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Applications I-D Internet-Draft This specification defines a set of requirements and facilities designed to improve email confidentiality between a mail user agent (MUA) and a mail submission or mail access server. This provides mechanisms intended to increase use of already deployed Transport Layer Security (TLS) technology and provides a model for a mail user agent's confidentiality assurance. This enables mail service providers to advertise strict transport security (STS) policies that request MUAs increase confidentiality assurance.

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 proposes changes to email software and deployments intended to increase the use of TLS and record when that use occurs. This adapts the strict transport security (STS) model described in to cover mail user agents (MUAs). In brief, this memo now recommends that: MUAs associate a confidentiality assurance level with each mail account, and the default level requires use of TLS with certificate validation for all TCP connections; TLS on a well-known port ("Implicit TLS") be supported for IMAP, POP, and SMTP Submission for all electronic mail user agents (MUAs), servers, and service providers; MUAs and mail protocol servers cooperate (via mechanisms defined in this specification) to upgrade security feature use and record/indicate that usage appropriately. The security upgrade model is aligned with the HTTP STS specification . This 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 . 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. This draft is subject to change. Implementation of this proposal is not recommended at this time. Please discuss this proposal on the ietf-uta mailing list.

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 . This specification expresses syntax using the Augmented Backus-Naur Form (ABNF) as described in , including the core rules in Appendix B and rules from . In examples, "C:" and "S:" indicate lines sent by the client and server respectively. If a single "C:" or "S:" label applies to multiple lines, then the line breaks between those lines are for editorial clarity only and are not part of the actual protocol exchange.

A "mail account" refers to the network services an end user uses to read, submit and manage email communications on the Internet. This typically involves at least one mail access server (IMAP or POP) and at least one SMTP submission server. An end users uses a mail user agent (MUA) to access a mail account and most MUAs support one or more mail accounts. This document uses the term "confidentiality assurance level" to indicate the degree to which the network connections between an MUA and a mail account have confidentiality protection from both passive and active attackers on the network. The configuration necessary for a mail account includes an email address, connection information and authentication credentials for network services. MUAs compliant with this specification MUST also associate a confidentiality assurance level with each mail account. MUAs MUST implement a high confidentiality assurance level as described in the next section. MUAs SHOULD continuously indicate to the user the confidentiality assurance level of the account currently in use when reading, submitting and managing mail (e.g., via a lock icon, background colors and indications similar to those commonly used in web browsers for a similar purpose) and SHOULD indicate the confidentiality assurance level for each account whenever displaying a list of mail accounts. Note that the displayed confidentiality assurance level could be higher than the level set at account configuration but never lower. If multiple active connections are associated with an account or view, the indication should match the level provided by the least confidential connection. Account configuration occurs when an MUA is first used to access a particular service, when a user wishes to access or submit mail through servers in addition to those specified or found during first use, or when a user explicitly requests to change account configuration parameters such as server names, user names, passwords, client certificates, etc. Account configuration can be entirely manual (entering server names explicitly) or partially automated via a mechanism such as DNS SRV records . MUAs SHOULD use the high confidentiality assurance level as the default for newly configured accounts.

A mail account has a high confidentiality assurance when the following conditions are met on all TCP server connections associated with an account. This includes connections to POP, IMAP and SMTP submission servers as well as any other associated protocols defined now or in the future. Examples of protocols associated with a mail account include managesieve and MTQP . TCP connections MUST attempt to negotiate TLS via either Implicit TLS or STARTTLS. MUAs MUST implement and PKIX . MUAs MAY implement DANE . User agents MUST abort a TLS session if the TLS negotiation fails or the server's certificate or identity fails to verify. A user may reconfigure the account to lower the expected level of confidentiality if he/she chooses. Reduction of expected account confidentiality MUST NOT be done on a click-through basis. The end user is part of the system that protects the user's confidentiality and security. As a result, it's critical not to present the end user with a simple action that reduces their confidentiality in response to certificate validation failure. An MUA which offers a user actions such as "connect anyway", "trust certificate for future connections" or "lower confidentiality assurance for this account" in response to certificate validation failure is not providing a high confidentiality assurance as defined in this section and thus does not comply with this document. Examples of acceptable actions to offer would be "work offline", "try again later", and "open service provider status web page".

