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Applications I-D Internet-Draft This specification outlines current recommendations for the use of Transport Layer Security (TLS) to provide confidentiality of email traffic between a mail user agent (MUA) and a mail submission or mail access server.

Software that provides email service via Internet Message Access Protocol (IMAP) , Post Office Protocol (POP) and/or Simple Mail Transfer Protocol (SMTP) Submission usually has Transport Layer Security (TLS) support but often does not use it in a way that maximizes end-user confidentiality. This specification describes current recommendations for the use of TLS in interactions between Mail User Agents and Mail Access Services, and between Mail User Agents and Mail Submission Services. In brief, this memo now recommends that: TLS version 1.2 or greater be used for all traffic between mail user agents (MUAs) and mail submission servers, and also between MUAs and mail access servers. MUAs and mail service providers discourage use of cleartext protocols for mail access and mail submission, and deprecate use of cleartext protocols for these purposes as soon as practicable. Use of "Implicit TLS" on ports reserved for that purpose, in preference to STARTTLS on a port that otherwise supports cleartext. This memo does not address use of TLS with SMTP for message relay (where Message Submission does not apply). Improved use of TLS with SMTP for message relay requires a different approach. One approach to address that topic is described in ; another is in . The recommendations in this memo do not replace the functionality of, and are not intended as a substitute for, end-to-end encryption of electronic mail.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here. The term "Implicit TLS" refers to the automatic negotiation of TLS whenever a TCP connection is made on a particular TCP port that is used exclusively by that server for TLS connections. The term "Implicit TLS" is intended to contrast with use of STARTTLS and similar commands in POP, IMAP, SMTP message submission, and other protocols, that are used by client and server to explicitly negotiate TLS on an established cleartext TCP connection. The term "Mail Access Services" includes POP, IMAP and any other protocol used to access or modify received messages, or to access or modify a mail user's account configuration. "Mail Submission Service" refers to the use of the protocol specified in (or one of its predecessors or successors) for submission of outgoing messages for delivery to recipients. The term "Mail Service Provider" (MSP) refers to a provider of Mail Access Services and/or Mail Submission Services. The term "Mail Account" refers to a user's identity with a Mail Service Provider, that user's authentication credentials, any user email that is stored by the MSP, and any other per-user configuration information maintained by the MSP (for example, spam filtering instructions). Most Mail User Agents (MUAs) support the ability to access multiple Mail Accounts. For each account that an MUA accesses on its user's behalf, it must have the server names, ports, authentication credentials, and other configuration information specified by the user. This information which is used by the MUA is referred to as "Mail Account Configuration" This specification expresses syntax using the Augmented Backus-Naur Form (ABNF) as described in , including the core rules in Appendix B and rules from .

Previous standards for use of email protocols with TLS used the STARTTLS mechanism: , , and . With STARTTLS, the client establishes a cleartext application session and determines whether to issue a STARTTLS command based on server capabilities and client configuration. If the client issues a STARTTLS command, a TLS handshake follows that can upgrade the connection. While this mechanism has been deployed, an alternate mechanism where TLS is negotiated immediately at connection start on a separate port (referred to in this document as "Implicit TLS") has been deployed more successfully. To encourage more widespread use of TLS, and to encourage a greater consistency for how TLS is used, this specification now recommends use of Implicit TLS for POP, IMAP, SMTP Submission, and all other protocols used between a Mail User Agent and a mail service.

When a TCP connection is established for the "pop3s" service (default port 995), a TLS handshake begins immediately. Clients MUST implement the certificate validation mechanism described in . Once the TLS session is established, POP3 protocol messages are exchanged as TLS application data for the remainder of the TCP connection. After the server sends a +OK greeting, the server and client MUST enter AUTHORIZATION state, even if a client certificate was supplied during the TLS handshake. See and for additional information on client certificate authentication. See for port registration information.

When a TCP connection is established for the "imaps" service (default port 993), a TLS handshake begins immediately. Clients MUST implement the certificate validation mechanism described in . Once the TLS session is established, IMAP protocol messages are exchanged as TLS application data for the remainder of the TCP connection. If a client certificate was provided during the TLS handshake that the server finds acceptable, the server MAY issue a PREAUTH greeting in which case both the server and client enter AUTHENTICATED state. If the server issues an OK greeting then both server and client enter NOT AUTHENTICATED state. See and for additional information on client certificate authentication. See and for port registration information.

