
A certification authority which is also a Mail Service Provider for a
hosted domain can use that knowdledge to validate SRV-IDs/DNS-IDs for
the domain.
5. Compliance Checklist for Mail Service Providers and Certificate
Signing Request generation tools
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. They 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
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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 hosting multiple domains
A server that hosts multiple domains needs to do one of the following
(or some combination thereof):
1. Use DNS SRV records to redirect each hosted email service to a
fixed domain, deploy TLS certificate(s) for that single domain,
and instruct users to configure their clients with appropriate
pinning (unless the SRV records can always be obtained via
DNSSEC). Some email clients come with preloaded list of pinned
certificates for some popular domains, which can avoid the need
for manual confirmation.
2. Use a single TLS certificate that includes a complete list of all
the domains it is serving.
3. Serve each domain on its own IP/port, using separate TLS
certificates on each IP/port.
4. Use Server Name Indication (SNI) TLS extension [RFC6066] to
select the right certificate to return during TLS negotiation.
Each domain has its own TLS certificate in this case.
Each of these deployment choices have their scaling disadvantages
when the list of domains changes. Use of DNS SRV without SRV-ID
requires manual confirmation from users. While preloading pinned
certificates avoids the need for manual confirmation, this
information can get stale quickly or would require support for a new
mechanism for distributing preloaded pinned certificates. A single
certificate (the second choice) requires that when a domain is added,
then a new Certificate Signing Request that includes a complete list
of all the domains needs to be issued and passed to a CA in order to
generate a new certificate. Separate IP/port can avoid regenerating
the certificate, but requires more transport layer resources. Use of
TLS SNI requires each email client to use it.
Several Mail Service Providers host hundreds and even thousands of
domains. This document, as well as its predecessors RFC 2595, RFC
3207, RFC 3501 and RFC 5804 don't address scaling issues caused by
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use of TLS in multi-tenanted environments. Further work is needed to
address this issue, possibly using DNSSEC or something like POSH
[RFC7711].
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
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(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. Operational Considerations
Section 5 covers operational considerations (in particular use of DNS
SRV for autoconfiguration) related to generating TLS certificiates
for email servers so that they can be successfully verified by email
clients. Additionally, Section 5.1 talks about operational
considerations related to hosting multiple domains.
8. IANA Considerations
This document doesn't require any action from IANA.
9. 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.
10. References
10.1. Normative References
[RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3",
STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996,
<http://www.rfc-editor.org/info/rfc1939>.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
February 2002, <http://www.rfc-editor.org/info/rfc3207>.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
<http://www.rfc-editor.org/info/rfc3501>.
[RFC4985] Santesson, S., "Internet X.509 Public Key Infrastructure
Subject Alternative Name for Expression of Service Name",
RFC 4985, DOI 10.17487/RFC4985, August 2007,
<http://www.rfc-editor.org/info/rfc4985>.
[RFC5804] Melnikov, A., Ed. and T. Martin, "A Protocol for Remotely
Managing Sieve Scripts", RFC 5804, DOI 10.17487/RFC5804,
July 2010, <http://www.rfc-editor.org/info/rfc5804>.
[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, DOI 10.17487/RFC5280, May 2008,
<http://www.rfc-editor.org/info/rfc5280>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
<http://www.rfc-editor.org/info/rfc6066>.
[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, DOI 10.17487/RFC6125, March
2011, <http://www.rfc-editor.org/info/rfc6125>.
[RFC6186] Daboo, C., "Use of SRV Records for Locating Email
Submission/Access Services", RFC 6186,
DOI 10.17487/RFC6186, March 2011,
<http://www.rfc-editor.org/info/rfc6186>.
[RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail",
STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011,
<http://www.rfc-editor.org/info/rfc6409>.
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10.2. Informative References
[RFC2595] Newman, C., "Using TLS with IMAP, POP3 and ACAP",
RFC 2595, DOI 10.17487/RFC2595, June 1999,
<http://www.rfc-editor.org/info/rfc2595>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<http://www.rfc-editor.org/info/rfc3986>.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005,
<http://www.rfc-editor.org/info/rfc4033>.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, DOI 10.17487/RFC4034, March 2005,
<http://www.rfc-editor.org/info/rfc4034>.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
<http://www.rfc-editor.org/info/rfc4035>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
2012, <http://www.rfc-editor.org/info/rfc6698>.
[RFC7711] Miller, M. and P. Saint-Andre, "PKIX over Secure HTTP
(POSH)", RFC 7711, DOI 10.17487/RFC7711, November 2015,
<http://www.rfc-editor.org/info/rfc7711>.
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Appendix A. Acknowledgements
Thank you to Chris Newman, Viktor Dukhovni, Sean Turner, Russ
Housley, Alessandro Vesely, Harald Alvestrand and John Levine 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 to RFC 2595, RFC 3207, RFC 3501, RFC 5804
This section lists detailed changes this document applies to RFC
2595, RFC 3207, RFC 3501 and RFC 5804.
The entire Section 2.4 of RFC 2595 is replaced with the following
text:
During the TLS negotiation, the client checks its understanding of
the server identity against the provided server's identity as
specified in Section 3.
The 3rd paragraph (and its subparagraphs) in Section 11.1 of RFC 3501
is replaced with the following text:
During the TLS negotiation, the IMAP client checks its
understanding of the server identity against the provided server's
identity as specified in Section 3.
The 3rd paragraph (and its subparagraphs) in Section 4.1 of RFC 3207
is replaced with the following text:
During the TLS negotiation, the Submission client checks its
understanding of the server identity against the provided server's
identity as specified in Section 3.
Sections 2.2.1 and 2.2.1.1 of RFC 5804 are replaced with the
following text:
During the TLS negotiation, the ManageSieve client checks its
understanding of the server identity against the server's identity
as specified in Section 3. When the reference identity is an IP
address, the iPAddress subjectAltName SHOULD be used by the client
for comparison. The comparison is performed as described in
Section 2.2.1.2 of RFC 5804.
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Appendix C. 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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