











Network Working Group                                 R. Siemborski, Ed.

Request for Comments: 4954                                  Google, Inc.

Obsoletes: 2554                                         A. Melnikov, Ed.

Updates: 3463                                              Isode Limited

Category: Standards Track                                      July 2007





               SMTP Service Extension for Authentication



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 IETF Trust (2007).



Abstract



   This document defines a Simple Mail Transport Protocol (SMTP)

   extension whereby an SMTP client may indicate an authentication

   mechanism to the server, perform an authentication protocol exchange,

   and optionally negotiate a security layer for subsequent protocol

   interactions during this session.  This extension includes a profile

   of the Simple Authentication and Security Layer (SASL) for SMTP.



   This document obsoletes RFC 2554.









































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Table of Contents



   1. Introduction ....................................................2

   2. How to Read This Document .......................................2

   3. The Authentication Service Extension ............................3

   4. The AUTH Command ................................................3

      4.1. Examples ...................................................7

   5. The AUTH Parameter to the MAIL FROM command .....................9

      5.1. Examples ..................................................10

   6. Status Codes ...................................................11

   7. Additional requirements on servers .............................12

   8. Formal Syntax ..................................................13

   9. Security Considerations ........................................14

   10. IANA Considerations ...........................................15

   11. Normative References ..........................................15

   12. Informative References ........................................16

   13. Acknowledgments ...............................................17

   14. Additional Requirements When Using SASL PLAIN over TLS ........17

   15. Changes since RFC 2554 ........................................18



1.  Introduction



   This document defines a Simple Mail Transport Protocol (SMTP)

   extension whereby an SMTP client may indicate an authentication

   mechanism to the server, perform an authentication protocol exchange,

   optionally negotiate a security layer for subsequent protocol

   interactions during this session and, during a mail transaction,

   optionally specify a mailbox associated with the identity that

   submitted the message to the mail delivery system.



   This extension includes a profile of the Simple Authentication and

   Security Layer (SASL) for SMTP.



   When compared to RFC 2554, this document deprecates use of the 538

   response code, adds a new Enhanced Status Code, adds a requirement to

   support SASLprep profile for preparing authorization identities,

   recommends use of RFC 3848 transmission types in the Received trace

   header field, and clarifies interaction with SMTP PIPELINING

   [PIPELINING] extension.



2.  How to Read This Document



   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 [KEYWORDS].



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

   server, respectively.







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3.  The Authentication Service Extension



   1.  The name of this [SMTP] service extension is "Authentication".



   2.  The EHLO keyword value associated with this extension is "AUTH".



   3.  The AUTH EHLO keyword contains as a parameter a space-separated

       list of the names of available [SASL] mechanisms.  The list of

       available mechanisms MAY change after a successful STARTTLS

       command [SMTP-TLS].



   4.  A new [SMTP] verb "AUTH" is defined.



   5.  An optional parameter using the keyword "AUTH" is added to the

       MAIL FROM command, and extends the maximum line length of the

       MAIL FROM command by 500 characters.



   6.  This extension is appropriate for the submission protocol

       [SUBMIT].



4.  The AUTH Command



   AUTH mechanism [initial-response]



      Arguments:

          mechanism: A string identifying a [SASL] authentication

          mechanism.



          initial-response: An optional initial client response.  If

          present, this response MUST be encoded as described in Section

          4 of [BASE64] or contain a single character "=".



      Restrictions:

          After an AUTH command has been successfully completed, no more

          AUTH commands may be issued in the same session.  After a

          successful AUTH command completes, a server MUST reject any

          further AUTH commands with a 503 reply.



          The AUTH command is not permitted during a mail transaction.

          An AUTH command issued during a mail transaction MUST be

          rejected with a 503 reply.