When a TCP connection is established for the "submissions" service (default port 465), a TLS handshake begins immediately. Clients MUST implement the certificate validation mechanism described in . Once a TLS session is established, message submission protocol data is exchanged as TLS application data for the remainder of the TCP connection. (Note: the "submissions" service name is defined in section 10.3 of this document, and follows the usual convention that the name of a service layered on top of Implicit TLS consists of the name of the service as used without TLS, with an "s" appended.) The STARTTLS mechanism on port 587 is relatively widely deployed due to the situation with port 465 (discussed in ). This differs from IMAP and POP services where Implicit TLS is more widely deployed on servers than STARTTLS. It is desirable to migrate core protocols used by MUA software to Implicit TLS over time for consistency as well as the additional reasons discussed in . However, to maximize use of encryption for submission it is desirable to support both mechanisms for Message Submission over TLS for a transition period of several years. As a result, clients and servers SHOULD implement both STARTTLS on port 587 and Implicit TLS on port 465 for this transition period. Note that there is no significant difference between the security properties of STARTTLS on port 587 and Implicit TLS on port 465 if the implementations are correct and both client and server are configured to require successful negotiation of TLS prior to message submission. Note that the "submissions" port provides access to a Mail Submission Agent (MSA) as defined in so requirements and recommendations for MSAs in that document apply to the submissions port, including the requirement to implement SMTP AUTH . See and for additional information on client certificate authentication. See for port registration information.

When a client or server wishes to close the connection, it SHOULD initiate the exchange of TLS close alerts before TCP connection termination. The client MAY, after sending a TLS close alert, gracefully close the TCP connection (e.g. call the close() function on the TCP socket or otherwise issue a TCP CLOSE ( section 3.5) without waiting for a TLS response from the server.

The following requirements and recommendations apply to Mail Access Services and Mail Submission Services: Mail Service Providers (MSPs) that support POP, IMAP, and/or Message Submission, MUST support TLS access for those services. Other services than POP, IMAP and/or Message Submission provided by MSPs SHOULD support TLS access, and MUST support TLS access for those services which support authentication via username and password. MSPs that support POP, IMAP, and/or Message Submission, SHOULD provide and support instances of those services which use Implicit TLS. (See .) For compatibility with existing MUAs and existing MUA configurations, MSPs SHOULD also, in the near term, provide instances of these services which support STARTTLS. This will permit legacy MUAs to discover new availability of TLS capability on servers, and may increase use of TLS by such MUAs. However, servers SHOULD NOT advertise STARTTLS if use of the STARTTLS command by a client is likely to fail (for example, if the server has no server certificate configured.) MSPs SHOULD advertise their Mail Access Services and Mail Submission Services using DNS SRV records according to . (In addition to making correct configuration easier for MUAs, this provides a way by which MUAs can discover when an MSP begins to offer TLS-based services.) Services supporting TLS SHOULD be advertised in preference to cleartext services (if offered). In addition, services using Implicit TLS SHOULD be advertised in preference to services supporting STARTTLS (if offered). (See also .) MSPs SHOULD deprecate use of cleartext Mail Access Services and Mail Submission Services as soon as practicable. (See .) MSPs currently supporting such use of cleartext SMTP (on port 25) as a means of message submission by their users (whether or not requiring authentication) SHOULD transition their users to using TLS (either Implicit TLS or STARTTLS) as soon as practicable. Mail services MUST support TLS 1.2 or later. All Mail services SHOULD implement the recommended TLS cipher suites described in or a future BCP or standards track revision of that document. Mail services currently supporting SSL 2.x, SSL 3.0, or TLS 1.0 SHOULD transition their users to later versions of TLS, and discontinue support for those versions of SSL and TLS, as soon as practicable. Mail Submission Servers accepting mail using TLS SHOULD include the TLS ciphersuite of the session in which the mail was received, in the Received field of the outgoing message. (See .) All Mail services implementing TLS SHOULD log TLS cipher information along with any connection or authentication logs that they maintain. Additional considerations and details appear below.

The specific means employed for deprecation of cleartext Mail Access Services and Mail Submission Services MAY vary from one MSP to the next in light of their user communities' needs and constraints. For example, an MSP MAY implement a gradual transition in which, over time, more and more users are forbidden to authenticate to cleartext instances of these services, thus encouraging those users to migrate to Implicit TLS. Access to cleartext services should eventually be either disabled, or limited strictly for use by legacy systems which cannot be upgraded. After a user's ability to authenticate to a service using cleartext is revoked, the server denying such access MUST NOT provide any indication over a cleartext channel of whether the user's authentication credentials were valid. An attempt to authenticate as such a user using either invalid credentials or valid credentials MUST both result in the same indication of access being denied. Also, users previously authenticating with passwords sent as cleartext SHOULD be required to change those passwords when migrating to TLS, if the old passwords were likely to have been compromised. (For any large community of users using public Internet to access mail without encryption, compromise of at least some of those passwords should be assumed.) Transition of users from SSL or TLS 1.0 to later versions of TLS MAY be accomplished by a means similar to that described above. There are multiple ways to accomplish this. One way is for the server to refuse a ClientHello message from any client sending a ClientHello.version field corresponding to any version of SSL or TLS 1.0. Another way is for the server to accept ClientHello messages from some client versions that it does not wish to support, but later refuse to allow the user to authenticate. The latter method may provide a better indication to the user of the reason for the failure but (depending on the protocol and method of authentication used) may also risk exposure of the user's password over an channel which is known to not provide adequate confidentiality. It is RECOMMENDED that new users be required to use TLS version 1.1 or greater from the start. However an MSP may find it necessary to make exceptions to accommodate some legacy systems which support only earlier versions of TLS, or only cleartext.

