| Network Working Group | A. Melnikov |
| Internet-Draft | Isode Ltd |
| Updates: 2595, 3207, 3501, 5804 (if | December 17, 2015 |
| approved) | |
| Intended status: Standards Track | |
| Expires: June 19, 2016 |
Updated TLS Server Identity Check Procedure for Email Related Protocols
draft-ietf-uta-email-tls-certs-08
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.
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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. [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.
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:
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:
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]:
The rules and guidelines defined in [RFC6125] apply to an email server certificate, with the following supplemental rules:
[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 SRV lookups to be protected by DNSSEC, but this solution depends on DNSSEC and thus is not discussed in this document. A future update to this document might rectify this.)
The ability to issue certificates that contain SRV-ID implies the ability to verify that entities requesting them are authorized to run email service for these SRV-IDs. In particular, certification authorities that can't verify such authorization MUST NOT include email SRV-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 for the domain.
A server that hosts multiple domains needs to do one of the following (or some combination thereof):
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 support 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 use of TLS in multi-tenanted environments. Further work is needed to address this issue, possibly using DNSSEC or something like POSH [RFC7711].
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.
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.
This document doesn't require any action from IANA.
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.
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.
[[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.