One document matched: draft-ietf-sasl-rfc2222bis-05.txt
Differences from draft-ietf-sasl-rfc2222bis-04.txt
Network Working Group A. Melnikov
Internet Draft Editor
Document: draft-ietf-sasl-rfc2222bis-05.txt January 2004
Obsoletes: RFC 2222 Expires in six months
Simple Authentication and Security Layer (SASL)
Status of this Memo
This document is an Internet Draft and is in full conformance with
all provisions of Section 10 of RFC 2026.
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. Internet Drafts may be updated, replaced, or obsoleted
by other documents at any time. It is not appropriate to use
Internet Drafts as reference material or to cite them other than as
``work in progress''.
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
A revised version of this draft document will be submitted to the RFC
editor as a Proposed Standard for the Internet Community. Discussion
and suggestions for improvement are requested. Distribution of this
draft is unlimited.
When published as an RFC this document will obsolete RFC 2222.
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1. Abstract
The Simple Authentication and Security Layer (SASL) provides a method
for adding authentication support with an optional security layer to
connection-based protocols. It also describes a structure for
authentication mechanisms. The result is an abstraction layer
between protocols and authentication mechanisms such that any SASL-
compatible authentication mechanism can be used with any SASL-
compatible protocol.
This document describes how a SASL authentication mechanism is
structured, describes how a protocol adds support for SASL, defines
the protocol for carrying a security layer over a connection, and
defines the EXTERNAL SASL authentication mechanism.
2. Organization of this document
2.1. How to read this document
This document is written to serve two different audiences, protocol
designers using this specification to support authentication in their
protocol, and implementors of clients or servers for those protocols
using this specification.
The sections "Overview", "Authentication Mechanisms", "Protocol
Profile Requirements", "Specific Issues", and "Security
Considerations" cover issues that protocol designers need to
understand and address in profiling this specification for use in a
specific protocol.
Implementors of a protocol using this specification need the
protocol-specific profiling information in addition to the
information in this document.
2.2. 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].
Character names in this document use the notation for code points and
names from the Unicode Standard [Unicode]. For example, the letter
"a" may be represented as either <U+0061> or <LATIN SMALL LETTER A>.
This document uses terms "integrity protection" and "confidentiality
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protection". The former references to a security layer that is able
to detect data modification by using some kind of hash. However,
integrity protection doesn't make the data unreadable to an attacker.
Confidentiality protection is a security layer that is able to make
the data unreadable to an attacker by using encryption.
Confidentiality protection usually implies integrity protection.
3. Overview
The Simple Authentication and Security Layer (SASL) is a method for
adding authentication support to connection-based protocols.
The SASL specification has three layers, as indicated in the diagram
below. At the top, a protocol definition using SASL specifies a
profile, including a command for identifying and authenticating a
user to a server and for optionally negotiating a security layer for
subsequent protocol interactions. At the bottom, a SASL mechanism
definition specifies an authentication mechanism. The SASL
framework, specified by this document, constrains the behavior of
protocol profiles and mechanisms, separating protocol from mechanism
and defining how they interact.
SMTP Protocol LDAP Protocol Etc
Profile Profile . . .
\----- | -----/
\ | /
SASL framework
/ | \
/----- | -----\
EXTERNAL DIGEST-MD5 Etc
SASL mechanism SASL mechanism . . .
This separation between the definition of protocols and the
definition of authentication mechanisms is crucial. It permits an
authentication mechanism to be defined once, making it usable by any
SASL protocol profile. In many implementations, the same SASL
mechanism code is used for multiple protocols.
4. Authentication mechanisms
SASL mechanisms are named by strings, from 1 to 20 characters in
length, consisting of ASCII [ASCII] upper-case letters, digits,
hyphens, and/or underscores. SASL mechanism names must be registered
with the Internet Assigned Numbers Authority (IANA). IETF standards
track documents may direct the IANA to reserve a portion of the SASL
mechanism namespace and may specify different registration criteria
for the reserved portion; the GSSAPI mechanism specification
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[SASL-GSSAPI] does this. Procedures for registering new SASL
mechanisms are given in section 8.
The "sasl-mech" production below defines the syntax of a SASL
mechanism name. This uses the Augmented Backus-Naur Form (ABNF)
notation as specified in [ABNF] and the ABNF core rules as specified
in Appendix A of the ABNF specification [ABNF].
sasl-mech = 1*20mech-char
mech-char = %x41-5A / DIGIT / "-" / "_"
; mech names restricted to ASCII uppercase letters,
; digits, "-" and "_"
4.1. Authentication protocol exchange
A SASL mechanism is responsible for conducting an authentication
protocol exchange. This consists of a series of server challenges
and client responses, the contents of which are specific to and
defined by the mechanism. To the protocol, the challenges and
responses are opaque binary tokens of arbitrary length. The
protocol's profile then specifies how these binary tokens are then
encoded for transfer over the connection.