      Discussion:

          The AUTH command initiates a [SASL] authentication exchange

          between the client and the server.  The client identifies the

          SASL mechanism to use with the first parameter of the AUTH

          command.  If the server supports the requested authentication

          mechanism, it performs the SASL exchange to authenticate the







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          user.  Optionally, it also negotiates a security layer for

          subsequent protocol interactions during this session.  If the

          requested authentication mechanism is invalid (e.g., is not

          supported or requires an encryption layer), the server rejects

          the AUTH command with a 504 reply.  If the server supports the

          [ESMTP-CODES] extension, it SHOULD return a 5.5.4 enhanced

          response code.



          The SASL authentication exchange consists of a series of

          server challenges and client responses that are specific to

          the chosen [SASL] mechanism.



          A server challenge is sent as a 334 reply with the text part

          containing the [BASE64] encoded string supplied by the SASL

          mechanism.  This challenge MUST NOT contain any text other

          than the BASE64 encoded challenge.



          A client response consists of a line containing a [BASE64]

          encoded string.  If the client wishes to cancel the

          authentication exchange, it issues a line with a single "*".

          If the server receives such a response, it MUST reject the

          AUTH command by sending a 501 reply.



          The optional initial response argument to the AUTH command is

          used to save a round-trip when using authentication mechanisms

          that support an initial client response.  If the initial

          response argument is omitted and the chosen mechanism requires

          an initial client response, the server MUST proceed as defined

          in Section 5.1 of [SASL].  In SMTP, a server challenge that

          contains no data is defined as a 334 reply with no text part.

          Note that there is still a space following the reply code, so

          the complete response line is "334 ".



          Note that the AUTH command is still subject to the line length

          limitations defined in [SMTP].  If use of the initial response

          argument would cause the AUTH command to exceed this length,

          the client MUST NOT use the initial response parameter (and

          instead proceed as defined in Section 5.1 of [SASL]).



          If the client is transmitting an initial response of zero

          length, it MUST instead transmit the response as a single

          equals sign ("=").  This indicates that the response is

          present, but contains no data.



          If the client uses an initial-response argument to the AUTH

          command with a SASL mechanism in which the client does not

          begin the authentication exchange, the server MUST reject the









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          AUTH command with a 501 reply.  Servers using the enhanced

          status codes extension [ESMTP-CODES] SHOULD return an enhanced

          status code of 5.7.0 in this case.



          If the server cannot [BASE64] decode any client response, it

          MUST reject the AUTH command with a 501 reply (and an enhanced

          status code of 5.5.2).  If the client cannot BASE64 decode any

          of the server's challenges, it MUST cancel the authentication

          using the "*" response.  In particular, servers and clients

          MUST reject (and not ignore) any character not explicitly

          allowed by the BASE64 alphabet, and MUST reject any sequence

          of BASE64 characters that contains the pad character ('=')

          anywhere other than the end of the string (e.g., "=AAA" and

          "AAA=BBB" are not allowed).



          Note that these [BASE64] strings can be much longer than

          normal SMTP commands.  Clients and servers MUST be able to

          handle the maximum encoded size of challenges and responses

          generated by their supported authentication mechanisms.  This

          requirement is independent of any line length limitations the

          client or server may have in other parts of its protocol

          implementation.  (At the time of writing of this document,

          12288 octets is considered to be a sufficient line length

          limit for handling of deployed authentication mechanisms.)

          If, during an authentication exchange, the server receives a

          line that is longer than the server's authentication buffer,

          the server fails the AUTH command with the 500 reply.  Servers

          using the enhanced status codes extension [ESMTP-CODES] SHOULD

          return an enhanced status code of 5.5.6 in this case.



          The authorization identity generated by this [SASL] exchange

          is a "simple username" (in the sense defined in [SASLprep]),

          and both client and server SHOULD (*) use the [SASLprep]

          profile of the [StringPrep] algorithm to prepare these names

          for transmission or comparison.  If preparation of the

          authorization identity fails or results in an empty string

          (unless it was transmitted as the empty string), the server

          MUST fail the authentication.



      (*) Note: Future revision of this specification may change this

      requirement to MUST.  Currently, the SHOULD is used in order to

      avoid breaking the majority of existing implementations.



