Section 5.1 for more information.) o 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 Section 5.2.) o MUAs MUST NOT treat a session as meeting a minimum level of confidentiality if the server's TLS certificate cannot be validated. (See Section 5.3.) o MUAs MAY impose other minimum confidentiality requirements in the future, e.g., in order to discourage the use of TLS versions or cryptographic algorithms in which weaknesses have been discovered. o 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) the user is prompted 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 asking 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. o MUAs MUST implement TLS 1.2 [RFC5246] or later. Earlier TLS and SSL versions MAY also be supported, so long as the MUA requires at least TLS 1.1 [RFC4346] when accessing accounts that are configured to impose minimum confidentiality requirements.
o All MUAs SHOULD implement the recommended TLS ciphersuites described in [RFC7525] or a future BCP or Standards Track revision of that document. o MUAs that are configured to not require minimum confidentiality for one or more accounts SHOULD detect when TLS becomes available on those accounts (using [RFC6186] or other means) and offer to upgrade the account to require TLS. Additional considerations and details appear below. RFC6186] 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 the use of [RFC6186] for account setup. However, when using configuration information obtained via 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 per [RFC6186], MUAs 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, an 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 that the user's 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 verify that either (a) the SRV records are signed using DNSSEC or (b) the target Fully Qualified Domain Name (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 of [RFC6186].) 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.
example of "easy 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 that 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. RFC7817] and PKIX [RFC5280]. MUAs MAY also support DNS-Based Authentication of Named Entities (DANE) [RFC6698] as a means of validating server certificates in order to meet minimum confidentiality requirements. MUAs MAY support the 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 Section 5.4.) Section 5.1 describes one way to do this). The certificates for these servers are verified using the rules described in [RFC7817] and PKIX [RFC5280]. In the event that the certificate does not validate due to an expired certificate, a lack of an appropriate chain of trust, or a lack of an identifier match, the MUA MAY offer to create a persistent binding between that certificate and the saved hostname 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 as described in [RFC7469]. 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, the 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 such confidentiality is provided. Additional advice on certificate pinning is presented in [RFC6125]. Section 7.4.6 of [RFC5246]. 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 the 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 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 [RFC4422], using the appropriate authentication command (AUTH for POP3 [RFC5034], AUTH for SMTP Submission [RFC4954], or AUTHENTICATE for IMAP [RFC3501]). RFC2979], 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, although 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. The use of AVAS services for submitted email therefore remains necessary. RFC6335]: Service Name: pop3s Transport Protocol: TCP Assignee: IESG <email@example.com> Contact: IETF Chair <firstname.lastname@example.org> Description: POP3 over TLS protocol Reference: RFC 8314 Port Number: 995
RFC6335]: Service Name: imaps Transport Protocol: TCP Assignee: IESG <email@example.com> Contact: IETF Chair <firstname.lastname@example.org> Description: IMAP over TLS protocol Reference: RFC 8314 Port Number: 993 No changes to existing UDP port assignments for pop3s or imaps are being requested. RFC6335]: Service Name: submissions Transport Protocol: TCP Assignee: IESG <email@example.com> Contact: IETF Chair <firstname.lastname@example.org> Description: Message Submission over TLS protocol Reference: RFC 8314 Port Number: 465 This is a one-time procedural exception to the rules in [RFC6335]. 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 has not assigned 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" [RFC6409] and used that port for email submission by default when an end user requested security during account setup. If a new port is assigned for the submissions service, either (a) email software will continue with unregistered use of port 465 (leaving the port registry inaccurate relative to
de facto practice and wasting a well-known port) or (b) confusion between the de facto and registered ports will cause harmful interoperability problems that will deter the use of TLS for Message Submission. The authors of this document believe that 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 been deployed, it has not replaced the deployed use of Implicit TLS submission on port 465. RFC5321], IANA has added two additional- registered-clauses for Received fields as defined in Section 4.3 of this document: o "tls": Indicates the TLS cipher used (if applicable) o "group": Indicates the Diffie-Hellman group used with the TLS cipher (if applicable) The descriptions and syntax of these additional clauses are provided in Section 4.3 of this document. TLS-1.3] appears to reduce this privacy risk somewhat.