After receiving an authentication command or any client response, a
server mechanism may issue a challenge, indicate failure, or indicate
completion. The server mechanism may return additional data with a
completion indication. The protocol's profile specifies how each of
these is then represented over the connection.
After receiving a challenge, a client mechanism may issue a response
or abort the exchange. The protocol's profile specifies how each of
these is then represented over the connection.
During the authentication protocol exchange, the mechanism performs
authentication, transmits an authorization identity (frequently known
as a userid) from the client to server, and negotiates the use of a
mechanism-specific security layer. If the use of a security layer is
agreed upon, then the mechanism must also define or negotiate the
maximum security layer buffer size that each side is able to receive.
4.2. Authorization and authentication identities
SASL authentication deals with two identities: the authorization
identity and the authentication identity. The transmitted
authorization identity may be an empty string (zero length), but the
transmitted authentication identity may not be an empty string.
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While some legacy mechanisms are incapable of transmitting an
authorization identity (which means that for these mechanisms the
authorization identity is always the empty string), newly defined
mechanisms SHOULD be capable of transmiting a non-empty authorization
identity.
Authentication identity is the identity derived from the client's
authentication credentials.
The authorization identity is used by the server as the primary
identity for making access policy decisions.
4.2.1. Authorization identities and proxy authentication
With any mechanism, transmitting an authorization identity of the
empty string directs the server to derive the authorization identity
from the client's authentication identity.
If the authorization identity transmitted during the authentication
protocol exchange is not the empty string, this is typically referred
to as "proxy authentication". This feature permits agents such as
proxy servers to authenticate using their own credentials, yet
request the access privileges of the identity for which they are
proxying.
The server makes an implementation defined policy decision as to
whether the authentication identity is permitted to have the access
privileges of the authorization identity and whether the
authorization identity is permitted to receive service. If it is
not, the server indicates failure of the authentication protocol
exchange.
As a client might not have the same information as the server,
clients SHOULD NOT derive authorization identities from
authentication identities. Instead, clients SHOULD provide no (or
empty) authorization identity when the user has not provided an
authorization identity.
The server SHOULD verify that a received authorization identity is in
the correct form. Profiles whose authorization identities are simple
user names (e.g. IMAP [RFC 3501]) SHOULD use "SASLPrep" profile
[SASLPrep] of the "stringprep" algorithm [StringPrep] to prepare
these names for matching. The profiles MAY use a stringprep profile
that is more strict than "SASLPrep". If the preparation of the
authorization identity fails or results in an empty string, the
server MUST fail the authentication exchange. The only exception to
this rule is when the received authorization identity is already the
empty string.
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4.2.2. Authorization Identity Format
An authorization identity is a string of zero or more Unicode
[Unicode] coded characters. The NUL <U+0000> character is not
permitted in authorization identities.
The character encoding scheme used for transmitting an authorization
identity over the protocol is specified in each authentication
mechanism All IETF-defined mechanisms MUST, and all other mechanisms
SHOULD, use UTF-8 [UTF-8]. (See [CHARSET-POLICY] for IETF policy
regarding character sets and encoding schemes.)
Mechanisms are expected to be capable of carrying the entire Unicode
repertoire (with the exception of the NUL character). An
authorization identity of the empty string and an absent
authorization identity MUST be treated as equivalent. A mechanism
which provides an optional field for an authorization identity,
SHOULD NOT allow that field, when present, to be empty. The meaning
of an authorization identity of the empty string is described in the
previous section.
4.3. Security layers
If use of a security layer is negotiated by the authentication
protocol exchange, the security layer is applied to all subsequent
data sent over the connection (until another security layer is
negotiated; see also section 6.3). The security layer takes effect
immediately following the last response of the authentication
exchange for data sent by the client and the completion indication
for data sent by the server.
Note that all SASL mechanisms that are unable to negotiate a security
layer automatically select no security layer.
Once the security layer is in effect the protocol stream is processed
by the security layer into buffers of security encoded data. Each
buffer of security encoded data is transferred over the connection as
a stream of octets prepended with a four octet field in network byte
order that represents the length of the following buffer. The length
of the security encoded data buffer MUST be no larger than the
maximum size that was either defined in the mechanism specification
or negotiated by the other side during the authentication protocol
exchange. Upon the receipt of a data buffer which is larger than the
defined/negotiated maximal buffer size the receiver SHOULD close the
connection. This might be a sign of an attack or a buggy
implementation.
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4.4. Character string issues
Authentication mechanisms SHOULD encode character strings in UTF-8
[UTF-8] (see [CHARSET-POLICY] for IETF policy regarding character
sets in IETF protocols). In order to avoid interoperability problems
due to differing normalizations, when a mechanisms specifies that
character data is to be used as input to a cryptographic and/or
comparison function, the mechanism specification MUST detail how the
data is to be represented, including any normalizations or other
preparations, to ensure proper function. Designers of mechanisms
SHOULD use the "SASLprep" profile [SASLPrep] of the "stringprep"
algorithm [StringPrep] where applicable.
