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Network Working Group C. Newman
Internet Draft: Plaintext Password SASL Mechanism Innosoft
Document: draft-newman-sasl-plaintrans-01.txt June 1997
Expires in six months
Plaintext Password SASL Mechanism and Transition Codes
Status of this memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other
documents at any time. It is inappropriate to use Internet-Drafts
as reference material or to cite them other than as "work in
progress."
To view the entire list of current Internet-Drafts, please check
the "1id-abstracts.txt" listing contained in the Internet-Drafts
Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net
(Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East
Coast), or ftp.isi.edu (US West Coast).
Abstract
While plaintext passwords have very poor security characteristics
by themselves, there are a number of contexts where they are useful
or necessary. This defines a plaintext password mechanism for SASL
[SASL] which is intended to be used in combination with an external
encryption layer, as a transition mechanism from a legacy
authentication database, or to use (insecurely) a legacy
authentication database which can not practically be replaced.
In hopes of promoting the future elimination of unencrypted
plaintext passwords, this defines error codes for use with POP3 and
IMAP4: one to indiciate the need for a transition to a stronger
mechanism, a second to indicate that plaintext passwords are no
longer accepted by a given service, and a third to indicate that
plaintext passwords are only accepted by a given service in
combination with an external strong encryption mechanism. Any
protocol offering the PLAIN mechanism should also support these
error codes.
Newman [Page 1]
Internet Draft Plaintext Password SASL Mechanism June 1997
1. Conventions Used in this Document
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].
2. Security Impact of Plaintext Passwords
Use of unencrypted plaintext passwords over the Internet is a
severe security risk. In particular, a passive observer can get
the password with any packet sniffer. This requires no technical
expertise, as one can simply plug a consumer level computer into
the network and run widely available network snoop programs. Such
attacks are difficult or impossible to detect, and can only be
prevented by complete physical and virtual security of the network
between the client and server -- something which is usually
impossible to achieve.
Unfortunately, most modern servers use legacy authentication
databases, usually tightly integrated with the operating system,
which apply a one-way function to the user's password preventing
the use of any mechanism other than plaintext passwords. This
means that plaintext passwords are the only authentication
technology today which will work with the vast majority of deployed
authentication databases. Often the cost of deploying a new
authentication technology or having different authentication
credentials for different services outweighs the security risks of
plaintext passwords.
Thus there are three situations where plaintext passwords are
necessary or useful:
(1) As a method to transition, on a per-user basis, from a legacy
authentication database a secure password mechanism such as
CRAM-MD5 [CRAM-MD5].
(2) As the only feasible mechanism when a legacy authentication
database is in use, multiple remote services are offered, there is
no way to transition all the remote services, and it is
unacceptable to have different passwords for different services.
(3) In conjunction with a strong encryption layer.
In all other cases, plaintext passwords SHOULD NOT be used. In
addition, any client or server supporting this mechanism SHOULD
also support a strong encryption layer or a stronger authentication
mechanism.
Newman [Page 2]
Internet Draft Plaintext Password SASL Mechanism June 1997
3. Plaintext Password SASL mechanism
The mechanism name associated with plaintext passwords is "PLAIN".
The mechanism consists of a single message from the client to the
server. The client sends the authorization identity, followed by a
US-ASCII NUL character, followed by the authentication identity,
followed by a US-ASCII NUL character, followed by the plaintext
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 these steps succeed, the user is
logged in.
When used as a transition mechanism, the password will be stored in
a new authentication database capable of supporting stronger
authentication mechanisms. Once this is completed, the server MAY
refuse future use of the PLAIN mechanism by that authentication
identity.
The formal grammar for the client message using Augmented BNF
[ABNF] follows.
message = [authorize-id] NUL authenticate-id NUL password
ACHAR = %x20..7E
PCHAR = %x01..09 / %x0B..0C / %x0E..7F
NUL = %x00
authenticate-id = 1*255ACHAR
authorize-id = 1*255ACHAR
password = 1*255PCHAR
4. New Error/Response Codes
When used as a transition mechanism, two new response codes are
helpful to provide guidance to clients. A third response code is
useful to indicate the need for an external encryption mechanism.
Newman [Page 3]
Internet Draft Plaintext Password SASL Mechanism June 1997
The failure codes for use with IMAP4 [IMAP4] are defined here:
TRANSITION-NEEDED
This IMAP response code occurs on the tagged NO response to an
AUTHENTICATE command after an attempt to use a mechanism other
than PLAIN. It indicates that that mechanism is not currently
usable, but will be usable once PLAIN or LOGIN is used.
PLAINTEXT-DENIED
This IMAP response code occurs on the tagged NO response to a
LOGIN command or "AUTHENTICATE PLAIN" command. It indicates
that a transition has already happened on the server and
plaintext passwords are no longer permitted. This may also be
used on the untagged OK response when an AUTHENTICATE command
using a stronger mechanism succeeds to indicate that plaintext
passwords will not be accepted from that user at a later date.
ENCRYPT-NEEDED
This IMAP response code occurs on the tagged NO response to a
LOGIN command or "AUTHENTICATE PLAIN" command. It indicates
that plaintext passwords may only be used in combination with
an external strong encryption service.
The failure codes for use with POP3 [POP3] are defined here:
-ERR [TRANSITION-NEEDED] A password transition is needed.
The portion between the "[" and "]" is a machine parsable
message, interpreted in a case insensitive fashion. The
remainer of the line may contain any human readable text.
