IMAP/POP3 over TLS: Using TLS with IMAP, POP3 and ACAP

Network Working Group                                          C. Newman
Request for Comments: 2595                                      Innosoft
Category: Standards Track                                      June 1999


                   Using TLS with IMAP, POP3 and ACAP


Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

1. Motivation

   The TLS protocol (formerly known as SSL) provides a way to secure an
   application protocol from tampering and eavesdropping.  The option of
   using such security is desirable for IMAP, POP and ACAP due to common
   connection eavesdropping and hijacking attacks [AUTH].  Although
   advanced SASL authentication mechanisms can provide a lightweight
   version of this service, TLS is complimentary to simple
   authentication-only SASL mechanisms or deployed clear-text password
   login commands.

   Many sites have a high investment in authentication infrastructure
   (e.g., a large database of a one-way-function applied to user
   passwords), so a privacy layer which is not tightly bound to user
   authentication can protect against network eavesdropping attacks
   without requiring a new authentication infrastructure and/or forcing
   all users to change their password.  Recognizing that such sites will
   desire simple password authentication in combination with TLS
   encryption, this specification defines the PLAIN SASL mechanism for
   use with protocols which lack a simple password authentication
   command such as ACAP and SMTP.  (Note there is a separate RFC for the
   STARTTLS command in SMTP [SMTPTLS].)

   There is a strong desire in the IETF to eliminate the transmission of
   clear-text passwords over unencrypted channels.  While SASL can be
   used for this purpose, TLS provides an additional tool with different
   deployability characteristics.  A server supporting both TLS with
   simple passwords and a challenge/response SASL mechanism is likely to
   interoperate with a wide variety of clients without resorting to
   unencrypted clear-text passwords.

   The STARTTLS command rectifies a number of the problems with using a
   separate port for a "secure" protocol variant.  Some of these are
   mentioned in section 7.

1.1. Conventions Used in this Document

   The key words "REQUIRED", "MUST", "MUST NOT", "SHOULD", "SHOULD NOT",
   "MAY", and "OPTIONAL" in this document are to be interpreted as
   described in "Key words for use in RFCs to Indicate Requirement
   Levels" [KEYWORDS].

   Terms related to authentication are defined in "On Internet
   Authentication" [AUTH].

   Formal syntax is defined using ABNF [ABNF].

   In examples, "C:" and "S:" indicate lines sent by the client and
   server respectively.

2. Basic Interoperability and Security Requirements

   The following requirements apply to all implementations of the
   STARTTLS extension for IMAP, POP3 and ACAP.

2.1. Cipher Suite Requirements

   Implementation of the TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA [TLS] cipher
   suite is REQUIRED.  This is important as it assures that any two
   compliant implementations can be configured to interoperate.

   All other cipher suites are OPTIONAL.

2.2. Privacy Operational Mode Security Requirements

   Both clients and servers SHOULD have a privacy operational mode which
   refuses authentication unless successful activation of an encryption
   layer (such as that provided by TLS) occurs prior to or at the time
   of authentication and which will terminate the connection if that
   encryption layer is deactivated.  Implementations are encouraged to
   have flexability with respect to the minimal encryption strength or
   cipher suites permitted.  A minimalist approach to this
   recommendation would be an operational mode where the
   TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA cipher suite is mandatory prior to
   permitting authentication.

   Clients MAY have an operational mode which uses encryption only when
   it is advertised by the server, but authentication continues
   regardless.  For backwards compatibility, servers SHOULD have an
   operational mode where only the authentication mechanisms required by
   the relevant base protocol specification are needed to successfully
   authenticate.

2.3. Clear-Text Password Requirements

   Clients and servers which implement STARTTLS MUST be configurable to
   refuse all clear-text login commands or mechanisms (including both
   standards-track and nonstandard mechanisms) unless an encryption
   layer of adequate strength is active.  Servers which allow
   unencrypted clear-text logins SHOULD be configurable to refuse
   clear-text logins both for the entire server, and on a per-user
   basis.