MUAs MAY implement a no confidentiality assurance level for accounts. At this level, the MUA MUST attempt to negotiate TLS, but MAY ignore server certificate validation failures. MUAs MAY support use of connections without TLS, but if they do they SHOULD attempt TLS first if available and MUST implement code to reconnect without TLS if TLS negotiation fails for reasons other than server certificate validity. Note that if the TLS certificate is not successfully validated as described in or a version of SSL/TLS prior to TLS 1.1 is used, the client MUST NOT present a high confidentiality indication for the account or connection.

This specification is not intended to limit experimentation and innovation with respect to user confidentiality. As a result more confidentiality assurance levels are permitted. However, levels below "no confidentiality assurance" described in the previous section are discouraged and implementers are cautioned that end users may be confused by too many confidentiality assurance levels.

Previous standards for use of email protocols with TLS used the STARTTLS mechanism: , , and . With STARTTLS, the client establishes a clear text 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 increase use of TLS, this specification recommends use of implicit TLS by new POP, IMAP and SMTP Submission software.

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 client credentials were supplied during the TLS handshake. See 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 and SHOULD 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 client credentials were 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 for additional information on client certificate authentication. See 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 (as required in ). 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 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 without waiting for a TLS response from the server.

Once an improved email security mechanism is deployed and ready for general use, it is desirable to continue using it for all future email service. For example, TLS is widely deployed in email software, but use of TLS is often not required. At the time this is written, deployed mail user agents (MUAs) usually make a determination if TLS is available when an account is first configured and may require use of TLS with that account if and only if it was initially available. If the service provider makes TLS available after initial client configuration, many MUAs will not notice the change. Alternatively, a security feature may be purely opportunistic and thus subject to downgrade attacks. For example, at the time this was written, most TLS stacks that support TLS 1.2 will use an older TLS version if the peer does not support TLS 1.2 and many do so without alerting the user of the reduced security. Thus a variety of active attacks could cause the loss of TLS 1.2 benefits. Only if client policy is upgraded to require TLS 1.2 can the client prevent all downgrade attacks. However, this sort of security policy upgrade will be ignored by most users unless it is automated. This section describes a mechanism, called "security directives", which is designed to permit an MUA to recognize when a service provider has committed to provide certain server security features, and that it's safe for the client to change its configuration for that account to require that such features be present in future sessions with that server. When an MUA implements both confidentiality assurance levels and security directives, then both the end-user and the service provider independently have the ability to improve the end-user's confidentiality. A security directive has the following formal syntax:

directive = directive-name [ "=" directive-value ] directive-name = token directive-value = token token = <As defined in RFC 7230>

This is a subset of the syntax used by HSTS as revised in ; but simplified for use by protocols other than HTTP.

Servers supporting this extension MUST advertise an STS policy. This includes a list of security directives the server administrator has explicitly configured as recommended for use by clients (the list MAY be empty). When a server advertises a security directive associated with a security facility, it is making a commitment to support that facility (or a revised version of that facility) indefinitely and recommending that the client save that directive with the account configuration and require that security facility for future connections to that server. Server STS policy may also include a "sts-url" directive with a value containing an https Uniform Resource Locator (URL) that the client can save and subsequently resolve for the user in the event of a security connection problem. Server STS policy has the following formal syntax:

Protocol extensions to advertise STS policy for email servers are defined in . The IANA Considerations defines a registry so that more directives can be defined in the future. Three initial directives are defined for use by MUAs in : tls-version, sts-url and tls-cert.

Before a client can consider storing any security directives, it MUST verify that the connection to the server uses TLS, the server has been authenticated, and any requirements for any previously saved security directives are met. Then the client performs the following steps for each security directive in the STS policy: If the security directive name is not known to the client, skip to the next directive. If the security directive is already saved with the same value (or a value considered greater than the current value in the directive's definition), the client skips the security directive and moves on to the next one. The client verifies the connection meets the requirements of the security directive. If the connection does not, then the directive will not be saved. If previous steps pass, the client SHOULD update the current account configuration to save the security directive. Once a security directive is saved, all subsequent connections to that host require any associated security feature. For this confidentiality protection to work as desired clients MUST NOT offer a click-through-to-connect action when unable to achieve connection security matching the saved security directives.

Suppose a server advertises the "tls-version" directive name with value "1.1". A client that successfully negotiates either TLS 1.1 or TLS 1.2 SHOULD save this directive. The server may subsequently change the value to "1.2". When a client with "1.1" saved value connects and negotiates TLS 1.2, it will upgrade the saved directive value to "1.2". However, a client that only supports TLS 1.1 will continue to require use of TLS 1.1 and work with that server as long as it permits TLS 1.1. This way individual clients can require the newer/stronger protocol (e.g., TLS 1.2), while older clients can continue to communicate securely (albeit potentially less so) using the older protocol.

