INTERNET-DRAFT Editor: Kurt D. Zeilenga
Intended Category: Standards Track OpenLDAP Foundation
Expires in six months 4 November 2002
Obsoletes: RFC 2245
Anonymous SASL Mechanism
<draft-zeilenga-sasl-anon-01.txt>
Status of Memo
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.
This document is intended to be, after appropriate review and
revision, submitted to the RFC Editor as a Standards Track document.
Distribution of this memo is unlimited. Technical discussion of this
document will take place on the IETF SASL mailing list
<ietf-sasl@imc.org>. Please send editorial comments directly to the
document editor <Kurt@OpenLDAP.org>.
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Copyright 2002, The Internet Society. All Rights Reserved.
Please see the Copyright section near the end of this document for
more information.
Abstract
It is common practice on the Internet to permit anonymous access to
various services. Traditionally, this has been done with a plain text
password mechanism using "anonymous" as the user name and optional
trace information, such as an email address, as the password. As
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plaintext login commands are not permitted in new IETF protocols, a
new way to provide anonymous login is needed within the context of the
Simple Authentication and Security Layer (SASL) framework.
1. Anonymous SASL mechanism
This document defines an anonymous mechanism for the Simple
Authentication and Security Layer ([SASL]) framework. The name
associated with this mechanism is "ANONYMOUS".
This document replaces RFC 2245. Changes since RFC 2245 are detailed
in Appendix A.
The mechanism consists of a single message from the client to the
server. The client sends optional trace information in the form of a
string of UTF-8 [UTF8] encoded printable characters from the Universal
Character Set ([UCS]). The trace information should take one of three
forms: an Internet email address, an opaque string which does not
contain the '@' (U+0040) character and can be interpreted by the
system administrator of the client's domain, or nothing. For privacy
reasons, an Internet email address should only be used with permission
from the user.
A server which permits anonymous access will announce support for the
ANONYMOUS mechanism, and allow anyone to log in using that mechanism,
usually with restricted access.
This mechanism does not provide a security layer.
The formal grammar for the client message using Augmented BNF [ABNF]
follows.
message = [ email / token ]
UTF1 = %x00-3F / %x41-7F ;; less '@' (U+0040)
UTF2 = %xC0-DF 1(UTF0)
UTF3 = %xE0-EF 2(UTF0)
UTF4 = %xF0-F7 3(UTF0)
UTF5 = %xF8-FB 4(UTF0)
UTF6 = %xFC-FD 5(UTF0)
UTF0 = %x80-BF
TCHAR = UTF1 / UTF2 / UTF3 / UTF3 / UTF4 / UTF5 / UTF6
;; any UTF-8 encoded UCS printable character
;; except '@' (U+0040)
email = addr-spec
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;; as defined in [IMAIL], except with no free
;; insertion of linear-white-space, and the
;; local-part MUST either be entirely enclosed in
;; quotes or entirely unquoted
token = 1*255TCHAR
The following characters are considered non-printable:
- control characters: U+0000..U+001F, U+007F..U+009F;
- replacement character: U+FFFD; and
- special characters and non-character: U+FEFF, U+FFFE, U+FFFF.
2. Example
Here is a sample ANONYMOUS login 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 opaque identification token is "sirhc".
S: * OK IMAP4 server ready
C: A001 CAPABILITY
S: * CAPABILITY IMAP4 IMAP4rev1 AUTH=DIGEST-MD5 AUTH=ANONYMOUS
S: A001 OK done
C: A002 AUTHENTICATE ANONYMOUS
S: +
C: c2lyaGM=
S: A003 OK Welcome, trace information has been logged.
3. Security Considerations
The ANONYMOUS mechanism grants access to information by anyone. For
this reason it should be disabled by default so the administrator can
make an explicit decision to enable it.
If the anonymous user has any write privileges, a denial of service
attack is possible by filling up all available space. This can be
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prevented by disabling all write access by anonymous users.
If anonymous users have read and write access to the same area, the
server can be used as a communication mechanism to anonymously
exchange information. Servers which accept anonymous submissions
should implement the common "drop box" model which forbids anonymous
read access to the area where anonymous submissions are accepted.