   If the server is unable to authenticate the client, it SHOULD reject

   the AUTH command with a 535 reply unless a more specific error code

   is appropriate.  Should the client successfully complete the

   exchange, the SMTP server issues a 235 reply.  (Note that the SMTP

   protocol doesn't support the SASL feature of returning additional







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   data with a successful outcome.)  These status codes, along with

   others defined by this extension, are discussed in Section 6 of this

   document.



   If a security layer is negotiated during the SASL exchange, it takes

   effect for the client on the octet immediately following the CRLF

   that concludes the last response generated by the client.  For the

   server, it takes effect immediately following the CRLF of its success

   reply.



   When a security layer takes effect, the SMTP protocol is reset to the

   initial state (the state in SMTP after a server issues a 220 service

   ready greeting).  The server MUST discard any knowledge obtained from

   the client, such as the EHLO argument, which was not obtained from

   the SASL negotiation itself.  Likewise, the client MUST discard any

   knowledge obtained from the server, such as the list of SMTP service

   extensions, which was not obtained from the SASL negotiation itself.

   (Note that a client MAY compare the advertised SASL mechanisms before

   and after authentication in order to detect an active down-

   negotiation attack).



   The client SHOULD send an EHLO command as the first command after a

   successful SASL negotiation that results in the enabling of a

   security layer.



   When an entity (whether it is the client or the server end) is

   sending data, and both [TLS] and SASL security layers are in effect,

   the TLS encoding MUST be applied after the SASL encoding, regardless

   of the order in which the layers were negotiated.



   The service name specified by this protocol's profile of SASL is

   "smtp".  This service name is also to be used for the [SUBMIT]

   protocol.



   If an AUTH command fails, the client MAY proceed without

   authentication.  Alternatively, the client MAY try another

   authentication mechanism or present different credentials by issuing

   another AUTH



   Note: A server implementation MUST implement a configuration in which

   it does NOT permit any plaintext password mechanisms, unless either

   the STARTTLS [SMTP-TLS] command has been negotiated or some other

   mechanism that protects the session from password snooping has been

   provided.  Server sites SHOULD NOT use any configuration which

   permits a plaintext password mechanism without such a protection

   mechanism against password snooping.











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   To ensure interoperability, client and server implementations of this

   extension MUST implement the [PLAIN] SASL mechanism running over TLS

   [TLS] [SMTP-TLS].  See also Section 15 for additional requirements on

   implementations of [PLAIN] over [TLS].



   Note that many existing client and server implementations implement

   CRAM-MD5 [CRAM-MD5] SASL mechanism.  In order to ensure

   interoperability with deployed software, new implementations MAY

   implement it; however, implementations should be aware that this SASL

   mechanism doesn't provide any server authentication.  Note that at

   the time of writing of this document the SASL Working Group is

   working on several replacement SASL mechanisms that provide server

   authentication and other features.



   When the AUTH command is used together with the [PIPELINING]

   extension, it MUST be the last command in a pipelined group of

   commands.  The only exception to this rule is when the AUTH command

   contains an initial response for a SASL mechanism that allows the

   client to send data first, the SASL mechanism is known to complete in

   one round-trip, and a security layer is not negotiated by the client.

   Two examples of such SASL mechanisms are PLAIN [PLAIN] and EXTERNAL

   [SASL].



4.1. Examples



   Here is an example of a client attempting AUTH using the [PLAIN] SASL

   mechanism under a TLS layer, and making use of the initial client

   response:



   S: 220-smtp.example.com ESMTP Server

   C: EHLO client.example.com

   S: 250-smtp.example.com Hello client.example.com

   S: 250-AUTH GSSAPI DIGEST-MD5

   S: 250-ENHANCEDSTATUSCODES

   S: 250 STARTTLS

   C: STARTTLS

   S: 220 Ready to start TLS

     ... TLS negotiation proceeds, further commands

         protected by TLS layer ...