When a security directive has been saved for connections from a client to a server and the facility identified by that directive is no longer available, this results in a connection failure. An MUA SHOULD inform the user of a potential threat to their confidentiality and offer to resolve a previously-recorded sts-url https URL if one is available. MUAs are discouraged from offering a lightweight option to reset or ignore directives as this defeats the benefit they provide to end users.

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. 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. The ABNF for the field follows:

tls-cipher-clause = CFWS "tls" FWS tls-cipher tls-cipher = tls-cipher-suite-name / tls-cipher-suite-hex tls-cipher-name = ALPHA *(ALPHA / DIGIT / "_") ; as registered in IANA cipher suite registry tls-cipher-hex = "0x" 4HEXDIG

This memo defines optional mechanisms for use by MUAs to communicate saved STS policy to servers and for servers to advertise policy. One purpose of such mechanisms is to permit servers to determine which and how many clients have saved security directives, and thus, to permit operators to be aware of potential impact to their users should support for such facilities be changed. For IMAP, the existing ID command is extended to provide this capability. For SMTP Submission, a new CLIENT command is defined. No similar mechanism is defined for POP in this version of the memo to keep POP simpler, but one may be added in the future if deemed necessary. In addition, for each of IMAP, POP, and SMTP, a new STS capability is defined so the client can access the server's STS policy.

When an IMAP server advertises the STS capability, that indicates the IMAP server implements IMAP4 ID with additional field values defined here. This is grouped with the ID command because that is the existing IMAP mechanism for clients to report data for server logging, and provides a way for the server to report the STS policy. From server to client, the argument to this ID field is the server STS policy. Servers MUST provide this information in response to an ID command. From client to server, this is a list of security directives the client has saved for this server (the client MAY omit the value for the sts-url directive in this context). Servers MAY record this information so administrators know the expected security properties of the client and can thus act to avoid security policy failures (e.g., by renewing server certificates on time, etc). From client to server, a list including one or more security directives the client has saved that the client was unable to achieve. This allows clients to report errors to the server prior to terminating the connection in the event an acceptable security level is unavailable. From client to server, this is a list of security directive names the client supports that are not saved. Server-side IMAP proxies that accept TLS connections from clients and connect in-the-clear over a fully private secure network to the server SHOULD use this field to report the tls-cipher (syntax as defined in ) to the server. IMAP clients SHOULD use the IMAP ID command to report policy failures and determine the server STS policy. Clients MAY use the ID command to report other security directive information. IMAP servers MUST implement the ID command at least to report STS policy to clients.

S: * OK [CAPABILITY IMAP4rev1 STS ID AUTH=PLAIN AUTH=SCRAM-SHA-1] hello C: a001 ID ("name" "Demo Mail" "version" "1.5" "saved" "tls-version=1.1; tls-cert" "directives" "tls-version=1.2") S: * ID ("name" "Demo Server" "version" "1.7" "sts-policy" "tls-version=1.1; tls-cert; sts-url=https://www.example.com/security-support.html") S: a001 OK ID completed ]]>

This example shows a client that successfully negotiated TLS version 1.1 or later and verified the server's certificate as required by IMAP. Even if the client successfully validates the server certificate, it will not require tls-version 1.2 in the future as the server does not advertise that version as policy. The client has not yet saved an STS URL, but if the client successfully validated the server certificate, it will save the provided URL.

S: * OK [CAPABILITY IMAP4rev1 DEEP ID AUTH=PLAIN AUTH=SCRAM-SHA-1] hello C: a001 ID ("name" "Demo Mail" "version" "1.5" "policy-failure" "tls-cert=pkix") S: * ID ("name" "Demo Server" "version" "1.7" "sts-policy" "tls-version=1.1; sts-url=") S: a001 OK ID completed C: a002 LOGOUT ]]>

This example shows a client that negotiated TLS, but was unable to verify the server's certificate. The policy-failure informs the server of this problem, at which point the client can disconnect. If the client had previously latched a URI for security problems from this server, it could offer to resolve that URI. However, the sts-policy in this exchange is ignored due to the latch failure.