2.4. Server Identity Check

   During the TLS negotiation, the client MUST check its understanding
   of the server hostname against the server's identity as presented in
   the server Certificate message, in order to prevent man-in-the-middle
   attacks.  Matching is performed according to these rules:

   - The client MUST use the server hostname it used to open the
     connection as the value to compare against the server name as
     expressed in the server certificate.  The client MUST NOT use any
     form of the server hostname derived from an insecure remote source
     (e.g., insecure DNS lookup).  CNAME canonicalization is not done.

   - If a subjectAltName extension of type dNSName is present in the
     certificate, it SHOULD be used as the source of the server's
     identity.

   - Matching is case-insensitive.

   - A "*" wildcard character MAY be used as the left-most name
     component in the certificate.  For example, *.example.com would
     match a.example.com, foo.example.com, etc. but would not match
     example.com.

   - If the certificate contains multiple names (e.g. more than one
     dNSName field), then a match with any one of the fields is
     considered acceptable.

   If the match fails, the client SHOULD either ask for explicit user
   confirmation, or terminate the connection and indicate the server's
   identity is suspect.

2.5. TLS Security Policy Check

   Both the client and server MUST check the result of the STARTTLS
   command and subsequent TLS negotiation to see whether acceptable
   authentication or privacy was achieved.  Ignoring this step
   completely invalidates using TLS for security.  The decision about
   whether acceptable authentication or privacy was achieved is made
   locally, is implementation-dependent, and is beyond the scope of this
   document.

3. IMAP STARTTLS extension

   When the TLS extension is present in IMAP, "STARTTLS" is listed as a
   capability in response to the CAPABILITY command.  This extension
   adds a single command, "STARTTLS" to the IMAP protocol which is used
   to begin a TLS negotiation.

3.1. STARTTLS Command

   Arguments:  none

   Responses:  no specific responses for this command

   Result:     OK - begin TLS negotiation
               BAD - command unknown or arguments invalid

      A TLS negotiation begins immediately after the CRLF at the end of
      the tagged OK response from the server.  Once a client issues a
      STARTTLS command, it MUST NOT issue further commands until a
      server response is seen and the TLS negotiation is complete.

      The STARTTLS command is only valid in non-authenticated state.
      The server remains in non-authenticated state, even if client
      credentials are supplied during the TLS negotiation.  The SASL
      [SASL] EXTERNAL mechanism MAY be used to authenticate once TLS
      client credentials are successfully exchanged, but servers
      supporting the STARTTLS command are not required to support the
      EXTERNAL mechanism.

      Once TLS has been started, the client MUST discard cached
      information about server capabilities and SHOULD re-issue the
      CAPABILITY command.  This is necessary to protect against
      man-in-the-middle attacks which alter the capabilities list prior
      to STARTTLS.  The server MAY advertise different capabilities
      after STARTTLS.

      The formal syntax for IMAP is amended as follows:

        command_any   =/  "STARTTLS"

   Example:    C: a001 CAPABILITY
               S: * CAPABILITY IMAP4rev1 STARTTLS LOGINDISABLED
               S: a001 OK CAPABILITY completed
               C: a002 STARTTLS
               S: a002 OK Begin TLS negotiation now
               <TLS negotiation, further commands are under TLS layer>
               C: a003 CAPABILITY
               S: * CAPABILITY IMAP4rev1 AUTH=EXTERNAL
               S: a003 OK CAPABILITY completed
               C: a004 LOGIN joe password
               S: a004 OK LOGIN completed

3.2. IMAP LOGINDISABLED capability

   The current IMAP protocol specification (RFC 2060) requires the
   implementation of the LOGIN command which uses clear-text passwords.
   Many sites may choose to disable this command unless encryption is
   active for security reasons.  An IMAP server MAY advertise that the
   LOGIN command is disabled by including the LOGINDISABLED capability
   in the capability response.  Such a server will respond with a tagged
   "NO" response to any attempt to use the LOGIN command.

   An IMAP server which implements STARTTLS MUST implement support for
   the LOGINDISABLED capability on unencrypted connections.

   An IMAP client which complies with this specification MUST NOT issue
   the LOGIN command if this capability is present.