There are three entities that have to deal with this issue: a client
(upon getting user input or retrieving a value from configuration), a
server (upon receiving the value from the client) and a utility that
is able to store passwords/hashes in a database that can be later
used by the server. SASL mechanisms must define which entity (or
entities) must perform the preparation. If preparation fails or
results in an empty string, the entity doing the preparation SHALL
fail the authentication exchange (or, in case of the utility, refuse
to store the data).
5. Protocol profile requirements
In order to use this specification, a protocol definition MUST supply
the following information:
1) A service name, to be selected from the IANA registry of "service"
elements for the GSSAPI host-based service name form [GSSAPI]. This
service name is made available to the authentication mechanism.
The registry is available at the URL
<http://www.iana.org/assignments/gssapi-service-names>.
2) A definition of the command to initiate the authentication
protocol exchange. This command must have as a parameter the name of
the mechanism being selected by the client.
The command SHOULD have an optional parameter giving an initial
response. This optional parameter allows the client to avoid a round
trip when using a mechanism which is defined to have the client send
data first. When this initial response is sent by the client and the
selected mechanism is defined to have the server start with an
initial challenge, the command fails. See section 6.1 of this
document for further information.
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3) A definition of the method by which the authentication protocol
exchange is carried out, including how the challenges and responses
are encoded, how the server indicates completion or failure of the
exchange, how the client aborts an exchange, and how the exchange
method interacts with any line length limits in the protocol.
The exchange method SHOULD allow the server to include an optional
data ("optional challenge") with a success notification. This allows
the server to avoid a round trip when using a mechanism which is
defined to have the server send additional data along with the
indication of successful completion. See section 6.2 of this
document for further information.
4) A protocol profile SHOULD specify a mechanism through which a
client may obtain the names of the SASL mechanisms available to it.
This is typically done through the protocol's extensions or
capabilities mechanism.
5) Identification of the octet where any negotiated security layer
starts to take effect, in both directions.
6) Specify if the protocol profile supports "multiple
authentications" (see section 6.3).
7) If both TLS and SASL security layer are allowed to be negotiated
by the protocol, the protocol profile MUST define in which order they
are applied to a cleartext data sent over the connection.
8) A protocol profile MAY further refine the definition of an
authorization identity by adding additional syntactic restrictions
and protocol-specific semantics. A protocol profile MUST specify the
form of the authorization identity (since it is protocol specific, as
opposed to the authentication identity, which is mechanism specific)
and how authorization identities are to be compared. Profiles whose
authorization identities are simple user names (e.g. IMAP [RFC 3501])
SHOULD use "SASLPrep" profile [SASLPrep] of the "stringprep"
algorithm [StringPrep] to prepare these names for matching. The
profiles MAY use a stringprep profile that is more strict than
SASLPrep.
A protocol profile SHOULD NOT attempt to amend the definition of
mechanisms or make mechanism-specific encodings. This breaks the
separation between protocol and mechanism that is fundamental to the
design of SASL. Likewise, SASL mechanisms SHOULD be profile neutral.
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6. Specific issues
6.1. Client sends data first
Some mechanisms specify that the first data sent in the
authentication protocol exchange is from the client to the server.
If a protocol's profile permits the command which initiates an
authentication protocol exchange to contain an initial client
response, this parameter SHOULD be used with such mechanisms.
If the initial client response parameter is not given, or if a
protocol's profile does not permit the command which initiates an
authentication protocol exchange to contain an initial client
response, then the server issues a challenge with no data. The
client's response to this challenge is then used as the initial
client response. (The server then proceeds to send the next
challenge, indicates completion, or indicates failure.)
6.1.1. Examples
The following are two examples of an SECURID authentication [SASL-
SECURID] in the SMTP protocol [SMTP]. In the first example below,
the client is trying fast reauthentication by sending the initial
response:
S: 220-smtp.example.com ESMTP Server
C: EHLO client.example.com
S: 250-smtp.example.com Hello client.example.com, pleased to meet you
S: 250-AUTH GSSAPI SECURID
S: 250 DSN
C: AUTH SECURID AG1hZ251cwAxMjM0NTY3OAA=
S: 235 Authentication successful
The example below is almost identical to the previous, but here the
client chooses not to use the initial response parameter.
S: 220-smtp.example.com ESMTP Server
C: EHLO client.example.com
S: 250-smtp.example.com Hello client.example.com, pleased to meet you
S: 250-AUTH GSSAPI SECURID
S: 250 DSN
C: AUTH SECURID
S: 334
C: AG1hZ251cwAxMjM0NTY3OAA=
S: 235 Authentication successful
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Section 7.2 contains an additional example.
