Network Working Group
Request for Comments: 2554
Category: Standards Track
J. Myers
Netscape Communications
March 1999

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 © The Internet Society (1999). All Rights Reserved.

1. Introduction

This document defines an SMTP service extension [ESMTP] 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. This extension is a profile of the Simple Authentication and Security Layer [SASL].

2. Conventions Used in this Document

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

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

3. The Authentication service extension

(1) the name of the 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 supported SASL mechanisms.

(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


4. The AUTH command

   AUTH mechanism [initial-response]


a string identifying a SASL authentication mechanism. an optional base64-encoded response


After an AUTH command has 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.


The AUTH command indicates an authentication mechanism to the server. If the server supports the requested authentication mechanism, it performs an authentication protocol exchange to authenticate and identify the user. Optionally, it also negotiates a security layer for subsequent protocol interactions. If the requested authentication mechanism is not supported, the server rejects the AUTH command with a 504 reply.

The authentication protocol exchange consists of a series of server challenges and client answers that are specific to the authentication mechanism. A server challenge, otherwise known as a ready response, is a 334 reply with the text part containing a BASE64 encoded string. The client answer consists of a line containing a BASE64 encoded string. If the client wishes to cancel an authentication exchange, it issues a line with a single "*". If the server receives such an answer, 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 are defined to send no data in the initial challenge. When the initial-response argument is used with such a mechanism, the initial empty challenge is not sent to the client and the server uses the data in the initial-response argument as if it were sent in response to the empty challenge. Unlike a zero-length client answer to a 334 reply, a zero- length initial response is sent as a single equals sign ("="). If the client uses an initial-response argument to the AUTH command with a mechanism that sends data in the initial challenge, the server rejects the AUTH command with a 535 reply.

If the server cannot BASE64 decode the argument, it rejects the AUTH command with a 501 reply. If the server rejects the authentication data, it SHOULD reject the AUTH command with a 535 reply unless a more specific error code, such as one listed in section 6, is appropriate. Should the client successfully complete the authentication exchange, the SMTP server issues a 235 reply.

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

If a security layer is negotiated through the SASL authentication exchange, it takes effect immediately following the CRLF that concludes the authentication exchange for the client, and the CRLF of the success reply for the server. Upon a security layer's taking 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 argument to the EHLO command, which was not obtained from the SASL negotiation itself. 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 (with the exception that a client MAY compare the list of 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 which results in the enabling of a security layer.

The server is not required to support any particular authentication mechanism, nor are authentication mechanisms required to support any security layers. If an AUTH command fails, the client may try another authentication mechanism by issuing another AUTH command.

If an AUTH command fails, the server MUST behave the same as if the client had not issued the AUTH command.

The BASE64 string may in general be arbitrarily long. Clients and servers MUST be able to support challenges and responses that are as long as are generated by the authentication mechanisms they support, independent of any line length limitations the client or server may have in other parts of its protocol implementation.


         S: 220 ESMTP server ready
         C: EHLO
         S: 250 AUTH CRAM-MD5 DIGEST-MD5
         C: AUTH FOOBAR
         S: 504 Unrecognized authentication type.
         C: AUTH CRAM-MD5
         S: 334
         C: ZnJlZCA5ZTk1YWVlMDljNDBhZjJiODRhMGMyYjNiYmFlNzg2ZQ==
         S: 235 Authentication successful.

5. The AUTH parameter to the MAIL FROM command



An addr-spec containing the identity which submitted the message to the delivery system, or the two character sequence "<>" indicating such an identity is unknown or insufficiently authenticated. To comply with the restrictions imposed on ESMTP parameters, the addr-spec is encoded inside an xtext. The syntax of an xtext is described in section 5 of [ESMTP-DSN].


The optional AUTH parameter to the MAIL FROM command allows cooperating agents in a trusted environment to communicate the authentication of individual messages.

If the server trusts the authenticated identity of the client to assert that the message was originally submitted by the supplied addr-spec, then the server SHOULD supply the same addr-spec in an AUTH parameter when relaying the message to any server which supports the AUTH extension.

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 client.

If the AUTH parameter to the MAIL FROM is not supplied, the client has authenticated, and the server believes the message is an original submission by the client, the server MAY supply the client's identity in the addr-spec in an AUTH parameter when relaying the message to any server which supports the AUTH extension.

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 the 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.