   C: EHLO client.example.com

   S: 250-smtp.example.com Hello client.example.com

   S: 250 AUTH GSSAPI DIGEST-MD5 PLAIN

   C: AUTH PLAIN dGVzdAB0ZXN0ADEyMzQ=

   S: 235 2.7.0 Authentication successful



   Here is another client that is attempting AUTH PLAIN under a TLS

   layer, this time without the initial response.  Parts of the

   negotiation before the TLS layer was established have been omitted:







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     ... TLS negotiation proceeds, further commands

         protected by TLS layer ...

   C: EHLO client.example.com

   S: 250-smtp.example.com Hello client.example.com

   S: 250 AUTH GSSAPI DIGEST-MD5 PLAIN

   C: AUTH PLAIN

    (note: there is a single space following the 334

     on the following line)

   S: 334

   C: dGVzdAB0ZXN0ADEyMzQ=

   S: 235 2.7.0 Authentication successful



   Here is an example using CRAM-MD5 [CRAM-MD5], a mechanism in which

   the client does not begin the authentication exchange, and includes a

   server challenge:



   S: 220-smtp.example.com ESMTP Server

   C: EHLO client.example.com

   S: 250-smtp.example.com Hello client.example.com

   S: 250-AUTH DIGEST-MD5 CRAM-MD5

   S: 250-ENHANCEDSTATUSCODES

   S: 250 STARTTLS

   C: AUTH CRAM-MD5

   S: 334 PDQxOTI5NDIzNDEuMTI4Mjg0NzJAc291cmNlZm91ci5hbmRyZXcuY211LmVk

      dT4=

   C: cmpzMyBlYzNhNTlmZWQzOTVhYmExZWM2MzY3YzRmNGI0MWFjMA==

   S: 235 2.7.0 Authentication successful



   Here is an example of a client attempting AUTH EXTERNAL under TLS,

   using the derived authorization ID (and thus a zero-length initial

   client response).



   S: 220-smtp.example.com ESMTP Server

   C: EHLO client.example.com

   S: 250-smtp.example.com Hello client.example.com

   S: 250-AUTH GSSAPI DIGEST-MD5

   S: 250-ENHANCEDSTATUSCODES

   S: 250 STARTTLS

   C: STARTTLS

   S: 220 Ready to start TLS

     ... TLS negotiation proceeds, further commands

         protected by TLS layer ...

   C: EHLO client.example.com

   S: 250-smtp.example.com Hello client.example.com

   S: 250 AUTH EXTERNAL GSSAPI DIGEST-MD5 PLAIN

   C: AUTH EXTERNAL =

   S: 235 2.7.0 Authentication successful









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5.  The AUTH Parameter to the MAIL FROM command



   AUTH=mailbox



   Arguments:

        A <mailbox> (see Section 4.1.2 of [SMTP]) that is associated

        with the identity that submitted the message to the delivery

        system, or the two character sequence "<>" indicating such an

        identity is unknown or insufficiently authenticated.  To comply

        with restrictions imposed on ESMTP parameters, the <mailbox> is

        encoded inside an xtext.  The syntax of an xtext is described in

        Section 4 of [ESMTP-DSN].



   Note:

        For the purposes of this discussion, "authenticated identity"

        refers to the identity (if any) derived from the authorization

        identity of previous AUTH command, while the terms "authorized

        identity" and "supplied <mailbox>" refer to the sender identity

        that is being associated with a particular message.  Note that

        one authenticated identity may be able to identify messages as

        being sent by any number of authorized identities within a

        single session.  For example, this may be the case when an SMTP

        server (one authenticated identity) is processing its queue

        (many messages with distinct authorized identities).



   Discussion:

        The optional AUTH parameter to the MAIL FROM command allows

        cooperating agents in a trusted environment to communicate the

        authorization identity associated with individual messages.



        If the server trusts the authenticated identity of the client to

        assert that the message was originally submitted by the supplied

        <mailbox>, then the server SHOULD supply the same <mailbox> in

        an AUTH parameter when relaying the message to any other server

        which supports the AUTH extension.



        For this reason, servers that advertise support for this

        extension MUST support the AUTH parameter to the MAIL FROM

        command even when the client has not authenticated itself to the

        server.