Mail servers MAY implement client certificate authentication on the Implicit TLS port. Servers MUST NOT request a client certificate during the TLS handshake unless the server is configured to accept some client certificates as sufficient for authentication and the server has the ability to determine a mail server authorization identity matching such certificates. How to make this determination is presently implementation specific. If the server accepts the client's certificate as sufficient for authorization, it MUST enable the SASL EXTERNAL mechanism. An IMAPS server MAY issue a PREAUTH greeting instead of enabling SASL EXTERNAL.

The ESMTPS transmission type provides trace information that can indicate TLS was used when transferring mail. However, TLS usage by itself is not a guarantee of confidentiality or security. The TLS cipher suite provides additional information about the level of security made available for a connection. This defines a new SMTP "tls" Received header additional-registered-clause that is used to record the TLS cipher suite that was negotiated for the connection. This clause SHOULD be included whenever a Submission server generates a Received header field for a message received via TLS. The value included in this additional clause SHOULD be the registered cipher suite name (e.g., TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) included in the TLS cipher suite registry. In the event the implementation does not know the name of the cipher suite (a situation that should be remedied promptly), a four-digit hexadecimal cipher suite identifier MAY be used. In addition, the Diffie-Hellman group name associated with the ciphersuite MAY be included (when applicable and known) following the ciphersuite name. The ABNF for the field follows:

tls-cipher-clause = CFWS "tls" FWS tls-cipher [ CFWS "group" FWS dh-group ] tls-cipher = tls-cipher-name / tls-cipher-hex tls-cipher-name = ALPHA *(ALPHA / DIGIT / "_") ; as registered in IANA cipher suite registry tls-cipher-hex = "0x" 4HEXDIG dh-group = ALPHA *(ALPHA / DIGIT / "_" / "-") ; as registered in IANA TLS Supported Groups Registry

MSPs MUST maintain valid server certificates for all servers. See for the recommendations and requirements necessary to achieve this. If a protocol server provides service for more than one mail domain, it MAY use a separate IP address for each domain and/or a server certificate that advertises multiple domains. This will generally be necessary unless and until it is acceptable to impose the constraint that the server and all clients support the Server Name Indication extension to TLS . Mail servers supporting SNI need to support the post-SRV hostname to interoperate with MUAs that have not implemented RFC 6186. For more discussion of this problem, see section 5.1 of .

This section discusses not only the DNS records that are recommended, but also implications of DNS records for server configuration and TLS server certificates.

It is recommended that MSPs advertise MX records for handling of inbound mail (instead of relying entirely on A or AAAA records), and that those MX records be signed using DNSSEC . This is mentioned here only for completeness, as handling of inbound mail is out of scope for this document.

MSPs SHOULD advertise SRV records to aid MUAs in determination of proper configuration of servers, per the instructions in . MSPs SHOULD advertise servers that support Implicit TLS in preference to those which support cleartext and/or STARTTLS operation.

All DNS records advertised by an MSP as a means of aiding clients in communicating with the MSP's servers, SHOULD be signed using DNSSEC if and when the parent DNS zone supports doing so.

MSPs SHOULD advertise TLSA records to provide an additional trust anchor for public keys used in TLS server certificates. However, TLSA records MUST NOT be advertised unless they are signed using DNSSEC.

When an MSP changes the Internet Facing Servers providing mail access and mail submission services, including SMTP-based spam/virus filters, it is generally necessary to support the same and/or a newer version of TLS and the same security directives that were previously advertised.