[RFC793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, DOI 10.17487/RFC0793, September 1981, <https://www.rfc-editor.org/info/rfc793>. [RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3", STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996, <https://www.rfc-editor.org/info/rfc1939>. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>. [RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207, February 2002, <https://www.rfc-editor.org/info/rfc3207>. [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, <https://www.rfc-editor.org/info/rfc3501>. [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "DNS Security Introduction and Requirements", RFC 4033, DOI 10.17487/RFC4033, March 2005, <https://www.rfc-editor.org/info/rfc4033>. [RFC5034] Siemborski, R. and A. Menon-Sen, "The Post Office Protocol (POP3) Simple Authentication and Security Layer (SASL) Authentication Mechanism", RFC 5034, DOI 10.17487/RFC5034, July 2007, <https://www.rfc-editor.org/info/rfc5034>. [RFC5234] Crocker, D., Ed., and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, <https://www.rfc-editor.org/info/rfc5234>. [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, August 2008, <https://www.rfc-editor.org/info/rfc5246>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, <https://www.rfc-editor.org/info/rfc5280>. [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, DOI 10.17487/RFC5322, October 2008, <https://www.rfc-editor.org/info/rfc5322>. [RFC6186] Daboo, C., "Use of SRV Records for Locating Email Submission/Access Services", RFC 6186, DOI 10.17487/RFC6186, March 2011, <https://www.rfc-editor.org/info/rfc6186>. [RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail", STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011, <https://www.rfc-editor.org/info/rfc6409>. [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication of Named Entities (DANE) Transport Layer Security (TLS) Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August 2012, <https://www.rfc-editor.org/info/rfc6698>. [RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre, "Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May 2015, <https://www.rfc-editor.org/info/rfc7525>. [RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via Opportunistic DNS-Based Authentication of Named Entities (DANE) Transport Layer Security (TLS)", RFC 7672, DOI 10.17487/RFC7672, October 2015, <https://www.rfc-editor.org/info/rfc7672>. [RFC7817] Melnikov, A., "Updated Transport Layer Security (TLS) Server Identity Check Procedure for Email-Related Protocols", RFC 7817, DOI 10.17487/RFC7817, March 2016, <https://www.rfc-editor.org/info/rfc7817>. [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[CERT-555316] CERT, "Vulnerability Note VU#555316: STARTTLS plaintext command injection vulnerability", Carnegie Mellon University Software Engineering Institute, September 2011, <https://www.kb.cert.org/vuls/id/555316>. [Email-TLS] Moore, K., "Recommendations for use of TLS by Electronic Mail Access Protocols", Work in Progress, draft-moore- email-tls-00, October 2013. [MTA-STS] Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A., and J. Jones, "SMTP MTA Strict Transport Security (MTA-STS)", Work in Progress, draft-ietf-uta-mta-sts-14, January 2018. [POP3-over-TLS] Melnikov, A., Newman, C., and M. Yevstifeyev, Ed., "POP3 over TLS", Work in Progress, draft-melnikov-pop3- over-tls-02, August 2011. [RFC2595] Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC 2595, DOI 10.17487/RFC2595, June 1999, <https://www.rfc-editor.org/info/rfc2595>. [RFC2979] Freed, N., "Behavior of and Requirements for Internet Firewalls", RFC 2979, DOI 10.17487/RFC2979, October 2000, <https://www.rfc-editor.org/info/rfc2979>. [RFC3848] Newman, C., "ESMTP and LMTP Transmission Types Registration", RFC 3848, DOI 10.17487/RFC3848, July 2004, <https://www.rfc-editor.org/info/rfc3848>. [RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.1", RFC 4346, DOI 10.17487/RFC4346, April 2006, <https://www.rfc-editor.org/info/rfc4346>. [RFC4422] Melnikov, A., Ed., and K. Zeilenga, Ed., "Simple Authentication and Security Layer (SASL)", RFC 4422, DOI 10.17487/RFC4422, June 2006, <https://www.rfc-editor.org/info/rfc4422>.
[RFC4954] Siemborski, R., Ed., and A. Melnikov, Ed., "SMTP Service Extension for Authentication", RFC 4954, DOI 10.17487/RFC4954, July 2007, <https://www.rfc-editor.org/info/rfc4954>. [RFC5068] Hutzler, C., Crocker, D., Resnick, P., Allman, E., and T. Finch, "Email Submission Operations: Access and Accountability Requirements", BCP 134, RFC 5068, DOI 10.17487/RFC5068, November 2007, <https://www.rfc-editor.org/info/rfc5068>. [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, DOI 10.17487/RFC5321, October 2008, <https://www.rfc-editor.org/info/rfc5321>. [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) Extensions: Extension Definitions", RFC 6066, DOI 10.17487/RFC6066, January 2011, <https://www.rfc-editor.org/info/rfc6066>. [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March 2011, <https://www.rfc-editor.org/info/rfc6125>. [RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S. Cheshire, "Internet Assigned Numbers Authority (IANA) Procedures for the Management of the Service Name and Transport Protocol Port Number Registry", BCP 165, RFC 6335, DOI 10.17487/RFC6335, August 2011, <https://www.rfc-editor.org/info/rfc6335>. [RFC7469] Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April 2015, <https://www.rfc-editor.org/info/rfc7469>. [TLS-1.3] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", Work in Progress, draft-ietf-tls-tls13-23, January 2018.
Email-TLS] ("Recommendations for use of TLS by Electronic Mail Access Protocols"). Subsequent versions merge ideas from both documents. One author of this document was also the author of RFC 2595, which 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 ciphersuite). However, they can result in TLS deployment where it would not otherwise happen, which is a sufficiently important goal that it overrides any 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 (Computer Emergency Readiness Team (CERT) vulnerability ID #555316 [CERT-555316]). Although there's nothing inherently wrong with STARTTLS, the fact that it resulted in a common implementation error (made independently by multiple implementers) suggests that 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 the use of SRV records as described in RFC 6186 resolves that critique for email. The second bullet is correct as well but is 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 TLS for all subsequent connections to
this server once TLS is successfully negotiated". The fourth bullet may be correct, but it 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 that 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 already widely used for that purpose 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 deployments, server deployments, and software to change significantly, which is contrary to the goal of promoting the increased use of TLS. Encouraging the use of STARTTLS on port 587 would not create interoperability problems, but it is unlikely to have any impact on the 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.
POP3-over-TLS], 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.