If the anonymous user can run many expensive operations (e.g., an IMAP
SEARCH BODY command), this could enable a denial of service attack.
Servers are encouraged to limit the number of anonymous users and
reduce their priority or limit their resource usage.
If there is no idle timeout for the anonymous user and there is a
limit on the number of anonymous users, a denial of service attack is
enabled. Servers should implement an idle timeout for anonymous
users.
The trace information is not authenticated so it can be falsified.
This can be used as an attempt to get someone else in trouble for
access to questionable information. Administrators trying to trace
abuse need to realize this information may be falsified.
A client which uses the user's correct email address as trace
information without explicit permission may violate that user's
privacy. Information about who accesses an anonymous archive on a
sensitive subject (e.g., sexual abuse) has strong privacy needs.
Clients should not send the email address without explicit permission
of the user and should offer the option of supplying no trace token --
thus only exposing the source IP address and time. Anonymous proxy
servers could enhance this privacy, but would have to consider the
resulting potential denial of service attacks.
Anonymous connections are susceptible to man in the middle attacks
which view or alter the data transferred. Clients and servers are
encouraged to support external integrity and encryption mechanisms.
Protocols which fail to require an explicit anonymous login are more
susceptible to break-ins given certain common implementation
techniques. Specifically, Unix servers which offer user login may
initially start up as root and switch to the appropriate user id after
an explicit login command. Normally such servers refuse all data
access commands prior to explicit login and may enter a restricted
security environment (e.g., the Unix chroot(2) function) for anonymous
users. If anonymous access is not explicitly requested, the entire
data access machinery is exposed to external security attacks without
the chance for explicit protective measures. Protocols which offer
restricted data access should not allow anonymous data access without
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an explicit login step.
General SASL security considerations apply to this mechanism.
4. IANA Considerations
It is requested that the SASL Mechanism registry [IANA-SASL] entry for
the ANONYMOUS mechanism be updated to reflect that this document now
provides its technical specification.
To: iana@iana.org
Subject: Updated Registration of SASL mechanism ANONYMOUS
SASL mechanism name: ANONYMOUS
Security considerations: See RFC XXXX.
Published specification (optional, recommended): RFC XXXX
Person & email address to contact for further information:
Kurt Zeilenga <kurt@openldap.org>
Chris Neuman <chris.newman@innosoft.com>
Intended usage: COMMON
Author/Change controller: IESG <iesg@ietf.org>
Note: Updates existing entry for ANONYMOUS
5. Acknowledgement
This document is a revision of RFC 2245 by Chris Newman.
6. Normative References
[ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[IMAIL] Crocker, D., "Standard for the Format of Arpa Internet
Text Messages", STD 11, RFC 822, August 1982.
[SASL] Myers, J., "Simple Authentication and Security Layer
(SASL)", RFC 2222bis (a work in progress).
[UCS] Universal Multiple-Octet Coded Character Set (UCS) -
Architecture and Basic Multilingual Plane, ISO/IEC 10646-1
: 1993.
[UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 2279, January 1998.
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7. Informative References
[IMAP4] Crispin, M., "Internet Message Access Protocol - Version
4rev1", RFC 2060, December 1996.
[IANA-SASL] IANA, "SIMPLE AUTHENTICATION AND SECURITY LAYER (SASL)
MECHANISMS", http://www.iana.org/assignments/sasl-
mechanisms.
8. Editor's Address
Kurt Zeilenga
OpenLDAP Foundation
Email: kurt@OpenLDAP.org
Appendix A. Changes since RFC 2245
This appendix is non-normative.
RFC 2245 allows the client to send optional trace information in the
form of a human readable string, but did not define the character
encoding nor character set to be used. This document mandates use of
UTF-8 encoded UCS printable characters. The grammar was updated to
reflect this.
Additionally, a number of editorial changes were made.
Full Copyright Statement
Copyright 2002, The Internet Society. All Rights Reserved.
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or as required to translate it into languages other than English.
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The limited permissions granted above are perpetual and will not be
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