Mail User Agent Strict Transport Security (MUA-STS)Network HereticsPO Box 1934KnoxvilleTNUS37901moore@network-heretics.comOracle440 E. Huntington Dr., Suite 400ArcadiaCA91006USchris.newman@oracle.com
Applications
I-DInternet-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 minimum confidentiality assurance level with each
mail account, and disconnections associated with that account that do
not provide the minimum confidentiality assurance level associated
with that account.By default, MUAs assign a minimum confidentiality assurance level
that 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 user uses a mail user
agent (MUA) to access a mail account. (Most MUAs support the ability
to access multiple 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 minimum confidentiality assurance level with each mail
account. If during a session with a network service, the requirements
for the minimum confidentiality assurance level associated with that
mail account are not met, the MUA MUST NOT continue the session with
the network service. MUAs MUST support at least the ability to detect
whether a session with a network service implements confidentiality
assurance level 1 as described in the next section. Note that the
minimum confidentiality assurance level associated with an account
applies to all protocol interactions and all servers associated with
the account.
MUAs SHOULD continuously indicate to the user the current
confidentiality assurance level of any account currently in use when
reading, submitting and managing mail (e.g., via a lock icon,
background colors, or other indications similar to those commonly used
in web browsers for a similar purpose) and SHOULD indicate the minimum
confidentiality assurance level for each account whenever displaying a
list of mail accounts. Note that the displayed confidentiality
assurance level for a current session could be higher than the minimum
confidentiality assurance level set at account configuration, but
never lower. If multiple active connections are associated with an
account or view, the indication of the current confidentiality
assurance level associated with the account should reflect the level
provided by the least confidential connection. It is therefore
possible that at any given instant some services associated with a
mail account meet the minimum confidentiality assurance level
associated with the account, and other services do not. An MUA MAY
continue to interact with those services for which the minimum
confidentiality assurance level is met, while refusing to interact
with those services for which the minimum confidentiality assurance
level is not met. For example, if the IMAP service associated with
a mail account meets the minimum confidentiality assurance level, but
the Mail Submission service associated with that account does not, the
MUA MAY continue to permit reading mail from that account but MUST NOT
send mail until it can do so using a Submission service that meets
the minimum confidentiality assurance level for that account.
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 require a minimum confidentiality assurance level of 1 as the
default for newly configured accounts.
This document defines two initial confidentiality assurance levels, 1
and 0. It is expected that other levels may be defined in the future,
as needed to thwart increasingly sophisticated and/or pervasive attacks.
A mail account has a confidentiality assurance level of 1 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 successfully negotiate TLS via either Implicit TLS
or STARTTLS.For protocols using TCP, both client and server must support, and
negotiate, a TLS version of 1.1 or greater.MUAs MUST implement and PKIX .MUAs MAY implement DANE as an alternate
means of verifying TLS server certificates. For confidentiality
assurance level 1, a certificate may be considered valid if it can be
validated using either DANE or PKIX.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 implementing a minimum confidentiality assurance of 1
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 support the ability to configure accounts with a minimum
confidentiality assurance level of 0. 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, or using
TLS versions prior to TLS 1.1, for accounts with a minimum
confidentiality assurance level of 0. Even for accounts with a
minimum confidentiality assurance level of 0, MUAs SHOULD attempt TLS
first if available, and MUST implement the ability to reconnect
without TLS if TLS negotiation fails for reasons other than server
certificate validity.
Note that if TLS is not used, or a version of TLS prior to TLS 1.1 is
negotiated, or the TLS server certificate is not successfully validated as
described in , the client MUST clearly
indicate to the user that there is currently no assurance of
confidentiality for the mail account or connection.
This specification is not intended to limit experimentation and
innovation with respect to user confidentiality. As a result, an
implementation MAY implement confidentiality assurance levels other
than those defined in this document, as long as those levels are
distinguished in user interfaces from those defined in this document,
and the ordering associated with them reflects the actual expectation
of confidentiality provided. However, implementation of levels below
confidentiality assurance level 0, as described in the previous
section, is discouraged. Implementers are also cautioned that end
users may be confused by too many confidentiality assurance levels.
As stated above, higher confidentiality assurance levels may be
standardized in the future. For example, a future confidentiality
assurance levels might require multiple independent trust anchors for
server certificate validation.
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. Once the client has changed the configuration for a
mail service to require specific server security features, those
features are said to be "latched".