The following requirements and recommendations apply to Mail User Agents: MUAs SHOULD be capable of using DNS SRV records to discover Mail Access Services and Mail Submission Services that are advertised by a MSP for an account being configured. Other means of discovering server configuration information (e.g. a database maintained by the MUA vendor) MAY also be supported. (See for more information.) MUAs SHOULD be configurable to require a minimum level of confidentiality for any particular Mail Account, and refuse to exchange information via any service associated with that Mail Account if the session does not provide that minimum level of confidentiality. (See .) MUAs MUST NOT treat a session as meeting a minimum level of confidentiality if the server's TLS certificate cannot be validated. (See .) MUAs MAY impose other minimum confidentiality requirements in the future, e.g. in order to discourage use of TLS versions or cryptographic algorithms in which weaknesses have been discovered. MUAs SHOULD provide a prominent indication of the level of confidentiality associated with an account configuration that is appropriate for the user interface (for example, a "lock" icon or changed background color for a visual interface, or some sort of audible indication for an audio user interface), at appropriate times and/or locations in order to inform the user of the confidentiality of the communications associated with that account. For example, this might be done whenever (a) prompting the user for authentication credentials, (b) the user is composing mail that will be sent to a particular submission server, (c) a list of accounts is displayed (particularly if the user can select from that list to read mail), or (d) the user is requesting to view or update any configuration data that will be stored on a remote server. If, however, an MUA provides such an indication, it MUST NOT indicate confidentiality for any connection that does not at least use TLS 1.1 with certificate verification and also meet the minimum confidentiality requirements associated with that account. MUAs MUST implement TLS 1.2 or later. Earlier TLS and SSL versions MAY also be supported so long as the MUA requires at least TLS 1.1 when accessing accounts that are configured to impose minimum confidentiality requirements. All MUAs SHOULD implement the recommended TLS cipher suites described in or a future BCP or standards track revision of that document. MUAs that are configured to not require minimum confidentiality for one or more accounts SHOULD detect when TLS becomes available on those accounts (using or other means), and offer to upgrade the account to require TLS. Additional considerations and details appear below.

This section updates by changing the preference rules and adding a new SRV service label _submissions._tcp to refer to Message Submission with Implicit TLS. User-configurable MUAs SHOULD support use of for account setup. However, when using configuration information obtained by this method, MUAs SHOULD ignore advertised services that do not satisfy minimum confidentiality requirements, unless the user has explicitly requested reduced confidentiality. This will have the effect of causing the MUA to default to ignoring advertised configurations that do not support TLS, even when those advertised configurations have a higher priority than other advertised configurations. When using configuration information, Mail User Agents SHOULD NOT automatically establish new configurations that do not require TLS for all servers, unless there are no advertised configurations using TLS. If such a configuration is chosen, prior to attempting to authenticate to the server or use the server for message submission, the MUA SHOULD warn the user that traffic to that server will not be encrypted and that it will therefore likely be intercepted by unauthorized parties. The specific wording is to be determined by the implementation, but it should adequately capture the sense of risk given the widespread incidence of mass surveillance of email traffic. Similarly, a MUA MUST NOT attempt to "test" a particular mail account configuration by submitting the user's authentication credentials to a server, unless a TLS session meeting minimum confidentiality levels has been established with that server. If minimum confidentiality requirements have not been satisfied, the MUA must explicitly warn the user that his password may be exposed to attackers before testing the new configuration. When establishing a new configuration for connecting to an IMAP, POP, or SMTP submission server, based on SRV records, an MUA SHOULD either verify that the SRV records are signed using DNSSEC, or that the target FQDN of the SRV record matches the original server FQDN for which the SRV queries were made. If the target FQDN is not in the queried domain, the MUA SHOULD verify with the user that the SRV target FQDN is suitable for use, before executing any connections to the host. (See section 6). An MUA MUST NOT consult SRV records to determine which servers to use on every connection attempt, unless those SRV records are signed by DNSSEC and have a valid signature. However, an MUA MAY consult SRV records from time to time to determine if an MSP's server configuration has changed, and alert the user if it appears that this has happened. This can also serve as a means to encourage users to upgrade their configurations to require TLS if and when their MSPs support it.