6.2. Server returns success with additional data
Some mechanisms may specify that additional data be sent to the
client along with an indication of successful completion of the
exchange. This data would, for example, authenticate the server to
the client.
If a protocol's profile does not permit this additional data to be
returned with a success indication, then the server issues the data
as a server challenge, without an indication of successful
completion. The client then responds with no data. After receiving
this empty response, the server then indicates successful completion
(with no additional data).
Client implementors should be aware of an additional failure case
that might occur when the profile supports sending the additional
data with success. Imagine that an active attacker is trying to
impersonate the server and sends faked data, which should be used to
authenticate the server to the client, with success. (A similar
situation can happen when either the server and/or the client has a
bug and they calculate different responses.) After checking the data,
the client will think that the authentication exchange has failed,
however the server will think that the authentication exchange has
completed successfully. At this point the client can not abort the
authentication exchange; it SHOULD close the connection instead.
However, if the profile did not support sending of additional data
with success, the client could have aborted the exchange at the very
last step of the authentication exchange.
6.2.1. Examples
The following are two examples of a DIGEST-MD5 authentication [SASL-
DIGEST] in the XMPP protocol [XMPP]. In the first example below, the
server is sending mutual authentication data with success.
C: <stream:stream
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
to='example.com'
version='1.0'>
S: <stream:stream
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
id='c2s_234'
from='example.com'
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version='1.0'>
S: <stream:features>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>DIGEST-MD5</mechanism>
<mechanism>CRAM-MD5</mechanism>
</mechanisms>
</stream:features>
C: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='DIGEST-MD5'/>
S: <challenge xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
cmVhbG09InNvbWVyZWFsbSIsbm9uY2U9Ik9BNk1HOXRFUUdtMmhoIixxb3A9ImF1dGgi
LGNoYXJzZXQ9dXRmLTgsYWxnb3JpdGhtPW1kNS1zZXNzCg==
</challenge>
C: <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
dXNlcm5hbWU9InNvbWVub2RlIixyZWFsbT0ic29tZXJlYWxtIixub25jZT0i
T0E2TUc5dEVRR20yaGgiLGNub25jZT0iT0E2TUhYaDZWcVRyUmsiLG5jPTAw
MDAwMDAxLHFvcD1hdXRoLGRpZ2VzdC11cmk9InhtcHAvZXhhbXBsZS5jb20i
LHJlc3BvbnNlPWQzODhkYWQ5MGQ0YmJkNzYwYTE1MjMyMWYyMTQzYWY3LGNo
YXJzZXQ9dXRmLTgK
</response>
S: <success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
cnNwYXV0aD1lYTQwZjYwMzM1YzQyN2I1NTI3Yjg0ZGJhYmNkZmZmZAo=
</success>
The example below is almost identical to the previous, but here
the server chooses not to use the additional data with success.
C: <stream:stream
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
to='example.com'
version='1.0'>
S: <stream:stream
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
id='c2s_234'
from='example.com'
version='1.0'>
S: <stream:features>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>DIGEST-MD5</mechanism>
<mechanism>CRAM-MD5</mechanism>
</mechanisms>
</stream:features>
C: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='DIGEST-MD5'/>
S: <challenge xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
cmVhbG09InNvbWVyZWFsbSIsbm9uY2U9Ik9BNk1HOXRFUUdtMmhoIixxb3A9ImF1dGgi
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LGNoYXJzZXQ9dXRmLTgsYWxnb3JpdGhtPW1kNS1zZXNzCg==
</challenge>
C: <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
dXNlcm5hbWU9InNvbWVub2RlIixyZWFsbT0ic29tZXJlYWxtIixub25jZT0i
T0E2TUc5dEVRR20yaGgiLGNub25jZT0iT0E2TUhYaDZWcVRyUmsiLG5jPTAw
MDAwMDAxLHFvcD1hdXRoLGRpZ2VzdC11cmk9InhtcHAvZXhhbXBsZS5jb20i
LHJlc3BvbnNlPWQzODhkYWQ5MGQ0YmJkNzYwYTE1MjMyMWYyMTQzYWY3LGNo
YXJzZXQ9dXRmLTgK
</response>
S: <challenge xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
cnNwYXV0aD1lYTQwZjYwMzM1YzQyN2I1NTI3Yjg0ZGJhYmNkZmZmZAo=
</challenge>
C: <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
S: <success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
6.3. Multiple authentications
Unless otherwise stated by the protocol's profile, only one
successful SASL negotiation may occur in a protocol session. In this
case, once an authentication protocol exchange has successfully
completed, further attempts to initiate an authentication protocol
exchange fail.
If a profile explicitly permits multiple successful SASL negotiations
to occur, then in no case may multiple security layers be
simultaneously in effect. If a security layer is in effect and a
subsequent SASL negotiation selects a second security layer, then the
second security layer replaces the first. If a security layer is in
effect and a subsequent SASL negotiation selects no security layer,
the original security layer remains in effect.