S: * OK [CAPABILITY IMAP4rev1 DEEP ID AUTH=PLAIN AUTH=SCRAM-SHA-1] hello C: a001 ID ("name" "Demo Mail" "version" "1.5" "saved" "tls-version=1.1; tls-cert=pkix" "tls" "TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256") S: * ID ("name" "Demo Server" "version" "1.7" "sts-policy" "tls-version=1.1; tls-cert=pkix; sts-url=https://www.example.com/support.html") S: a001 OK ID completed ]]>

This example shows the connection from an IMAP proxy to a back-end server. The client connected to the proxy and sent the ID command shown in example 1, and the proxy has added the "tls" item to the ID command so the back-end server can log the cipher suite that was used on the connection from the client.

POP servers supporting this specification MUST implement the POP3 extension mechanism . POP servers MUST advertise the DEEP capability with an argument indicating the server's DEEP status.

S: +OK POP server ready C: CAPA S: +OK Capability list follows S: TOP S: SASL PLAIN SCRAM-SHA-1 S: RESP-CODES S: PIPELINING S: UIDL S: STS tls-version=1.2 sts-url=https://www.example.com/security-support.html> S: . ]]>

After verifying the TLS server certificate and issuing CAPA, the client can save any or all of the STS policy. If the client connects to this same server later and has a security failure, the client can direct the user's browser to the previously-saved URL where the service provider can provide advice to the end user.

SMTP Submission servers supporting this specification MUST implement the MSTS SMTP extension. The name of this extension is MSTS. The EHLO keyword value is MSTS and the sts-policy ABNF is the syntax of the EHLO keyword parameters. This does not add parameters to the MAIL FROM or RCPT TO commands. This also adds a CLIENT command to SMTP which is used to report client information to the server. The formal syntax for the command follows:

String = ]]>

The CLIENT command parameters listed here have the same meaning as the parameters used in the IMAP STS extension (). The server responds to the CLIENT command with a "250" if the command has correct syntax and a "501" if the command has incorrect syntax.

S: 220 example.com Demo SMTP Submission Server C: EHLO client.example.com S: 250-example.com S: 250-8BITMIME S: 250-PIPELINING S: 250-DSN S: 250-AUTH PLAIN LOGIN S: 250-MSTS tls-version=1.2; tls-cert; sts-url= S: 250-BURL imap S: 250 SIZE 0 C: CLIENT name demo_submit version 1.5 saved "tls-version=1.1; tls-cert=pkix" directives "tls-version=1.2" S: 250 OK ]]>

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 default to a high confidentiality assurance level, unless the user has explicitly requested reduced confidentiality. This will have the effect of causing the MUA to ignore 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. When establishing a new configuration for connecting to an IMAP, POP, or SMTP Submission server, an MUA SHOULD NOT blindly trust SRV records unless they are signed by DNSSEC and have a valid signature. Instead, the MUA SHOULD warn the user that the DNS-advertised mechanism for connecting to the server is not authenticated, and request the user to manually verify the connection details by reference to his or her mail service provider's documentation. Similarly, 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.

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 create a persistent binding between that certificate and the saved host name for the server. This is called certificate pinning. Certificate pinning is only appropriate during account setup and MUST NOT be offered in response to a failed certificate validation for an existing account. An MUA that allows certificate pinning MUST NOT allow a certificate pinned for one account to validate connections for other accounts. A pinned certificate is subject to a man-in-the-middle attack at account setup time, and lacks a mechanism to revoke or securely refresh the certificate. Therefore use of a pinned certificate does not provide a high confidentiality assurance and an MUA MUST NOT indicate a high level for an account or connection using a pinned certificate. Additional advice on certificate pinning is present in .

This section details requirements for implementations of electronic mail protocol clients and servers. A requirement for a client or server implementation to support a particular feature is not the same thing as a requirement that a client or server running a conforming implementation be configured to use that feature. Requirements for Mail Service Providers (MSPs) are distinct from requirements for protocol implementations, and are listed in a separate section.