MUAs SHOULD, by default, require a minimum level of confidentiality for services accessed by each account. For MUAs supporting the ability to access multiple mail accounts, this requirement SHOULD be configurable on a per-account basis. The default minimum expected level of confidentiality for all new accounts MUST require successful validation of the server's certificate and SHOULD require negotiation of TLS version 1.1 or greater. (Future revisions to this specification may raise these requirements or impose additional requirements to address newly-discovered weaknesses in protocols or cryptographic algorithms.) MUAs MAY permit the user to disable this minimum confidentiality requirement during initial account configuration, or subsequently editing an account configuration, but MUST warn users that such a configuration will not assure privacy for either passwords or messages. An MUA which is configured to require a minimum level of confidentiality for a mail account MUST NOT attempt to perform any operation other than capability discovery, or STARTTLS for servers not using Implicit TLS, unless the minimum level of confidentiality is provided by that connection. MUAs SHOULD NOT allow users to easily access or send mail via an connection, or authenticate to any service using a password, if that account is configured to impose minimum confidentiality requirements and that connection does not meet all of those requirements. An example of "easily access" would be to display a dialog informing the user that the security requirements of the account were not met by the connection, but allowing the user to "click through" to send mail or access the service anyway. Experience indicates that users presented with such an option often "click through" without understanding the risks that they're accepting by doing so. Furthermore, users who frequently find the need to "click through" to use an insecure connection may become conditioned to do so as a matter of habit, before considering whether the risks are reasonable in each specific instance. An MUA which is not configured to require a minimum level of confidentiality for a mail account SHOULD still attempt to connect to the services associated with that account using the most secure means available, e.g. by using Implicit TLS or STARTTLS.

MUAs MUST validate TLS server certificates according to and PKIX . MUAs MAY also support DANE as a means of validating server certificates in order to meet minimum confidentiality requirements. MUAs MAY support use of certificate pinning but MUST NOT consider a connection in which the server's authenticity relies on certificate pinning, as providing the minimum level of confidentiality. (See .)

During account setup, the MUA will identify servers that provide account services such as mail access and mail submission (the previous section describes one way to do this). The certificates for these servers are verified using the rules described in and PKIX . In the event the certificate does not validate due to an expired certificate, lack of appropriate chain of trust, or lack of identifier match, the MUA MAY offer to create a persistent binding between that certificate and the saved host name for the server, for use when accessing that account's servers. This is called certificate pinning. (Note: This use of the term "certificate pinning" means something subtly different than "HTTP Public Key Pinning" . The dual use of the same term is confusing, but unfortunately both uses are well-established.) Certificate pinning is only appropriate during mail account setup and MUST NOT be offered as an option in response to a failed certificate validation for an existing mail account. An MUA that allows certificate pinning MUST NOT allow a certificate pinned for one account to validate connections for other accounts. An MUA that allows certificate pinning MUST also allow a user to undo the pinning, i.e. to revoke trust in a certificate that has previously been pinned. A pinned certificate is subject to a man-in-the-middle attack at account setup time, and typically lacks a mechanism to automatically revoke or securely refresh the certificate. Note also that a man-in-the-middle attack at account setup time will expose the user's password to the attacker (if a password is used). Therefore use of a pinned certificate does not meet the requirement for a minimum confidentiality level, and an MUA MUST NOT indicate to the user that the such confidentiality is provided. Additional advice on certificate pinning is present in .

MUAs MAY implement client certificate authentication on the Implicit TLS port. An MUA MUST NOT provide a client certificate during the TLS handshake unless the server requests one and the MUA has been authorized to use that client certificate with that account. Having the end-user explicitly configure a client certificate for use with a given account is sufficient to meet this requirement. However, installing a client certificate for use with one account MUST NOT automatically authorize use of that certificate with other accounts. This is not intended to prohibit site-specific authorization mechanisms, such as a site-administrator-controlled mechanism to authorize use of a client certificate with a given account, or a domain-name matching mechanism. Note: The requirement that the server request a certificate is just a restatement of the TLS protocol rules, e.g. section 7.4.6. The requirement that the client not send a certificate not known to be acceptable to the server is pragmatic in multiple ways: the current TLS protocol provides no way for the client to know which of potentially multiple certificates it should use; also, when the client sends a certificate it is potentially disclosing its identity (or its user's identity) to both the server and to any party with access to the transmission medium, perhaps unnecessarily and for no useful purpose. A client supporting client certificate authentication with Implicit TLS MUST implement the SASL EXTERNAL mechanism using the appropriate authentication command (AUTH for POP3 , AUTH for SMTP Submission , AUTHENTICATE for IMAP ).

There are multiple ways to connect an Anti-Virus and/or Anti-Spam (AVAS) service to a mail server. Some mechanisms, such as the de-facto milter protocol, are out of scope for this specification. However, some services use an SMTP relay proxy that intercepts mail at the application layer to perform a scan and proxy or forward to another MTA. Deploying AVAS services in this way can cause many problems including direct interference with this specification, and other forms of confidentiality or security reduction. An AVAS product or service is considered compatible with this specification if all IMAP, POP and SMTP-related software (including proxies) it includes are compliant with this specification. Note that end-to-end email encryption prevents AVAS software and services from using email content as part of a spam or virus assessment. Furthermore, while a minimum confidentiality level can prevent a man-in-the-middle from introducing spam or virus content between the MUA and Submission server, it does not prevent other forms of client or account compromise. Use of AVAS services for submitted email therefore remains necessary.