Note that keeping the original security layer is a subject to a class
of security attack described in section 6.3.1. However, at the time
of the writing of this document the Working Group consensus is not to
change SASL handling of security layers, as the risk of such attacks
is considered to be low and specific to only certain classes of
implementations. The protocol profiles that allow for
reauthentication SHOULD recommend that another security layer is
negotiated once a security layer was installed.
Also note, that if a subsequent authentication fails, the protocol
profile MAY allow the connection state to return to non-
authenticated, however the previously negotiated security layer MUST
NOT be removed. Only a successful reauthentication is able
replace/remove the previously negotiated security layer.
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6.3.1. Description of Multiple Authentication attack
Let's assume that the protected resources on a server are partitioned
into a set of protection spaces, each with its own authentication
mechanisms and/or authorization database. Let's use the term
"partition" to reference any such protected space. An example of a
partition might be an HTTP "realm". Also a proxy/frontend can use
different partitions for different servers/backends it represents.
Now consider the following scenario. A client has already
authenticated and established a security layer with "Partition A"
which is managed by the server AA. Now the same client authenticates
to "Partition B" (managed by the server BB) without negotiating a new
security layer, while the security layer negotiated with "Partition
A" remains in effect. The server BB is now able to observe how known
cleartext is encrypted. This scenario enables the server BB to make
guesses about previously observed ciphertext between the client and
the server AA using the server's SASL engine as an oracle. This
scenario is illustrated below:
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+---------+ +---------+
| | | |
|Partition| |Partition|
| B | | A |
+---------+ +---------+
| ^ |
| : +-----------+ |
Traffic from | : | Encryption| | Traffic from A
B to client +-------->| end point |<-------+ to client
: | (SSL/SASL)|
: +-----------+
: |
: |
: +---+
: | |
: | |
: | | Encryption tunnel, e.g. SASL or SSL,
: | | between the server
(1) Recording +---------:| | and a single client only.
encrypted | | Separate tunnels to different
traffic between | | clients.
Partition A and client +---+
|
|
+-----------> Traffic to clients
<<Some text about trust relationship here.
Where this suitation cannot be managed through trust relationship, it
may be approrpiate for the server implementation to not support
multiple authentications. >>
7. The EXTERNAL mechanism
The mechanism name associated with external authentication is
"EXTERNAL".
The client sends an initial response with the UTF-8 encoding of the
authorization identity. The form of the authorization identity is
further restricted by the application-level protocol's SASL profile.
The server uses information, external to SASL, to determine whether
the client is authorized to authenticate as the authorization
identity. If the client is so authorized, the server indicates
successful completion of the authentication exchange; otherwise the
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server indicates failure.
The system providing this external information may be, for example,
IPSec or TLS. However, the client can make no assumptions as to what
information the server can use in determining client authorization.
E.g., just because TLS was established, doesn't mean that the server
will use the information provided by TLS.
If the client sends the empty string as the authorization identity
(thus requesting that the authorization identity be derived from the
client's authentication credentials), the authorization identity is
to be derived from authentication credentials which exist in the
system that is providing the external authentication.
7.1. Formal syntax
The following syntax specification uses the augmented Backus-Naur
Form (BNF) notation as specified in [ABNF]. This uses the ABNF core
rules as specified in Appendix A of the ABNF specification [ABNF].
Non-terminals referenced but not defined below are as defined by
[UTF-8].
The "extern-init-resp" rule below defines the initial response sent
from client to server.
extern-init-resp = *( UTF8-char-no-nul )
UTF8-char-no-nul = UTF8-1-no-nul / UTF8-2 / UTF8-3 / UTF8-4
UTF8-1-no-nul = %x01-7F
7.2. Example
The following is an example of an EXTERNAL authentication in the SMTP
protocol [SMTP]. In this example, the client is proxy
authenticating, sending the authorization identity "fred" using in
the (optional) initial response. The server has determined the
client's identity through IPsec and has a security policy that
permits that identity to proxy authenticate as any other identity.
To the protocol profile, the four octet sequence "fred" is an opaque
binary data. The SASL protocol profile for SMTP [SMTP-AUTH] specifies
that server challenges and client responses are encoded in BASE64
[BASE64]; the BASE64 encoding of "fred" is "ZnJlZA==".
S: 220 smtp.example.com ESMTP server ready
C: EHLO jgm.example.com
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S: 250-smtp.example.com
S: 250 AUTH DIGEST-MD5 EXTERNAL
C: AUTH EXTERNAL ZnJlZA==
S: 235 Authentication successful.
The following example is almost identical to the one above, but the
client doesn't use proxy authentication.
S: 220 smtp.example.com ESMTP server ready
C: EHLO jgm.example.com
S: 250-smtp.example.com
S: 250 AUTH DIGEST-MD5 EXTERNAL
C: AUTH EXTERNAL
S: 235 Authentication successful.
