Network Working Group K. Zeilenga, Ed.
Request for Comments: 4505 OpenLDAP Foundation
Obsoletes: 2245 June 2006
Category: Standards Track
Anonymous Simple Authentication and Security Layer (SASL) Mechanism
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 (2006).
On the Internet, it is common practice 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 using
optional trace information, such as an email address, as the
password. As plain-text 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)
This document defines an anonymous mechanism for the Simple
Authentication and Security Layer ([SASL]) framework. The name
associated with this mechanism is "ANONYMOUS".
Unlike many other SASL mechanisms, whose purpose is to authenticate
and identify the user to a server, the purpose of this SASL mechanism
is to allow the user to gain access to services or resources without
requiring the user to establish or otherwise disclose their identity
to the server. That is, this mechanism provides an anonymous login
This mechanism does not provide a security layer.
This document replaces RFC 2245. Changes since RFC 2245 are detailed
in Appendix A.
2. The Anonymous Mechanism
The mechanism consists of a single message from the client to the
server. The client may include in this message trace information in
the form of a string of [UTF-8]-encoded [Unicode] characters prepared
in accordance with [StringPrep] and the "trace" stringprep profile
defined in Section 3 of this document. The trace information, which
has no semantical value, should take one of two forms: an Internet
email address, or an opaque string that does not contain the '@'
(U+0040) character and that can be interpreted by the system
administrator of the client's domain. For privacy reasons, an
Internet email address or other information identifying the user
should only be used with permission from the user.
A server that permits anonymous access will announce support for the
ANONYMOUS mechanism and allow anyone to log in using that mechanism,
usually with restricted access.
A formal grammar for the client message using Augmented BNF [ABNF] is
provided below as a tool for understanding this technical
message = [ email / token ]
;; to be prepared in accordance with Section 3
UTF1 = %x00-3F / %x41-7F ;; less '@' (U+0040)
UTF2 = %xC2-DF UTF0
UTF3 = %xE0 %xA0-BF UTF0 / %xE1-EC 2(UTF0) /
%xED %x80-9F UTF0 / %xEE-EF 2(UTF0)
UTF4 = %xF0 %x90-BF 2(UTF0) / %xF1-F3 3(UTF0) /
%xF4 %x80-8F 2(UTF0)
UTF0 = %x80-BF
TCHAR = UTF1 / UTF2 / UTF3 / UTF4
;; any UTF-8 encoded Unicode character
;; except '@' (U+0040)
email = addr-spec
;; as defined in [IMAIL]
token = 1*255TCHAR
Note to implementors:
The <token> production is restricted to 255 UTF-8-encoded Unicode
characters. As the encoding of a characters uses a sequence of 1
to 4 octets, a token may be as long as 1020 octets.
3. The "trace" Profile of "Stringprep"
This section defines the "trace" profile of [StringPrep]. This
profile is designed for use with the SASL ANONYMOUS Mechanism.
Specifically, the client is to prepare the <message> production in
accordance with this profile.
The character repertoire of this profile is Unicode 3.2 [Unicode].
No mapping is required by this profile.
No Unicode normalization is required by this profile.
The list of unassigned code points for this profile is that provided
in Appendix A of [StringPrep]. Unassigned code points are not
Characters from the following tables of [StringPrep] are prohibited:
- C.2.1 (ASCII control characters)
- C.2.2 (Non-ASCII control characters)
- C.3 (Private use characters)
- C.4 (Non-character code points)
- C.5 (Surrogate codes)
- C.6 (Inappropriate for plain text)
- C.8 (Change display properties are deprecated)
- C.9 (Tagging characters)
No additional characters are prohibited.
This profile requires bidirectional character checking per Section 6
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. Additionally, protocols with SASL profiles permitting
an initial client response will be able to avoid the extra round trip
below (the server response with an empty "+ ").
In this example, the trace information 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: A003 OK Welcome, trace information has been logged.
5. Security Considerations
The ANONYMOUS mechanism grants access to services and/or resources by
anyone. For this reason, it should be disabled by default so that
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
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 exchange
information anonymously. Servers that 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 reduce the priority of anonymous
users or limit their resource usage.
While servers may impose a limit on the number of anonymous users,
note that such limits enable denial-of-service attacks and should be
used with caution.
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 investigating
abuse need to realize that this trace information may be falsified.
A client that uses the user's correct email address as trace
information without explicit permission may violate that user's
privacy. Anyone who accesses an anonymous archive on a sensitive
subject (e.g., sexual abuse) likely has strong privacy needs.
Clients should not send the email address without the explicit
permission of the user and should offer the option of supplying no
trace information, 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
that view or alter the data transferred. Clients and servers are
encouraged to support external data security services.
Protocols that fail to require an explicit anonymous login are more
susceptible to break-ins given certain common implementation
techniques. Specifically, Unix servers that 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 that offer restricted data access should not allow
anonymous data access without an explicit login step.
General [SASL] security considerations apply to this mechanism.
[StringPrep] security considerations and [Unicode] security
considerations discussed in [StringPrep] apply to this mechanism.
[UTF-8] security considerations also apply.
6. IANA Considerations
The SASL Mechanism registry [IANA-SASL] entry for the ANONYMOUS
mechanism has been updated by the IANA to reflect that this document
now provides its technical specification.
Subject: Updated Registration of SASL mechanism ANONYMOUS
SASL mechanism name: ANONYMOUS
Security considerations: See RFC 4505.
Published specification (optional, recommended): RFC 4505
Person & email address to contact for further information:
Kurt Zeilenga <Kurt@OpenLDAP.org>
Chris Newman <Chris.Newman@sun.com>
Intended usage: COMMON
Author/Change controller: IESG <firstname.lastname@example.org>
Note: Updates existing entry for ANONYMOUS
The [StringPrep] profile "trace", first defined in this RFC, has been
Subject: Initial Registration of Stringprep "trace" profile
Stringprep profile: trace
Published specification: RFC 4505
Person & email address to contact for further information:
Kurt Zeilenga <email@example.com>
This document is a revision of RFC 2245 by Chris Newman. Portions of
the grammar defined in Section 1 were borrowed from RFC 3629 by
This document is a product of the IETF SASL WG.
8. Normative References
[ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[IMAIL] Resnick, P., "Internet Message Format", RFC 2822, April
[SASL] Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
Authentication and Security Layer (SASL)", RFC 4422,
[StringPrep] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ('stringprep')", RFC 3454,
[Unicode] The Unicode Consortium, "The Unicode Standard, Version
3.2.0" is defined by "The Unicode Standard, Version 3.0"
(Reading, MA, Addison-Wesley, 2000. ISBN 0-201-61633-5),
as amended by the "Unicode Standard Annex #27: Unicode
3.1" (http://www.unicode.org/reports/tr27/) and by the
"Unicode Standard Annex #28: Unicode 3.2"
[UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 3629 (also STD 63), November 2003.
Appendix A. Changes since RFC 2245
This appendix is non-normative.
RFC 2245 allows the client to include optional trace information in
the form of a human readable string. RFC 2245 restricted this string
to US-ASCII. As the Internet is international, this document uses a
string restricted to UTF-8 encoded Unicode characters. A
"stringprep" profile is defined to precisely define which Unicode
characters are allowed in this string. While the string remains
restricted to 255 characters, the encoded length of each character
may now range from 1 to 4 octets.
Additionally, a number of editorial changes were made.
Kurt D. Zeilenga
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