These requirements apply to MUAs as well as POP, IMAP and SMTP Submission servers. All implementations MUST be configurable to support implicit TLS using the TLS 1.2 protocol or later . All implementations MUST implement the recommended cipher suites described in or a future BCP or standards track revision of that document. All implementations MUST be configurable to require TLS before performing any operation other than capability discovery and STARTTLS. The IMAP specification is hereby modified to revoke the second paragraph of section 11.1 and replace it with the text from the first three bullet items in this list. See Appendix B of to see additional modifications to IMAP certificate validation rules. The standard for use of TLS with IMAP, POP3 and ACAP is modified to revoke section 2.1 and replace it with the text from the first three bullet items in this list. See Appendix B of to see additional modifications to RFC 2595 certificate validation rules. The standard for Message Submission is updated to add the first three bullet items above to section 4.3 as well as to require implementation of the TLS server identity check as described in and PKIX .

MUAs and 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. Clients MUST NOT provide a client certificate during the TLS handshake unless the server requests one and the client has determined the certificate can be safely used with that specific server, OR the client has been explicitly configured by the user to use that particular certificate with that server. 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. 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 ).

These requirements apply to servers that implement POP, IMAP or SMTP Submission. Servers MUST implement the DEEP extension described in IMAP and SMTP submission servers SHOULD implement and be configurable to support STARTTLS. This enables discovery of new TLS availability, and can increase usage of TLS by legacy clients. Servers MUST NOT advertise STARTTLS if it is unlikely to succeed based on server configuration (e.g., there is no server certificate installed). SMTP message submission servers that have negotiated TLS SHOULD add a Received header field to the message including the tls clause described in . Servers MUST be configurable to include the TLS cipher information in any connection or user logging or auditing facility they provide.

This section describes requirements on Mail User Agents (MUAs) using IMAP, POP, and/or Submission protocols. Note: Requirements pertaining to use of Submission servers are also applicable to use of SMTP servers (e.g., port 25) for mail submission. User agents SHOULD indicate to users at configuration time, the expected level of confidentiality based on appropriate security inputs such as which security directives are pre-set, the number of trust anchors, certificate validity, use of an extended validation certificate, TLS version supported, and TLS cipher suites supported by both server and client. This indication SHOULD also be present when editing or viewing account configuration. MUAs SHOULD detect when STARTTLS and/or implicit TLS becomes available for a protocol and set the tls-version=1.1 directive if the server advertises the tls-version=1.1 security policy after a successful TLS negotiation. Whenever requested to establish any configuration that does not require both TLS and server certificate verification to talk to a server or account, an MUA SHOULD warn its user that his or her mail traffic (including password, if applicable) will be exposed to attackers, and give the user an opportunity to abort the connection prior to transmission of any such password or traffic. MUAs SHOULD implement the "tls-version=1.2" security directive (the TLS library has to provide an API that controls permissible TLS versions and communicates the negotiated TLS protocol version to the application for this to be possible). See for additional requirements.

MUAs which are not configurable to use user-specified servers MUST implement TLS or similarly other strong encryption mechanism when communicating with their mail servers. This generally applies to MUAs that are pre-configured to operate with one or more specific services, whether or not supplied by the vendor of those services. MUAs using protocols other than IMAP, POP, and Submission to communicate with mail servers, MUST implement TLS or other similarly robust encryption mechanism in conjunction with those protocols.

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 do not impact DEEP. 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 DEEP and confidentiality or security reduction. An AVAS product or service is considered DEEP compliant if all IMAP, POP and SMTP-related software it includes is DEEP compliant and it advertises and supports all security directives that the actual servers advertise. 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 DEEP high confidentiality assurance 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 so use of AVAS services for submitted email remains necessary.

This section details requirements for providers of IMAP, POP, and/or SMTP submission services, for providers who claim to conform to this specification.

Mail Service Providers MUST use server implementations that conform to this specification.

This document updates the advice in by making Implicit TLS on port 465 the preferred submission port. Mail Service Providers that accept mail submissions from end-users using the Internet Protocol MUST provide one or more SMTP Submission services, separate from the SMTP MTA services used to process incoming mail. Those submission services MUST be configured to support Implicit TLS on port 465 and SHOULD support STARTTLS if port 587 is used. MSPs MAY also support submission of messages via one or more designated SMTP servers to facilitate compatibility with legacy MUAs. Discussion: SMTP servers used to accept incoming mail or to relay mail are expected to accept mail in cleartext. This is incompatible with the purpose of this memo which is to encourage encryption of traffic between mail servers. There is no such requirement for mail submission servers to accept mail in cleartext or without authentication. For other reasons, use of separate SMTP submission servers has been best practice for many years.

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 . 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.

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.