8. IANA Considerations
8.1. Guidelines for IANA
It is requested that IANA updates the SASL mechanisms registry as
follows:
Change the "Intended usage" of the KERBEROS_V4 and SKEY mechanism
registrations to OBSOLETE. Change the "Published specification"
of the EXTERNAL mechanism to this document. Updated registration
is provided in Section 8.6.
8.2. Registration procedure
Registration of a SASL mechanism is done by filling in the template
in section 8.5 and sending it via electronic mail to <iana@iana.org>.
IANA has the right to reject obviously bogus registrations, but will
perform no review of claims made in the registration form. SASL
mechanism registrations are currently available at the URL
<http://www.iana.org/assignments/sasl-mechanisms>.
There is no naming convention for SASL mechanisms; any name that
conforms to the syntax of a SASL mechanism name can be registered.
An IETF Standards Track document may reserve a portion of the SASL
mechanism namespace ("family of SASL mechanisms") for its own use,
amending the registration rules for that portion of the namespace.
Each family of SASL mechanisms MUST be identified by a prefix.
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While the registration procedures do not require it, authors of SASL
mechanisms are encouraged to seek community review and comment
whenever that is feasible. Authors may seek community review by
posting a specification of their proposed mechanism as an Internet-
Draft. SASL mechanisms intended for widespread use should be
standardized through the normal IETF process, when appropriate.
8.3. Comments on SASL mechanism registrations
Comments on registered SASL mechanisms should first be sent to the
"owner" of the mechanism and/or to the SASL WG mailing list.
Submitters of comments may, after a reasonable attempt to contact the
owner, request IANA to attach their comment to the SASL mechanism
registration itself. If IANA approves of this, the comment will be
made accessible in conjunction with the SASL mechanism registration
itself.
8.4. Change control
Once a SASL mechanism registration has been published by IANA, the
author may request a change to its definition. The change request
follows the same procedure as the registration request.
The owner of a SASL mechanism may pass responsibility for the SASL
mechanism to another person or agency by informing IANA; this can be
done without discussion or review.
The IESG may reassign responsibility for a SASL mechanism. The most
common case of this will be to enable changes to be made to
mechanisms where the author of the registration has died, moved out
of contact or is otherwise unable to make changes that are important
to the community.
SASL mechanism registrations may not be deleted; mechanisms which are
no longer believed appropriate for use can be declared OBSOLETE by a
change to their "intended use" field; such SASL mechanisms will be
clearly marked in the lists published by IANA.
The IESG is considered to be the owner of all SASL mechanisms which
are on the IETF standards track.
8.5. Registration template
Subject: Registration of SASL mechanism X
Family of SASL mechanisms: (YES or NO)
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SASL mechanism name (or prefix for the family):
Security considerations:
Published specification (optional, recommended):
Person & email address to contact for further information:
Intended usage:
(One of COMMON, LIMITED USE or OBSOLETE)
Owner/Change controller:
(Any other information that the author deems interesting may be
added below this line.)
8.6. The EXTERNAL mechanism registration
It is requested that the SASL Mechanism registry [IANA-SASL] entry
for the EXTERNAL mechanism be updated to reflect that this document
now provides its technical specification.
Subject: Updated Registration of SASL mechanism EXTERNAL
Family of SASL mechanisms: NO
SASL mechanism name: EXTERNAL
Security considerations: See RFC XXXX, section 9.
Published specification (optional, recommended): RFC XXXX
Person & email address to contact for further information:
Alexey Melnikov <Alexey.Melnikov@isode.com>
Intended usage: COMMON
Owner/Change controller: IESG <iesg@ietf.org>
Note: Updates existing entry for EXTERNAL
9. Security considerations
Security issues are discussed throughout this memo.
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SASL implementations might be subject to online dictionary attacks
(e.g. username harvesting, password cracking). In order to mitigate
the attacks a server implementation is encouraged to implement a
policy when after a number of failed authentication attempts it
returns errors to all subsequent authentication attempts on the same
connection. Alternatively, the server may implement a policy whereby
the connection is dropped after a number of failed authentication
attempts. If a server implementation chooses to do so, it should not
drop the connection until at least 3 authentication attempts have
failed.
The mechanisms that support integrity protection are designed such
that the negotiation of the security layer and authorization identity
is integrity protected. When the client selects a security layer
with at least integrity protection, this protects against an active
attacker hijacking the connection and modifying the authentication
exchange to negotiate a plaintext connection.
When a server or client supports multiple authentication mechanisms,
each of which has a different security strength, it is possible for
an active attacker to cause a party to use the least secure mechanism
supported. To protect against this sort of attack, a client or
server which supports mechanisms of different strengths should have a
configurable minimum strength that it will use. It is not sufficient
for this minimum strength check to only be on the server, since an
active attacker can change which mechanisms the client sees as being
supported, causing the client to send authentication credentials for
its weakest supported mechanism.
