|Title||SMTP Service Extension for Authentication
Network Working Group J. Myers
Request for Comments: 2554 Netscape Communications
Category: Standards Track March 1999
SMTP Service Extension
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 (C) The Internet Society (1999). All Rights Reserved.
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
2. Conventions Used in this Document
In examples, "C:" and "S:" indicate lines sent by the client and
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
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
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
The service name specified by this protocol's profile of SASL
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
S: 220 smtp.example.com ESMTP server ready
C: EHLO jgm.example.com
S: 250 AUTH CRAM-MD5 DIGEST-MD5
C: AUTH FOOBAR
S: 504 Unrecognized authentication type.
C: AUTH CRAM-MD5
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
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:<email@example.com> AUTH=e+3Dmc2@example.com
S: 250 OK
6. Error Codes
The following error codes may be used to indicate various conditions
432 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.
534 Authentication mechanism is too weak
This response to the AUTH command indicates that the selected
authentication mechanism is weaker than server policy permits for
538 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.
454 Temporary authentication failure
This response to the AUTH command indicates that the authentication
failed due to a temporary server failure.
530 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 / "+" / "/"
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
[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
(3) it authenticates the message submission, not authorship of the
(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
10. Author's Address
John Gardiner Myers
501 East Middlefield Road
Mail Stop MV-029
Mountain View, CA 94043
11. Full Copyright Statement
Copyright (C) The Internet Society (1999). All Rights Reserved.
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