        A MAIL FROM parameter of AUTH=<> indicates that the original

        submitter of the message is not known.  The server MUST NOT

        treat the message as having been originally submitted by the

        authenticated identity that resulted from the AUTH command.













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        If the AUTH parameter to the MAIL FROM command is not supplied,

        the client has authenticated, and the server believes the

        message is an original submission, the server MAY generate a

        <mailbox> from the user's authenticated identity for use in an

        AUTH parameter when relaying the message to any server which

        supports the AUTH extension.  The generated <mailbox> is

        implementation specific, but it MUST conform to the syntax of

        [SMTP].  If the implementation cannot generate a valid

        <mailbox>, it MUST transmit AUTH=<> when relaying this message.



        If the server does not sufficiently trust the authenticated

        identity of the client, or if the client is not authenticated,

        then the server MUST behave as if the AUTH=<> parameter was

        supplied.  The server MAY, however, write the value of any

        supplied AUTH parameter to a log file.



        If an AUTH=<> parameter was supplied, either explicitly or due

        to the requirement in the previous paragraph, then the server

        MUST supply the AUTH=<> parameter when relaying the message to

        any server which it has authenticated to using the AUTH

        extension.



        A server MAY treat expansion of a mailing list as a new

        submission, setting the AUTH parameter to the mailing list

        address or mailing list administration address when relaying the

        message to list subscribers.



        Note that an implementation which is hard-coded to treat all

        clients as being insufficiently trusted is compliant with this

        specification.  In that case, the implementation does nothing

        more than parse and discard syntactically valid AUTH parameters

        to the MAIL FROM command, and supply AUTH=<> parameters to any

        servers that it authenticates to.



5.1.  Examples



   An example where the original identity of the sender is trusted and

   known:



   C: MAIL FROM:<e=mc2@example.com> AUTH=e+3Dmc2@example.com

   S: 250 OK



   One example where the identity of the sender is not trusted or is

   otherwise being suppressed by the client:



   C: MAIL FROM:<john+@example.org> AUTH=<>

   S: 250 OK









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6.  Status Codes



   The following error codes may be used to indicate various success or

   failure conditions.  Servers that return enhanced status codes

   [ESMTP-CODES] SHOULD use the enhanced codes suggested here.



   235 2.7.0  Authentication Succeeded



   This response to the AUTH command indicates that the authentication

   was successful.



   432 4.7.12  A password transition is needed



   This response to the AUTH command indicates that the user needs to

   transition to the selected authentication mechanism.  This is

   typically done by authenticating once using the [PLAIN]

   authentication mechanism.  The selected mechanism SHOULD then work

   for authentications in subsequent sessions.



   454 4.7.0  Temporary authentication failure



   This response to the AUTH command indicates that the authentication

   failed due to a temporary server failure.  The client SHOULD NOT

   prompt the user for another password in this case, and should instead

   notify the user of server failure.



   534 5.7.9  Authentication mechanism is too weak



   This response to the AUTH command indicates that the selected

   authentication mechanism is weaker than server policy permits for

   that user.  The client SHOULD retry with a new authentication

   mechanism.



   535 5.7.8  Authentication credentials invalid



   This response to the AUTH command indicates that the authentication

   failed due to invalid or insufficient authentication credentials.  In

   this case, the client SHOULD ask the user to supply new credentials

   (such as by presenting a password dialog box).



   500 5.5.6  Authentication Exchange line is too long



   This response to the AUTH command indicates that the authentication

   failed due to the client sending a [BASE64] response that is longer

   than the maximum buffer size available for the currently selected

   SASL mechanism.











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   530 5.7.0  Authentication required



   This response SHOULD be returned by any command other than AUTH,

   EHLO, HELO, NOOP, RSET, or QUIT when server policy requires

   authentication in order to perform the requested action and

   authentication is not currently in force.