Note that security directives are a separate mechanism from minimum
confidentiality assurance levels. A connection between a client and a
service MUST meet the requirements of both the minimum confidentiality
assurance level associated with the account, and the conditions of any
security directives established for that service.
Otherwise the client MUST abandon the connection.
When an MUA implements both
minimum 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:
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. For example, a security directive claiming that
the server supports tls-version 1.2 will not be saved by a client if
the currently negotiated TLS session is using TLS 1.1.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:
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.
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.This example shows a client that negotiated TLS, but was unable to
verify the server's certificate using PKIX. The policy-failure informs
the server of this problem, at which point the client can disconnect. If the
client had previously saved the sts-url security directive from this
server, it could offer to resolve that URI. However, the sts-policy in
this exchange is ignored due to the failure to meet the conditions of
the tls-version security directive.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. (Note: DEEP is an ancronym for the original name of this
specification, before the terms were changed to align better with
those used in HSTS.)
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:
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.
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 minimum
confidentiality assurance level of 1, 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 meet the requirement for a minimum confidentiality assurance level
of 1, and an MUA MUST NOT indicate a confidentiality assurance level
of 1 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 implement TLS 1.2 or later, and 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 appropriate STS Policy and Reporting
extensions 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 capability 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 when using SMTP servers
(e.g., port 25) for mail submission.
User agents SHOULD indicate to users at configuration time, the minimum
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.For any mail service not initially configured to require TLS, MUAs
SHOULD detect when STARTTLS and/or implicit TLS becomes available for
a protocol and set the tls-version security directive if the server
advertises the tls-version=1.1 or higher security policy after a
successful negotiation (including certificate validation) of TLS 1.1.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 support the ability to save 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, 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 compliant with
this specification if all IMAP, POP and SMTP-related software
(including proxies) it includes are compliant with this specification,
and each of these services advertise and support all security
directives that the actual end-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 a minimum confidentiality assurance
level of 1 or better 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.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 STS policies that include at least tls11,
tls-cert and sts-url, with the latter having an associated 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. A MSP
MAY also specifically indicate a commitment to support PKIX
validation, DANE validation, or both, using tls-cert=pkix,
tls-cert=dane, or tls-cert=pkix+dane, respectively.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-versionMandatory; 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 onlyCOMMONRFC XXXX (this document once published)Authors of this documentIESGtls-certOptional; pkix refers to PKIX certificate
validation; dane refers to DANE certificate validation; pkix+dane
refers to use of both PKIX and DANE validation; any refers to
any validation method the client considers acceptable. If no value is
supplied, "any" is assumed.This directive indicates that TLS was
successfully negotiated and the server certificate was successfully
verified by the client 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 using a means consistent with the associated (or
implied) value. Note that an advertisement of either tls-cert=pkix
or tls-cert=pkix+dane in a server's STS policy indicates that the server
commits to using certificates that are verifiable using PKIX in the
future, but tls-cert=pkix implies no commitment regarding DANE support.
Similarly, an advertisement of either tls-cert=dane or
tls-cert=pkix+dane indicates that the server commits to using
certificates that are verifiable using DANE in the future, but
tls-cert=dane implies no commitment regarding PKIX support. An
advertisement of tls-cert or tls-cert=any indicates only that the
server will continue to provide valid server certificates, but makes
no commitment about the means of verifiability.
(For the HSTS protocol, the presence of a Strict-Transport-Security
response header serves as an indication that the certificate should be
valid, so the tls-cert directive is never specified in that protocol.)
MUA onlyCOMMONRFC XXXX (this document once published)Authors of this documentIESGsts-urlMandatory 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 onlyCOMMONRFC XXXX (this document once published)Authors of this documentIESGmax-agesee .see .HSTS onlyCOMMONAuthors of this documentIESGincludeSubDomainsNonesee .HSTS onlyCOMMONAuthors of this documentIESG
IANA is asked to update the registration of the TCP well-known port 995 using the following template ():
IANA is asked to update the registration of the TCP well-known port 993 using the following template ():
IANA is asked to assign an alternate usage of port 465 in addition to the current assignment using the following template ():
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.
STSsts-policynonenoneboth / may change after STLSN/AThis documentSee .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 :
tlsIndicates 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-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 minimium 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 requrements 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 referenceChanges 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 abstractuse 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 feedbackChanges 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 sectionRemove 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.