   This capability is useful to prevent clients compliant with this
   specification from sending an unencrypted password in an environment
   subject to passive attacks.  It has no impact on an environment
   subject to active attacks as a man-in-the-middle attacker can remove
   this capability.  Therefore this does not relieve clients of the need
   to follow the privacy mode recommendation in section 2.2.

   Servers advertising this capability will fail to interoperate with
   many existing compliant IMAP clients and will be unable to prevent
   those clients from disclosing the user's password.

4. POP3 STARTTLS extension

   The POP3 STARTTLS extension adds the STLS command to POP3 servers.
   If this is implemented, the POP3 extension mechanism [POP3EXT] MUST
   also be implemented to avoid the need for client probing of multiple
   commands.  The capability name "STLS" indicates this command is
   present and permitted in the current state.

      STLS

         Arguments: none

         Restrictions:
             Only permitted in AUTHORIZATION state.

         Discussion:
             A TLS negotiation begins immediately after the CRLF at the
             end of the +OK response from the server.  A -ERR response
             MAY result if a security layer is already active.  Once a
             client issues a STLS command, it MUST NOT issue further
             commands until a server response is seen and the TLS
             negotiation is complete.

             The STLS command is only permitted in AUTHORIZATION state
             and the server remains in AUTHORIZATION state, even if
             client credentials are supplied during the TLS negotiation.
             The AUTH command [POP-AUTH] with the EXTERNAL mechanism
             [SASL] MAY be used to authenticate once TLS client
             credentials are successfully exchanged, but servers
             supporting the STLS command are not required to support the
             EXTERNAL mechanism.

             Once TLS has been started, the client MUST discard cached
             information about server capabilities and SHOULD re-issue
             the CAPA command.  This is necessary to protect against
             man-in-the-middle attacks which alter the capabilities list
             prior to STLS.  The server MAY advertise different
             capabilities after STLS.

         Possible Responses:
             +OK -ERR

         Examples:
             C: STLS
             S: +OK Begin TLS negotiation
             <TLS negotiation, further commands are under TLS layer>
               ...
             C: STLS
             S: -ERR Command not permitted when TLS active

5. ACAP STARTTLS extension

   When the TLS extension is present in ACAP, "STARTTLS" is listed as a
   capability in the ACAP greeting.  No arguments to this capability are
   defined at this time.  This extension adds a single command,
   "STARTTLS" to the ACAP protocol which is used to begin a TLS
   negotiation.

5.1. STARTTLS Command

   Arguments:  none

   Responses:  no specific responses for this command

   Result:     OK - begin TLS negotiation
               BAD - command unknown or arguments invalid

      A TLS negotiation begins immediately after the CRLF at the end of
      the tagged OK response from the server.  Once a client issues a
      STARTTLS command, it MUST NOT issue further commands until a
      server response is seen and the TLS negotiation is complete.

      The STARTTLS command is only valid in non-authenticated state.
      The server remains in non-authenticated state, even if client
      credentials are supplied during the TLS negotiation.  The SASL
      [SASL] EXTERNAL mechanism MAY be used to authenticate once TLS
      client credentials are successfully exchanged, but servers
      supporting the STARTTLS command are not required to support the
      EXTERNAL mechanism.

      After the TLS layer is established, the server MUST re-issue an
      untagged ACAP greeting.  This is necessary to protect against
      man-in-the-middle attacks which alter the capabilities list prior
      to STARTTLS.  The client MUST discard cached capability
      information and replace it with the information from the new ACAP
      greeting.  The server MAY advertise different capabilities after
      STARTTLS.

      The formal syntax for ACAP is amended as follows:

        command_any   =/  "STARTTLS"

   Example:    S: * ACAP (SASL "CRAM-MD5") (STARTTLS)
               C: a002 STARTTLS
               S: a002 OK "Begin TLS negotiation now"
               <TLS negotiation, further commands are under TLS layer>
               S: * ACAP (SASL "CRAM-MD5" "PLAIN" "EXTERNAL")

6. PLAIN SASL mechanism

   Clear-text passwords are simple, interoperate with almost all
   existing operating system authentication databases, and are useful
   for a smooth transition to a more secure password-based
   authentication mechanism.  The drawback is that they are unacceptable
   for use over an unencrypted network connection.