This occurs in response to an APOP or AUTH [POP-AUTH] command
that is not currently available, but will be available after
the use of the PLAIN mechanism or the USER/PASS commands.
-ERR [PLAINTEXT-DENIED] Plaintext passwords forbidden
This occurs in response to a USER, PASS or AUTH PLAIN command
and indicates that the server no longer permits plaintext
passwords because a transition has occurred. It may also
occure on a "+OK" line after a successful APOP or AUTH command
to indicate that plaintext passwords will not be accepted from
that user at a later date.
Newman [Page 4]
Internet Draft Plaintext Password SASL Mechanism June 1997
-ERR [ENCRYPT-NEEDED] Plaintext passwords need encryption
This occurs in response to a USER, PASS or AUTH PLAIN command
and indicates that the server does not permit plaintext
passwords without an external encryption mechanism for the
specified user.
5. Example
Here is a sample transition exchange between an IMAP client and
server. In this example, "C:" and "S:" indicate lines sent by the
client and server respectively. If such lines are wrapped without
a new "C:" or "S:" label, then the wrapping is for editorial
clarity and is not part of the command.
Note that this example uses the IMAP profile [IMAP4] of SASL. The
base64 encoding of challenges and responses, as well as the "+ "
preceding the responses are part of the IMAP4 profile, not part of
SASL itself. Newer profiles of SASL will include the client
message with the AUTHENTICATE command itself so the extra round
trip below (the server response with an empty "+ ") can be
eliminated.
In this example, the user's authentication identifier is "tim", his
authorization identifier is the same, and his password is
"tanstaaftanstaaf".
S: * OK IMAP4 server ready
C: A001 CAPABILITY
S: * CAPABILITY IMAP4 IMAP4rev1 AUTH=CRAM-MD5 AUTH=PLAIN
S: A001 OK done
C: A002 AUTHENTICATE CRAM-MD5
S: + PDE4OTYuNjk3MTcwOTUyQHBvc3RvZmZpY2UucmVzdG9uLm1jaS5uZXQ+
C: dGltIGI5MTNhNjAyYzdlZGE3YTQ5NWI0ZTZlNzMzNGQzODkw
S: A002 NO [TRANSITION-NEEDED] You can't login securely until
you've changed your password on the server
C: A003 AUTHENTICATE PLAIN
S: +
C: AHRpbQB0YW5zdGFhZnRhbnN0YWFm
S: A003 OK You can now login securely in the future.
C: A004 SELECT INBOX
...
6. Security Considerations
Security considerations are discussed throughout this document.
A man in the middle or a spoof server may be able to aquire the
Newman [Page 5]
Internet Draft Plaintext Password SASL Mechanism June 1997
user's password by pretending no strong authentication mechanisms
are available. The are two steps the client can take to help
defeat such attacks. First, clients SHOULD record the occurance of
a PLAINTEXT-DENIED or ENCRYPT-NEEDED error for a given user, server
and protocol combination and refuse to use unencrypted plaintext
passwords for that combination in the future. Second, clients
SHOULD get permission from the user prior to using an unencrypted
plaintext password. If the user normally makes a secure connection
and suddenly sees a warning, this might prevent a password
compromise.
Unencrypted plaintext passwords are visible to any network snooper,
as discussed in section 2. Servers SHOULD have the ability to
enable the PLAINTEXT-DENIED or ENCRYPT-NEEDED errors on a per-user
basis to ease the transition away from unencrypted plaintext
passwords.
7. References
[ABNF] Crocker, D., "Augmented BNF for Syntax Specifications:
ABNF", Work in progress: draft-ietf-drums-abnf-xx.txt
[CRAM-MD5] Klensin, Catoe, Krumviede, "IMAP/POP AUTHorize Extension
for Simple Challenge/Response", RFC 2095, MCI, January 1997.
<ftp://ds.internic.net/rfc/rfc2095.txt>
[IMAP4] Crispin, M., "Internet Message Access Protocol - Version
4rev1", RFC 2060, University of Washington, December 1996.
<ftp://ds.internic.net/rfc/rfc2060.txt>
[KEYWORDS] Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, Harvard University, March 1997.
<ftp://ds.internic.net/rfc/rfc2119.txt>
[POP3] Myers, J., Rose, M., "Post Office Protocol - Version 3", RFC
1939, Carnegie Mellon, Dover Beach Consulting, Inc., May 1996.
<ftp://ds.internic.net/rfc/rfc1939.txt>
[POP-AUTH] Myers, "POP3 AUTHentication command", RFC 1734, Carnegie
Mellon, December 1994.
<ftp://ds.internic.net/rfc/rfc1734.txt>
Newman [Page 6]
Internet Draft Plaintext Password SASL Mechanism June 1997
[SASL] Myers, "Simple Authentication and Security Layer (SASL)",
work in progress.
[UTF8] Yergeau, F. "UTF-8, a transformation format of Unicode and
ISO 10646", RFC 2044, Alis Technologies, October 1996.
<ftp://ds.internic.net/rfc/rfc2044.txt>
8. Acknowledgements
Thanks to John Myers and Larry Osterman for providing feedback on
the initial version of this specification. Special thanks to Ned
Freed and Kevin Carosso for coming up with the basic plaintext
transition idea.
9. Author's Address
Chris Newman
Innosoft International, Inc.
1050 Lakes Drive
West Covina, CA 91790 USA
Email: chris.newman@innosoft.com
Newman [Page 7]