MSPs SHOULD regularly and frequently monitor their various servers to make sure that: TLS server certificates remain valid and are not about to expire, TLSA records match the public keys advertised in server certificates, are signed using DNSSEC, server configurations are consistent with SRV advertisements, and DNSSEC signatures are valid and verifiable. Failure to detect expired certificates and DNS configuration errors in a timely fashion can result in significant loss of service for an MSP's users and a significant support burden for the MSP.

MSPs SHOULD advertise a DEEP status that includes tls11, tls-cert and an HTTPS URL that can be used to inform clients of service outages or problems impacting client confidentiality. Note that advertising tls-cert is a commitment to maintain and renew server certificates.

New servers and services SHOULD be configured to require TLS unless it's necessary to support legacy clients or existing client configurations.

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.

IANA shall create (has created) the registry "STS Security Directives". This registry is a single table and will use an expert review process . Each registration will contain the following fields: The name of the security directive. This follows the directive-name ABNF. The permitted values of the security directive. This should also explain if the value is optional or mandatory and what to do if the value is not recognized. This describes the meaning of the security directive and the conditions under which the directive is saved. The protocols to which this security directive applies. Presently this may be MSTS (for MUA STS), HSTS (for HTTP STS), or ALL. One of COMMON, LIMITED USE or OBSOLETE. Optional reference to specification. The identify of the submitter or submitters. The identity of the change controller for the registration. This will be "IESG" in case of registrations in IETF-produced documents. The expert reviewer will verify the directive name follows the ABNF, and that the value and description fields are clear, unambiguous, do not overlap existing deployed technology, do not create security problems and appropriately considers interoperability issues. Email security directives intended for LIMITED USE have a lower review bar (interoperability and overlap issues are less of a concern). The reviewer may approve a registration, reject for a stated reason or recommend the proposal have standards track review due to importance or difficult subtleties. Standards-track registrations may be updated if the relevant standards are updated as a consequence of that action. Non-standards-track entries may be updated by the listed change controller. The entry's name and submitter may not be changed. In exceptional cases, any aspect of any registered entity may be updated at the direction of the IESG (for example, to correct a conflict).

This document defines three initial security directives for the registry as follows, and registers the two additional directives specified in . tls-version Mandatory; 1.1 refers to or later and 1.2 refers to or later. Future versions may be added; this is ignored if the version is unrecognized. This directive indicates that the TLS version negotiated must be the specified version or later. In the event this directive is saved and only an older TLS version is available, that results in STS policy failure. MUA only COMMON RFC XXXX (this document once published) Authors of this document IESG tls-cert Optional; pkix refers to PKIX certificate validation, dane refers to DANE certificate validation, any refers to any validation mechanism the client considers acceptable. If no value is provided, "any" is assumed. This directive indicates that TLS was successfully negotiated and the server certificate was successfully verified by the client using PKIX or DANE and the server certificate identity was verified using the algorithm appropriate for the protocol (see ). This directive is saved if the client sees this in the advertised server STS policy after successfully negotiating TLS and verifying the certificate and server identity. For the HSTS protocol, this directive (with the "any" value) is implied by the presence of the Strict-Transport-Security header, as a result it is never specified in that protocol. MUA only COMMON RFC XXXX (this document once published) Authors of this document IESG sts-url Mandatory for server-policy, optional for client reporting. The value is an https URL. This directive indicates that the client SHOULD resolve (with appropriate certificate validation) and display the URL in the event of a policy failure. MUA only COMMON RFC XXXX (this document once published) Authors of this document IESG max-age see . see . HSTS only COMMON Authors of this document IESG includeSubDomains None see . HSTS only COMMON Authors of this document IESG

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 template ():

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

IANA is asked to assign an alternate usage of 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. 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.

This document adds the STS capability to the IMAP capabilities registry. This is described in .

This document adds the STS capability to the POP3 capabilities registry. STS sts-policy none none both / may change after STLS N/A This document See .

This document adds the MSTS EHLO Keyword to the SMTP Service Extension registry. This is described in .

This document adds the following entry to the "Additional-registered-clauses" sub-registry of the "MAIL Parameters" registry, created by : tls Indicates the TLS cipher suite used for a transport connection. See tls-cipher ABNF 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. It could be argued that sharing the name and version of the client software with the server has privacy implications. Although providing this information is not required, it is encouraged so that mail service providers can more effectively inform end-users running old clients that they need to upgrade to protect their security, or know which clients to use in a test deployment prior to upgrading a server to have higher security requirements.

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. 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-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.