   This defines the "PLAIN" SASL mechanism for use with ACAP and other
   protocols with no clear-text login command.  The PLAIN SASL mechanism
   MUST NOT be advertised or used unless a strong encryption layer (such
   as the provided by TLS) is active or backwards compatibility dictates
   otherwise.

   The mechanism consists of a single message from the client to the
   server.  The client sends the authorization identity (identity to
   login as), followed by a US-ASCII NUL character, followed by the
   authentication identity (identity whose password will be used),
   followed by a US-ASCII NUL character, followed by the clear-text
   password.  The client may leave the authorization identity empty to
   indicate that it is the same as the authentication identity.

   The server will verify the authentication identity and password with
   the system authentication database and verify that the authentication
   credentials permit the client to login as the authorization identity.
   If both steps succeed, the user is logged in.

   The server MAY also use the password to initialize any new
   authentication database, such as one suitable for CRAM-MD5
   [CRAM-MD5].

   Non-US-ASCII characters are permitted as long as they are represented
   in UTF-8 [UTF-8].  Use of non-visible characters or characters which
   a user may be unable to enter on some keyboards is discouraged.

   The formal grammar for the client message using Augmented BNF [ABNF]
   follows.

   message         = [authorize-id] NUL authenticate-id NUL password
   authenticate-id = 1*UTF8-SAFE      ; MUST accept up to 255 octets
   authorize-id    = 1*UTF8-SAFE      ; MUST accept up to 255 octets
   password        = 1*UTF8-SAFE      ; MUST accept up to 255 octets
   NUL             = %x00
   UTF8-SAFE       = %x01-09 / %x0B-0C / %x0E-7F / UTF8-2 /
                     UTF8-3 / UTF8-4 / UTF8-5 / UTF8-6
   UTF8-1          = %x80-BF
   UTF8-2          = %xC0-DF UTF8-1
   UTF8-3          = %xE0-EF 2UTF8-1
   UTF8-4          = %xF0-F7 3UTF8-1
   UTF8-5          = %xF8-FB 4UTF8-1
   UTF8-6          = %xFC-FD 5UTF8-1

   Here is an example of how this might be used to initialize a CRAM-MD5
   authentication database for ACAP:

   Example:    S: * ACAP (SASL "CRAM-MD5") (STARTTLS)
               C: a001 AUTHENTICATE "CRAM-MD5"
               S: + "<1896.697170952@postoffice.reston.mci.net>"
               C: "tim b913a602c7eda7a495b4e6e7334d3890"
               S: a001 NO (TRANSITION-NEEDED)
                  "Please change your password, or use TLS to login"
               C: a002 STARTTLS
               S: a002 OK "Begin TLS negotiation now"
               <TLS negotiation, further commands are under TLS layer>
               S: * ACAP (SASL "CRAM-MD5" "PLAIN" "EXTERNAL")
               C: a003 AUTHENTICATE "PLAIN" {21+}
               C: <NUL>tim<NUL>tanstaaftanstaaf
               S: a003 OK CRAM-MD5 password initialized

   Note: In this example, <NUL> represents a single ASCII NUL octet.

7. imaps and pop3s ports

   Separate "imaps" and "pop3s" ports were registered for use with SSL.
   Use of these ports is discouraged in favor of the STARTTLS or STLS
   commands.

   A number of problems have been observed with separate ports for
   "secure" variants of protocols.  This is an attempt to enumerate some
   of those problems.

   - Separate ports lead to a separate URL scheme which intrudes into
     the user interface in inappropriate ways.  For example, many web
     pages use language like "click here if your browser supports SSL."
     This is a decision the browser is often more capable of making than
     the user.