IANA is asked to update the registration of the TCP well-known port 995 using the following template ():

Service Name: pop3s Transport Protocol: TCP Assignee: IETF <iesg@ietf.org> Contact: IESG <iesg@ietf.org> Description: POP3 over TLS protocol Reference: RFC XXXX (this document once published) Port Number: 995

IANA is asked to update the registration of the TCP well-known port 993 using the following templates ():

Service Name: imaps Transport Protocol: TCP Assignee: IETF <iesg@ietf.org> Contact: IESG <iesg@ietf.org> Description: IMAP over TLS protocol Reference: RFC XXXX (this document once published) Port Number: 993

No changes to existing UDP port assignments for pop3s or imaps are being requested.

IANA is asked to assign an alternate usage of TCP port 465 in addition to the current assignment using the following template ():

Service Name: submissions Transport Protocol: TCP Assignee: IETF <iesg@ietf.org> Contact: IESG <iesg@ietf.org> Description: Message Submission over TLS protocol Reference: RFC XXXX (this document once published) Port Number: 465

This is a one-time procedural exception to the rules in RFC 6335. This requires explicit IESG approval and does not set a precedent. Note: Since the purpose of this alternate usage assignment is to align with widespread existing practice, and there is no known usage of UDP port 465 for message submission over TLS, IANA is not being asked to assign an alternate usage of UDP port 465. Historically, port 465 was briefly registered as the "smtps" port. This registration made no sense as the SMTP transport MX infrastructure has no way to specify a port, so port 25 is always used. As a result, the registration was revoked and was subsequently reassigned to a different service. In hindsight, the "smtps" registration should have been renamed or reserved rather than revoked. Unfortunately, some widely deployed mail software interpreted "smtps" as "submissions" and used that port for email submission by default when an end-user requests security during account setup. If a new port is assigned for the submissions service, email software will either continue with unregistered use of port 465 (leaving the port registry inaccurate relative to de-facto practice and wasting a well-known port), or confusion between the de-facto and registered ports will cause harmful interoperability problems that will deter use of TLS for message submission. The authors believe both of these outcomes are less desirable than a wart in the registry documenting real-world usage of a port for two purposes. Although STARTTLS-on-port-587 has deployed, it has not replaced deployed use of Implicit TLS submission on port 465.

Per the provisions in , IANA is requested to add two additional-registered-clauses for Received fields as defined in of this document: "tls" indicating the TLS cipher used (if applicable), and "group" indicating the Diffie-Hellman group used with the TLS cipher (if applicable) The descriptions and syntax of these additional clauses are in of this document.

This entire document is about security considerations. In general, this is targeted to improve mail confidentiality and to mitigate threats external to the email system such as network-level snooping or interception; this is not intended to mitigate active attackers who have compromised service provider systems. Implementers should be aware that use of client certificates with TLS 1.2 reveals the user's identity to any party with ability to read packets from the transmission medium, and therefore may compromise the user's privacy. There seems to be no easy fix with TLS 1.2 or earlier versions other than to avoid presenting client certificates except when there is explicit authorization to do so. TLS 1.3 appears to reduce the privacy risk somewhat.

This section is not normative. The first version of this was written independently from draft-moore-email-tls-00.txt; subsequent versions merge ideas from both drafts. One author of this document was also the author of RFC 2595 that became the standard for TLS usage with POP and IMAP, and the other author was perhaps the first to propose that idea. In hindsight both authors now believe that that approach was a mistake. At this point the authors believe that while anything that makes it easier to deploy TLS is good, the desirable end state is that these protocols always use TLS, leaving no need for a separate port for cleartext operation except to support legacy clients while they continue to be used. The separate port model for TLS is inherently simpler to implement, debug and deploy. It also enables a "generic TLS load-balancer" that accepts secure client connections for arbitrary foo-over-TLS protocols and forwards them to a server that may or may not support TLS. Such load-balancers cause many problems because they violate the end-to-end principle and the server loses the ability to log security-relevant information about the client unless the protocol is designed to forward that information (as this specification does for the cipher suite). However, they can result in TLS deployment where it would not otherwise happen which is a sufficiently important goal that it overrides the problems. Although STARTTLS appears only slightly more complex than separate-port TLS, we again learned the lesson that complexity is the enemy of security in the form of the STARTTLS command injection vulnerability (CERT vulnerability ID #555316). Although there's nothing inherently wrong with STARTTLS, the fact it resulted in a common implementation error (made independently by multiple implementers) suggests it is a less secure architecture than Implicit TLS. Section 7 of RFC 2595 critiques the separate-port approach to TLS. The first bullet was a correct critique. There are proposals in the http community to address that, and use of SRV records as described in RFC 6186 resolves that critique for email. The second bullet is correct as well, but not very important because useful deployment of security layers other than TLS in email is small enough to be effectively irrelevant. (Also it's less correct than it used to be because "export" ciphersuites are no longer supported in modern versions of TLS.) The third bullet is incorrect because it misses the desirable option of "use and latch-on TLS if available". The fourth bullet may be correct, but is not a problem yet with current port consumption rates. The fundamental error was prioritizing a perceived better design based on a mostly valid critique over real-world deployability. But getting security and confidentiality facilities actually deployed is so important it should trump design purity considerations. Port 465 is presently used for two purposes: for submissions by a large number of clients and service providers and for the "urd" protocol by one vendor. Actually documenting this current state is controversial as discussed in the IANA considerations section. However, there is no good alternative. Registering a new port for submissions when port 465 is widely used for that purpose already will just create interoperability problems. Registering a port that's only used if advertised by an SRV record (RFC 6186) would not create interoperability problems but would require all client and server deployments and software to change significantly which is contrary to the goal of promoting more TLS use. Encouraging use of STARTTLS on port 587 would not create interoperability problems, but is unlikely to have impact on current undocumented use of port 465 and makes the guidance in this document less consistent. The remaining option is to document the current state of the world and support future use of port 465 for submission as this increases consistency and ease-of-deployment for TLS email submission.