   538 5.7.11  Encryption required for requested authentication

               mechanism



   This response to the AUTH command indicates that the selected

   authentication mechanism may only be used when the underlying SMTP

   connection is encrypted.  Note that this response code is documented

   here for historical purposes only.  Modern implementations SHOULD NOT

   advertise mechanisms that are not permitted due to lack of

   encryption, unless an encryption layer of sufficient strength is

   currently being employed.



   This document adds several new enhanced status codes to the list

   defined in [ENHANCED]:



   The following 3 Enhanced Status Codes were defined above:



       5.7.8 Authentication credentials invalid

       5.7.9 Authentication mechanism is too weak

       5.7.11 Encryption required for requested authentication mechanism



   X.5.6     Authentication Exchange line is too long



   This enhanced status code SHOULD be returned when the server fails

   the AUTH command due to the client sending a [BASE64] response which

   is longer than the maximum buffer size available for the currently

   selected SASL mechanism.  This is useful for both permanent and

   persistent transient errors.



7.  Additional Requirements on Servers



   As described in Section 4.4 of [SMTP], an SMTP server that receives a

   message for delivery or further processing MUST insert the

   "Received:" header field at the beginning of the message content.

   This document places additional requirements on the content of a

   generated "Received:" header field.  Upon successful authentication,

   a server SHOULD use the "ESMTPA" or the "ESMTPSA" [SMTP-TT] (when

   appropriate) keyword in the "with" clause of the Received header

   field.













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8.  Formal Syntax



   The following syntax specification uses the Augmented Backus-Naur

   Form notation as specified in [ABNF].  Non-terminals referenced but

   not defined below are as defined by [ABNF] or [SASL].  The non-

   terminal <mailbox> is defined in [SMTP].



   Except as noted otherwise, all alphabetic characters are case-

   insensitive.  The use of upper or lower case characters to define

   token strings is for editorial clarity only.  Implementations MUST

   accept these strings in a case-insensitive fashion.



      hexchar         = "+" HEXDIG HEXDIG



      xchar           = %x21-2A / %x2C-3C / %x3E-7E

                        ;; US-ASCII except for "+", "=", SP, and CTL



      xtext           = *(xchar / hexchar)

                        ;; non-US-ASCII is only allowed as hexchar



      auth-command    = "AUTH" SP sasl-mech [SP initial-response]

                        *(CRLF [base64]) [CRLF cancel-response]

                        CRLF

                        ;; <sasl-mech> is defined in [SASL]



      auth-param      = "AUTH=" xtext

                        ;; Parameter to the MAIL FROM command.

                        ;; This non-terminal complies with

                        ;; syntax defined by esmtp-param [SMTP].

                        ;;

                        ;; The decoded form of the xtext MUST be

                        ;; either a <mailbox> or the two

                        ;; characters "<>"



      base64          = base64-terminal /

                        ( 1*(4base64-char) [base64-terminal] )



      base64-char     = ALPHA / DIGIT / "+" / "/"

                        ;; Case-sensitive



      base64-terminal = (2base64-char "==") / (3base64-char "=")



      continue-req    = "334" SP [base64] CRLF

                        ;; Intermediate response to the AUTH

                        ;; command.

                        ;; This non-terminal complies with

                        ;; syntax defined by Reply-line [SMTP].









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      initial-response= base64 / "="



      cancel-response = "*"



9.  Security Considerations



   Security issues are discussed throughout this memo.



   If a client uses this extension to get an encrypted tunnel through an

   insecure network to a cooperating server, it needs to be configured

   to never send mail to that server when the connection is not mutually

   authenticated and encrypted.  Otherwise, an attacker could steal the

   client's mail by hijacking the [SMTP] connection and either

   pretending the server does not support the Authentication extension

   or causing all AUTH commands to fail.



   Before the [SASL] negotiation has begun, any protocol interactions

   are performed in the clear and may be modified by an active attacker.

   For this reason, clients and servers MUST discard any knowledge

   obtained prior to the start of the SASL negotiation upon the

   establishment of a security layer.