   - Separate ports imply a model of either "secure" or "not secure."
     This can be misleading in a number of ways.  First, the "secure"
     port may not in fact be acceptably secure as an export-crippled
     cipher suite might be in use.  This can mislead users into a false
     sense of security.  Second, the normal port might in fact be
     secured by using a SASL mechanism which includes a security layer.
     Thus the separate port distinction makes the complex topic of
     security policy even more confusing.  One common result of this
     confusion is that firewall administrators are often misled into
     permitting the "secure" port and blocking the standard port.  This
     could be a poor choice given the common use of SSL with a 40-bit
     key encryption layer and plain-text password authentication is less
     secure than strong SASL mechanisms such as GSSAPI with Kerberos 5.

   - Use of separate ports for SSL has caused clients to implement only
     two security policies: use SSL or don't use SSL.  The desirable
     security policy "use TLS when available" would be cumbersome with
     the separate port model, but is simple with STARTTLS.

   - Port numbers are a limited resource.  While they are not yet in
     short supply, it is unwise to set a precedent that could double (or
     worse) the speed of their consumption.


8. IANA Considerations

   This constitutes registration of the "STARTTLS" and "LOGINDISABLED"
   IMAP capabilities as required by section 7.2.1 of RFC 2060 [IMAP].

   The registration for the POP3 "STLS" capability follows:

   CAPA tag:                   STLS
   Arguments:                  none
   Added commands:             STLS
   Standard commands affected: May enable USER/PASS as a side-effect.
     CAPA command SHOULD be re-issued after successful completion.
   Announced states/Valid states: AUTHORIZATION state only.
   Specification reference:    this memo

   The registration for the ACAP "STARTTLS" capability follows:

   Capability name:            STARTTLS
   Capability keyword:         STARTTLS
   Capability arguments:       none
   Published Specification(s): this memo
   Person and email address for further information:
       see author's address section below

   The registration for the PLAIN SASL mechanism follows:

   SASL mechanism name:        PLAIN
   Security Considerations:    See section 9 of this memo
   Published specification:    this memo
   Person & email address to contact for further information:
       see author's address section below
   Intended usage:             COMMON
   Author/Change controller:   see author's address section below

9. Security Considerations

   TLS only provides protection for data sent over a network connection.
   Messages transferred over IMAP or POP3 are still available to server
   administrators and usually subject to eavesdropping, tampering and
   forgery when transmitted through SMTP or NNTP.  TLS is no substitute
   for an end-to-end message security mechanism using MIME security
   multiparts [MIME-SEC].

   A man-in-the-middle attacker can remove STARTTLS from the capability
   list or generate a failure response to the STARTTLS command.  In
   order to detect such an attack, clients SHOULD warn the user when
   session privacy is not active and/or be configurable to refuse to
   proceed without an acceptable level of security.

   A man-in-the-middle attacker can always cause a down-negotiation to
   the weakest authentication mechanism or cipher suite available.  For
   this reason, implementations SHOULD be configurable to refuse weak
   mechanisms or cipher suites.

   Any protocol interactions prior to the TLS handshake are performed in
   the clear and can be modified by a man-in-the-middle attacker.  For
   this reason, clients MUST discard cached information about server
   capabilities advertised prior to the start of the TLS handshake.

   Clients are encouraged to clearly indicate when the level of
   encryption active is known to be vulnerable to attack using modern
   hardware (such as encryption keys with 56 bits of entropy or less).

   The LOGINDISABLED IMAP capability (discussed in section 3.2) only
   reduces the potential for passive attacks, it provides no protection
   against active attacks.  The responsibility remains with the client
   to avoid sending a password over a vulnerable channel.

   The PLAIN mechanism relies on the TLS encryption layer for security.
   When used without TLS, it is vulnerable to a common network
   eavesdropping attack.  Therefore PLAIN MUST NOT be advertised or used
   unless a suitable TLS encryption layer is active or backwards
   compatibility dictates otherwise.

   When the PLAIN mechanism is used, the server gains the ability to
   impersonate the user to all services with the same password
   regardless of any encryption provided by TLS or other network privacy
   mechanisms.  While many other authentication mechanisms have similar
   weaknesses, stronger SASL mechanisms such as Kerberos address this
   issue.  Clients are encouraged to have an operational mode where all
   mechanisms which are likely to reveal the user's password to the
   server are disabled.