Changes since draft-ietf-uta-email-deep-07: After discussion with the WG in Prague, removed BCP language and once again made unambiguous that this is intended as a standards-track document. Server implementations now MUST implement TLS 1.2, consistent with RFC 7525. MUAs may still consider a TLS 1.1 session as meeting minimum confidentiality requirements. MSPs now MUST support TLS for POP, IMAP, Submission, and any other services that use username/password authentication. Added text to clarify the purpose of recommending that MSPs use DNS SRV records to advertise services. Changed text about MUAs not blindly trusting unsigned SRV records, to instead restate RFC 6186 requirements. Changes since draft-ietf-uta-email-deep-06: On the recommendation of one of the co-chairs and some working group members, rewrote document with the intended status of BCP. This involved removing a great deal of text that consisted essentially of new protocol specification, especially the STS features, on the theory that a BCP should base its recommendations on current practice, and that new protocol features should be subject to the interoperability test requirements associated with normal standards-track documents. Changes since draft-ietf-uta-email-deep-05: Clarify throughout that the confidentiality assurance level associated with a mail account is a minimum level; attempt to distinguish this from the current confidentiality level provided by a connection between client and server. Change naming for confidentiality assurance levels: instead of "high" or "no" confidence, assign numbers 1 and 0 to them respectively. This because it seems likely that in the not-too-distant future, what was defined in -05 as "high" confidence will be considered insufficient, and calling that "high" confidence will become misleading. For example, relying entirely on a list of trusted CAs to validate server certificates from arbitrary parties, appears to be less and less reliable in practice at thwarting MITM attacks. Clarify that if some services associated with a mail account don't meet the minimum confidentiality assurance level assigned to that account, other services that do meet that minimum confidentiality assurance level may continue to be used. Clarify that successful negotiation of at least TLS version 1.1 is required as a condition of meeting confidentiality assurance level 1. Clarify that validation of a server certificate using either DANE or PKIX is sufficient to meet the certificate validation requirement of confidentiality assurance level 1. Clarify that minimum confidentiality assurance levels are separate from security directives, and that the requirements of both mechanisms must be met. Explicitly cite an example that a security directive of tls-version=1.2 won't be saved if the currently negotiated tls-version is 1.1. (This example already appeared a bit later in the text, but for author KM it seemed to make the mechanism clearer to use this example earlier.) Clarify some protocol examples as to whether PKIX or DANE was used to verify a server's certificate. Remove most references to DEEP as the conversion from DEEP to MUA-STS seemed incomplete, but kept the DEEP command for use in POP3 on the assumption that author CN wanted it that way. Removed most references to "latch" and derivative words. Added pkix+dane as a value for the tls-cert directive, to indicate (from a server) that both PKIX and DANE validation will be supported, or (from a client) that both PKIX and DANE were used to validate a certificate. Also clarified what each of any, pkix, dane, and pkix+dane mean when advertised by a server and in particular that tls-cert=any provides no assurance of future PKIX verifiability in contrast to tls-cert=pkix or tls-cert=pkix+dane. It seemed important to support the ability to evolve to using multiple trust anchors for certificate validation, but also to allow servers to have the option to migrate from PKIX to DANE if that made sense for them. This change seemed less disruptive than either defining additional directives, or allowing multiple instances of the same directive with different values to appear in the same advertisement. Clarify interaction of this specification with anti-virus / anti-spam mechanisms. Changes since draft-ietf-uta-email-deep-04: Swap sections 5.1 and 5.3 ("Email Security Tags" and "Server DEEP Status") as that order may aid understanding of the model. Also rewrote parts of these two sections to try to make the model clearer. Add text about versioning of security tags to make the model clearer. Add example of security tag upgrade. Convert remaining mention of TLS 1.0 to TLS 1.1. Change document title from DEEP to MUA STS to align with SMTP relay STS. Slight updates