The client's selection of a SASL mechanism is done in the clear and
may be modified by an active attacker. It is important for any new
SASL mechanisms to be designed such that an active attacker cannot
obtain an authentication with weaker security properties by modifying
the SASL mechanism name and/or the challenges and responses.
In order to detect Man-in-the-middle (MITM) attacks the client MAY
list available SASL mechanisms both before and after the SASL
security layer is negotiated. This allows the client to detect
active attacks that remove mechanisms from the server's list of
supported mechanisms, and allows the client to ensure that it is
using the best mechanism supported by both client and server. New
protocol profiles SHOULd require servers to make the list of SASL
mechanisms available for the initial authentication available to the
client after security layers are established. Some older protocols
do not require this (or don't support listing of SASL mechanisms once
authentication is complete); for these protocols clients MUST NOT
treat an empty list of SASL mechanisms after authentication as a MITM
attack.
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Any protocol interactions prior to authentication are performed in
the clear and may be modified by an active attacker. In the case
where a client selects integrity protection, it is important that any
security-sensitive protocol negotiations be performed after
authentication is complete. Protocols should be designed such that
negotiations performed prior to authentication should be either
ignored or revalidated once authentication is complete.
When use of a security layer is negotiated by the authentication
protocol exchange, the receiver should handle gracefully any security
encoded data buffer larger than the defined/negotiated maximal size.
In particular, it must not blindly allocate the amount of memory
specified in the buffer size field, as this might cause the "out of
memory" condition. If the receiver detects a large block, it SHOULD
close the connection.
Distributed server implementations need to be careful in how they
trust other parties and, in particular, authentication secrets should
only be disclosed to other parties that are trusted to manage and use
those secrets in manner acceptable to disclosing party. It should be
noted that where those secrets are used to providing data
confidentiality protections, if a third party (other then the
discloser/declosee) has knowledge of some portion of the protected
information, it can use this knowledge in an attack upon other
portions of the protected information.
Section 6.3.1 contains a description of a potential class of attack
on a distributed server implementation. The section also gives some
recommendations about mitigating such attacks.
"stringprep" and Unicode security considerations apply to
authentication identities, authorization identities and passwords.
The EXTERNAL mechanism provides no security protection; it is
vulnerable to spoofing by either client or server, active attack, and
eavesdropping. It should only be used when external security
mechanisms are present and have sufficient strength.
10. References
10.1. Normative References
[ABNF] Crocker, D. (Ed.), Overell, P., "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997
[ASCII] American National Standards Institute, "Code Extension
Techniques for Use with the 7-bit Coded Character Set of American
National Standard Code (ASCII) for Information Interchange", FIPS PUB
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35, 1974
[CHARSET-POLICY] Alvestrand, H., "IETF Policy on Character Sets and
Languages", RFC 2277, BCP 18, January 1998
[GSSAPI] Linn, J., "Generic Security Service Application Program
Interface, Version 2, Update 1", RFC 2743, January 2000
[ISO-10646] "Universal Multiple-Octet Coded Character Set (UCS) -
Architecture and Basic Multilingual Plane", ISO/IEC 10646-1 : 1993.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, BCP 19, March 1997
[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" (http://www.unicode.org/reports/tr28/).
[Stringprep] Hoffman, P., Blanchet, M., "Preparation of
Internationalized Strings ("stringprep")", RFC 3454, December 2002.
[SASLPrep] Zeilenga, K., "SASLprep: Stringprep profile for user names
and passwords", Work in progress, draft-ietf-sasl-saslprep-XX.txt.
[UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO 10646",
RFC 3629, STD 63, November 2003.
10.2. Informative References
[SASL-GSSAPI] Melnikov, A., "SASL GSSAPI mechanisms", work in
progress, draft-ietf-sasl-gssapi-XX.txt, November 2003
[SASL-DIGEST] Leach, P., Newman, C., Melnikov, A., "Using Digest
Authentication as a SASL Mechanism", work in progress, draft-ietf-
sasl-rfc2831bis-XX.txt, replaces RFC 2831
[SASL-OTP] Newman, C., "The One-Time-Password SASL Mechanism", RFC
2444, October 1998.
[SASL-SECURID] Nystrom, M., "The SecurID(r) SASL Mechanism", RFC
2808, April 2000.
[SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, April
2001.
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[SMTP-AUTH] Myers, J., "SMTP Service Extension for Authentication",
RFC 2554, March 1999.
Being revised by Siemborski, R., "SMTP Service Extension for
Authentication", work in progress, draft-siemborski-rfc2554bis-
XX.txt.
[XMPP] Saint-Andre, P., "Extensible Messaging and Presence Protocol
(XMPP): Core", work in progress, draft-ietf-xmpp-core-XX.txt.
[BASE64] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 3548, July 2003.