   The security considerations for TLS apply to STARTTLS and the
   security considerations for SASL apply to the PLAIN mechanism.
   Additional security requirements are discussed in section 2.

10. References

   [ABNF]     Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 2234, November 1997.

   [ACAP]     Newman, C. and J. Myers, "ACAP -- Application
              Configuration Access Protocol", RFC 2244, November 1997.

   [AUTH]     Haller, N. and R. Atkinson, "On Internet Authentication",
              RFC 1704, October 1994.

   [CRAM-MD5] Klensin, J., Catoe, R. and P. Krumviede, "IMAP/POP
              AUTHorize Extension for Simple Challenge/Response", RFC
              2195, September 1997.

   [IMAP]     Crispin, M., "Internet Message Access Protocol - Version
              4rev1", RFC 2060, December 1996.

   [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [MIME-SEC] Galvin, J., Murphy, S., Crocker, S. and N. Freed,
              "Security Multiparts for MIME: Multipart/Signed and
              Multipart/Encrypted", RFC 1847, October 1995.

   [POP3]     Myers, J. and M. Rose, "Post Office Protocol - Version 3",
              STD 53, RFC 1939, May 1996.

   [POP3EXT]  Gellens, R., Newman, C. and L. Lundblade, "POP3 Extension
              Mechanism", RFC 2449, November 1998.

   [POP-AUTH] Myers, J., "POP3 AUTHentication command", RFC 1734,
              December 1994.

   [SASL]     Myers, J., "Simple Authentication and Security Layer
              (SASL)", RFC 2222, October 1997.

   [SMTPTLS]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
              TLS", RFC 2487, January 1999.

   [TLS]      Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
              RFC 2246, January 1999.

   [UTF-8]    Yergeau, F., "UTF-8, a transformation format of ISO
              10646", RFC 2279, January 1998.


11. Author's Address

   Chris Newman
   Innosoft International, Inc.
   1050 Lakes Drive
   West Covina, CA 91790 USA

   EMail: chris.newman@innosoft.com


A. Appendix -- Compliance Checklist

   An implementation is not compliant if it fails to satisfy one or more
   of the MUST requirements for the protocols it implements.  An
   implementation that satisfies all the MUST and all the SHOULD
   requirements for its protocols is said to be "unconditionally
   compliant"; one that satisfies all the MUST requirements but not all
   the SHOULD requirements for its protocols is said to be
   "conditionally compliant".

   Rules                                                 Section
   -----                                                 -------
   Mandatory-to-implement Cipher Suite                      2.1
   SHOULD have mode where encryption required               2.2
   server SHOULD have mode where TLS not required           2.2
   MUST be configurable to refuse all clear-text login
     commands or mechanisms                                 2.3
   server SHOULD be configurable to refuse clear-text
     login commands on entire server and on per-user basis  2.3
   client MUST check server identity                        2.4
   client MUST use hostname used to open connection         2.4
   client MUST NOT use hostname from insecure remote lookup 2.4
   client SHOULD support subjectAltName of dNSName type     2.4
   client SHOULD ask for confirmation or terminate on fail  2.4
   MUST check result of STARTTLS for acceptable privacy     2.5
   client MUST NOT issue commands after STARTTLS
      until server response and negotiation done        3.1,4,5.1
   client MUST discard cached information             3.1,4,5.1,9
   client SHOULD re-issue CAPABILITY/CAPA command       3.1,4
   IMAP server with STARTTLS MUST implement LOGINDISABLED   3.2
   IMAP client MUST NOT issue LOGIN if LOGINDISABLED        3.2
   POP server MUST implement POP3 extensions                4
   ACAP server MUST re-issue ACAP greeting                  5.1
   client SHOULD warn when session privacy not active and/or
     refuse to proceed without acceptable security level    9
   SHOULD be configurable to refuse weak mechanisms or
     cipher suites                                          9

   The PLAIN mechanism is an optional part of this specification.
   However if it is implemented the following rules apply:

   Rules                                                 Section
   -----                                                 -------
   MUST NOT use PLAIN unless strong encryption active
     or backwards compatibility dictates otherwise         6,9
   MUST use UTF-8 encoding for characters in PLAIN          6

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