   This mechanism does not protect the TCP port, so an active attacker

   may redirect a relay connection attempt (i.e., a connection between

   two Mail Transfer Agents (MTAs)) to the submission port [SUBMIT].

   The AUTH=<> parameter prevents such an attack from causing a relayed

   message and, in the absence of other envelope authentication, from

   picking up the authentication of the relay client.



   A message submission client may require the user to authenticate

   whenever a suitable [SASL] mechanism is advertised.  Therefore, it

   may not be desirable for a submission server [SUBMIT] to advertise a

   SASL mechanism when use of that mechanism grants the clients no

   benefits over anonymous submission.



   Servers MAY implement a policy whereby the connection is dropped

   after a number of failed authentication attempts.  If they do so,

   they SHOULD NOT drop the connection until at least 3 attempts to

   authenticate have failed.



   If an implementation supports SASL mechanisms that are vulnerable to

   passive eavesdropping attacks (such as [PLAIN]), then the

   implementation MUST support at least one configuration where these

   SASL mechanisms are not advertised or used without the presence of an

   external security layer such as [TLS].













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   This extension is not intended to replace or be used instead of end-

   to-end message signature and encryption systems such as [S/MIME] or

   [PGP].  This extension addresses a different problem than end-to-end

   systems; it has the following key differences:



   1.  It is generally useful only within a trusted enclave.



   2.  It protects the entire envelope of a message, not just the

       message's body.



   3.  It authenticates the message submission, not authorship of the

       message content.



   4.  When mutual authentication is used along with a security layer,

       it can give the sender some assurance that the message was

       successfully delivered to the next hop.



   Additional security considerations are mentioned in the [SASL]

   specification.  Additional security considerations specific to a

   particular SASL mechanism are described in the relevant

   specification.  Additional security considerations for [PLAIN] over

   [TLS] are mentioned in Section 15 of this document.



10.  IANA Considerations



   IANA updated the entry for the "smtp" SASL protocol name to point at

   this document.



   IANA updated the registration of the Authentication SMTP service

   extension as defined in Section 3 of this document.  This registry is

   currently located at <http://www.iana.org/assignments/mail-

   parameters>.



11.  Normative References



   [ABNF]        Crocker, D. and P. Overell, "Augmented BNF for Syntax

                 Specifications: ABNF", RFC 4234, October 2005.



   [BASE64]      Josefsson, S., "The Base16, Base32, and Base64 Data

                 Encodings", RFC 4648, October 2006.



   [ESMTP-CODES] Freed, N., "SMTP Service Extension for Returning

                 Enhanced Error Codes", RFC 2034, October 1996.



   [ENHANCED]    Vaudreuil, G., "Enhanced Mail System Status Codes", RFC

                 3463, January 2003.











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   [ESMTP-DSN]   Moore, K., "Simple Mail Transfer Protocol (SMTP)

                 Service Extension Delivery Status Notifications

                 (DSNs)", RFC 3461, January 2003.



   [KEYWORDS]    Bradner, S., "Key words for use in RFCs to Indicate

                 Requirement Levels", BCP 14, RFC 2119, March 1997.



   [SASL]        Melnikov, A. and K. Zeilenga, "Simple Authentication

                 and Security Layer (SASL)", RFC 4422, June 2006.



   [SASLprep]    Zeilenga, K., "SASLprep: Stringprep Profile for User

                 Names and Passwords", RFC 4013, February 2005.



   [SMTP]        Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,

                 April 2001.



   [SMTP-TLS]    Hoffman, P., "SMTP Service Extension for Secure SMTP

                 over Transport Layer Security", RFC 3207, February

                 2002.



   [StringPrep]  Hoffman, P. and M. Blanchet, "Preparation of

                 Internationalized Strings ("stringprep")", RFC 3454,

                 December 2002.



   [SUBMIT]      Gellens, R. and J. Klensin, "Message Submission for

                 Mail", RFC 4409, April 2006.



   [SMTP-TT]     Newman, C., "ESMTP and LMTP Transmission Types

                 Registration", RFC 3848, July 2004.