to abstract and introductions. Rename security latches/tags to security directives. Rename server DEEP status to STS policy. Change syntax to use directive-style HSTS syntax. Make HSTS reference normative. Remove SMTP DSN header as that belongs in SMTP relay STS document. Changes since draft-ietf-uta-email-deep-03: Add more references to ietf-uta-email-tls-certs in implementation requirements section. Replace primary reference to RFC 6125 with ietf-uta-email-tls-certs, so move RFC 6125 to informative list for this specification. Changes since draft-ietf-uta-email-deep-02: Make reference to design considerations explicit rather than "elsewhere in this document". Change provider requirement so SMTP submission services are separate from SMTP MTA services as opposed to the previous phrasing that required the servers be separate (which is too restrictive). Update DANE SMTP reference Changes since draft-ietf-uta-email-deep-01: Change text in tls11 and tls12 registrations to clarify certificate rules, including additional PKIX and DANE references. Change from tls10 to tls11 (including reference) as the minimum. Fix typo in example 5. Remove open issues section; enough time has passed so not worth waiting for more input. Changes since draft-ietf-uta-email-deep-00: Update and clarify abstract use term confidentiality instead of privacy in most cases. update open issues to request input for missing text. move certificate pinning sub-section to account setup section and attempt to define it more precisely. Add note about end-to-end encryption in AVAS section. swap order of DNSSEC and TLSA sub-sections. change meaning of 'tls10' and 'tls12' latches to require certificate validation. Replace cipher suite advice with reference to RFC 7525. Change examples to use TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 as cipher suite. Add text to update IMAP, POP3 and Message Submission standards with newer TLS advice. Add clearer text in introduction that this does not cover SMTP relay. Update references to uta-tls-certs. Add paragraph to Implicit TLS for SMTP Submission section recommending that STARTTLS also be implemented. Changes since draft-newman-email-deep-02: Changed "privacy assurance" to "confidentiality assurance" Changed "low privacy assurance" to "no confidentiality assurance" Attempt to improve definition of confidentiality assurance level. Add SHOULD indicate when MUA is showing list of mail accounts. Add SHOULD NOT latch tls10, tls12 tags until TLS negotiated. Removed sentence about deleting and re-creating the account in latch failure section. Remove use of word "fallback" with respect to TLS version negotiation. Added bullet about changes to Internet facing servers to MSP section. minor wording improvements based on feedback Changes since -01: Updated abstract, introduction and document structure to focus more on mail user agent privacy assurance. Added email account privacy section, also moving section on account setup using SRV records to that section. Finished writing IANA considerations section Remove provisional concept and instead have server explicitly list security tags clients should latch. Added note that rules for the submissions port follow the same rules as those for the submit port. Reference and update advice in . Fixed typo in Client Certificate Authentication section. Removed tls-pfs security latch and all mention of perfect forward secrecy as it was controversial. Added reference to HSTS. Changes since -00: Rewrote introduction to merge ideas from draft-moore-email-tls-00. Added Implicit TLS section, Account configuration section and IANA port registration updates based on draft-moore-email-tls-00. Add protocol details necessary to standardize implicit TLS for POP/IMAP/submission, using ideas from draft-melnikov-pop3-over-tls. Reduce initial set of security tags based on feedback. Add deep status concept to allow a window for software updates to be backed out before latches make that problematic, as well as to provide service providers with a mechanism they can use to assist customers in the event of a privacy failure. Add DNS SRV section from draft-moore-email-tls-00. Write most of the missing IANA considerations section. Rewrite most of implementation requirements section based more on draft-moore-email-tls-00. Remove new cipher requirements for now because those may be dealt with elsewhere.

Thanks to Ned Freed for discussion of the initial latch concepts in this document. Thanks to Alexey Melnikov for draft-melnikov-pop3-over-tls-02, which was the basis of the POP3 Implicit TLS text. Thanks to Russ Housley, Alexey Melnikov and Dan Newman for review feedback. Thanks to Paul Hoffman for interesting feedback in initial conversations about this idea.