It is conforming for an implementation to be hard-coded to treat all clients as being insufficiently trusted. 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 to which it authenticates using the AUTH extension.


       C: MAIL FROM:<>
       S: 250 OK

6. Error Codes

The following error codes may be used to indicate various conditions as described.

  1. A password transition is needed

This response to the AUTH command indicates that the user needs to transition to the selected authentication mechanism. This typically done by authenticating once using the PLAIN authentication mechanism.

  1. 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.

  1. 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.

  1. Temporary authentication failure

This response to the AUTH command indicates that the authentication failed due to a temporary server failure.

  1. Authentication required

This response may be returned by any command other than AUTH, EHLO, HELO, NOOP, RSET, or QUIT. It indicates that server policy requires authentication in order to perform the requested action.

7. Formal Syntax

The following syntax specification uses the augmented Backus-Naur Form (BNF) notation as specified in [ABNF].

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.

   UPALPHA         = %x41-5A            ;; Uppercase: A-Z
   LOALPHA         = %x61-7A            ;; Lowercase: a-z
   ALPHA           = UPALPHA / LOALPHA  ;; case insensitive
   DIGIT           = %x30-39            ;; Digits 0-9
   HEXDIGIT        = %x41-46 / DIGIT    ;; hexidecimal digit (uppercase)
   hexchar         = "+" HEXDIGIT HEXDIGIT
   xchar           = %x21-2A / %x2C-3C / %x3E-7E
                     ;; US-ASCII except for "+", "=", SPACE and CTL
   xtext           = *(xchar / hexchar)
   AUTH_CHAR       = ALPHA / DIGIT / "-" / "_"
   auth_type       = 1*20AUTH_CHAR
   auth_command    = "AUTH" SPACE auth_type [SPACE (base64 / "=")]
                     *(CRLF [base64]) CRLF
   auth_param      = "AUTH=" xtext
                       ;; The decoded form of the xtext MUST be either
                       ;; an addr-spec or the two characters "<>"
   base64          = base64_terminal /
                     ( 1*(4base64_CHAR) [base64_terminal] )
   base64_char     = UPALPHA / LOALPHA / DIGIT / "+" / "/"
                     ;; Case-sensitive
   base64_terminal = (2base64_char "==") / (3base64_char "=")
   continue_req    = "334" SPACE [base64] CRLF
   CR              = %x0C           ;; ASCII CR, carriage return
   CRLF            = CR LF
   CTL             = %x00-1F / %x7F ;; any ASCII control character and DEL
   LF              = %x0A           ;; ASCII LF, line feed
   SPACE           = %x20           ;; ASCII SP, space

8. References

   [ABNF]      Crocker, D. and P. Overell, "Augmented BNF for Syntax
               Specifications: ABNF", RFC 2234, November 1997.
   [CRAM-MD5]  Klensin, J., Catoe, R. and P. Krumviede, "IMAP/POP
               AUTHorize Extension for Simple Challenge/Response", RFC
               2195, September 1997.
   [ESMTP]     Klensin, J., Freed, N., Rose, M., Stefferud, E. and D.
               Crocker, "SMTP Service Extensions", RFC 1869, November
   [ESMTP-DSN] Moore, K, "SMTP Service Extension for Delivery Status
               Notifications", RFC 1891, January 1996.
   [KEYWORDS]  Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.
   [SASL]      Myers, J., "Simple Authentication and Security Layer
               (SASL)", RFC 2222, October 1997.
   [SUBMIT]    Gellens, R. and J. Klensin, "Message Submission", RFC
               2476, December 1998.
   [RFC821]    Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC
               821, August 1982.
   [RFC822]    Crocker, D., "Standard for the Format of ARPA Internet
               Text Messages", STD 11, RFC 822, August 1982.

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 completion of a SASL negotiation which results in a security layer.

This mechanism does not protect the TCP port, so an active attacker may redirect a relay connection attempt to the submission port [SUBMIT]. The AUTH=<> parameter prevents such an attack from causing an relayed message without an envelope authentication to pick 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 client no benefits over anonymous submission.

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) it can give the sender some assurance the message was

delivered to the next hop in the case where the sender

mutually authenticates with the next hop and negotiates an

appropriate security layer.

Additional security considerations are mentioned in the SASL specification [SASL].

10. Author's Address

John Gardiner Myers
Netscape Communications
501 East Middlefield Road
Mail Stop MV-029
Mountain View, CA 94043


11. Full Copyright Statement

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

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