   [PLAIN]       Zeilenga, K., Ed., "The PLAIN Simple Authentication and

                 Security Layer (SASL) Mechanism", RFC 4616, August

                 2006.



   [X509]        Housley, R., Polk, W., Ford, W., and D. Solo, "Internet

                 X.509 Public Key Infrastructure Certificate and

                 Certificate Revocation List (CRL) Profile", RFC 3280,

                 April 2002.



12.  Informative References



   [PGP]         Elkins, M., "MIME Security with Pretty Good Privacy

                 (PGP)", RFC 2015, October 1996.



   [S/MIME]      Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail

                 Extensions (S/MIME) Version 3.1 Message Specification",

                 RFC 3851, July 2004.









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   [TLS]         Dierks, T. and E. Rescorla, "The Transport Layer

                 Security (TLS) Protocol Version 1.1", RFC 4346, April

                 2006.



   [PIPELINING]  Freed, N., "SMTP Service Extension for Command

                 Pipelining", STD 60, RFC 2920, September 2000.



   [CRAM-MD5]    Klensin, J., Catoe, R., and P. Krumviede, "IMAP/POP

                 AUTHorize Extension for Simple Challenge/Response", RFC

                 2195, September 1997.



13.  Acknowledgments



   The editors would like to acknowledge the contributions of John Myers

   and other contributors to RFC 2554, on which this document draws from

   heavily.



   The editors would also like to thank Ken Murchison, Mark Crispin,

   Chris Newman, David Wilson, Dave Cridland, Frank Ellermann, Ned

   Freed, John Klensin, Tony Finch, Abhijit Menon-Sen, Philip Guenther,

   Sam Hartman, Russ Housley, Cullen Jennings, and Lisa Dusseault for

   the time they devoted to reviewing of this document and/or for the

   comments received.



14.  Additional Requirements When Using SASL PLAIN over TLS



   This section is normative for SMTP implementations that support SASL

   [PLAIN] over [TLS].



   If an SMTP client is willing to use SASL PLAIN over TLS to

   authenticate to the SMTP server, the client verifies the server

   certificate according to the rules of [X509].  If the server has not

   provided any certificate, or if the certificate verification fails,

   the client MUST NOT attempt to authenticate using the SASL PLAIN

   mechanism.



   After a successful [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.  If the match fails, the client MUST NOT

   attempt to authenticate using the SASL PLAIN mechanism.  Matching is

   performed according to the following 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











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        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 leftmost 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.



15.  Changes since RFC 2554



   1.  Clarified that servers MUST support the use of the AUTH=mailbox

       parameter to MAIL FROM, even when the client is not

       authenticated.



   2.  Clarified the initial-client-send requirements, and give

       additional examples.



   3.  Updated references to newer versions of various specifications.



   4.  Required SASL PLAIN (over TLS) as mandatory-to-implement.



   5.  Clarified that the mechanism list can change.



   6.  Deprecated the use of the 538 response code.



   7.  Added the use of the SASLprep profile for preparing authorization

       identities.



   8.  Substantial cleanup of response codes and indicated suggested

       enhanced response codes.  Also indicated what response codes

       should result in a client prompting the user for new credentials.



   9.  Updated ABNF section to use RFC 4234.



   10. Clarified interaction with SMTP PIPELINING extension.



   11. Added a reference to RFC 3848.







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   12. Added a new Enhanced Status Code for "authentication line too

       long" case.



   13. Other general editorial clarifications.



Editors' Addresses



   Robert Siemborski

   Google, Inc.

   1600 Ampitheatre Parkway

   Mountain View, CA 94043, USA



   Phone: +1 650 623 6925

   EMail: robsiemb@google.com





   Alexey Melnikov

   Isode Limited

   5 Castle Business Village, 36 Station Road,

   Hampton, Middlesex, TW12 2BX, UK



   EMail: Alexey.Melnikov@isode.com



























































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Full Copyright Statement



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Acknowledgement



   Funding for the RFC Editor function is currently provided by the

   Internet Society.















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