[RFC-INSTRUCTIONS] Postel, J., Reynolds, J., "Instructions to RFC
Authors", RFC 2223, October 1997.
[IANA-SASL] IANA, "SIMPLE AUTHENTICATION AND SECURITY LAYER (SASL)
MECHANISMS", http://www.iana.org/assignments/sasl-mechanisms.
11. Editor's Address
Alexey Melnikov
Isode Limited
Email: Alexey.Melnikov@isode.com
12. Acknowledgments
This document is a revision of RFC 2222 written by John G. Myers. He
also contributed significantly to this revision.
Magnus Nystrom provided the ASCII art used in Section 6.3.
Definition of partition was extracted from RFC 2617 ("HTTP
Authentication: Basic and Digest Access Authentication").
Contributions of many members of the SASL mailing list are gratefully
acknowledged, in particular Kurt D. Zeilenga, Peter Saint-Andre, Rob
Siemborski, Jeffrey Hutzelman, Hallvard B Furuseth and Tony Hansen
for proofreading the document and various editorial suggestions.
13. Full Copyright Statement
Copyright (C) The Internet Society (2004). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
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or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Appendix A. Relation of SASL to transport security
Questions have been raised about the relationship between SASL and
various services (such as IPsec and TLS) which provide a secured
connection.
Two of the key features of SASL are:
The separation of the authorization identity from the identity in
the client's credentials. This permits agents such as proxy
servers to authenticate using their own credentials, yet request
the access privileges of the identity for which they are proxying.
Upon successful completion of an authentication exchange, the
server knows the authorization identity the client wishes to use.
This allows servers to move to a "user is authenticated" state in
the protocol.
These features are extremely important to some application protocols,
yet Transport Security services do not always provide them. To
define SASL mechanisms based on these services would be a very messy
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task, as the framing of these services would be redundant with the
framing of SASL and some method of providing these important SASL
features would have to be devised.
Sometimes it is desired to enable within an existing connection the
use of a security service which does not fit the SASL model. (TLS is
an example of such a service.) This can be done by adding a command,
for example "STARTTLS", to the protocol. Such a command is outside
the scope of SASL, and should be different from the command which
starts a SASL authentication protocol exchange.
In certain situations, it is reasonable to use SASL underneath one of
these Transport Security services. The transport service would
secure the connection, either service would authenticate the client,
and SASL would negotiate the authorization identity. The SASL
negotiation would be what moves the protocol from "unauthenticated"
to "authenticated" state. The "EXTERNAL" SASL mechanism is
explicitly intended to handle the case where the transport service
secures the connection and authenticates the client and SASL
negotiates the authorization identity.
Appendix B. Changes since RFC 2222
The GSSAPI mechanism was removed. It is now specified in a separate
document [SASL-GSSAPI].
The "KERBEROS_V4" mechanism defined in RFC 2222 is obsolete and has
been removed.
The "SKEY" mechanism described in RFC 2222 is obsolete and has been
removed. It has been replaced by the OTP mechanism [SASL-OTP].
The overview has been substantially reorganized and clarified.
Clarified the definition and semantics of the authorization identity.
Prohibited the NUL character in authorization identities.
Added a section on character string issues.
The word "must" in the first paragraph of the "Protocol profile
requirements" section was changed to "MUST".
Specified that protocol profiles SHOULD provide a way for clients to
discover available SASL mechanisms.
Made the requirement that protocol profiles specify the semantics of
the authorization identity optional to the protocol profile.
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Clarified that such a specification is a refinement of the definition
in the base SASL spec.
Added a requirement discouraging protocol profiles from breaking the
separation between protocol and mechanism.
Mentioned that standards track documents may carve out their own
portions of the SASL mechanism namespace and may amend registration
rules for the portion. However registration of individual SASL
mechanisms is still required.
Specified that the authorization identity in the EXTERNAL mechanism
is encoded in UTF-8.
Added a statement that a protocol profile SHOULD allow challenge data
to be sent with a success indication.
Added a security consideration for the EXTERNAL mechansim.
Clarified sections concerning success with additional data.
Cleaned up IANA considerations/registrations and assembled them in
one place.
Updated references and split them into Informative and Normative.
Added text to the Security Considerations section regarding handling
of extremely large SASL blocks.
Replaced UTF-8 ABNF with the reference to the UTF-8 document.
Added text about SASLPrep for authentication identities and
passwords. Described where SASLPrep preparation should take place.
Added paragraph about verifying authorization identities.
Added a protocol profile requirement to specify interaction between
SASL and TLS security layers.
Added a protocol profile requirement to specify if it supports
reauthentication.
Removed the text that seemed to suggest that SASL security layer must
not be used when TLS is available.
Created two subsections in 4.2 to talk separately about proxy
authorization and format of the authorization identities.
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Made requirement to verify that an authorization identity is correct
by performing SASLPrep a SHOULD, instead of a MUST.
Clarified that each SASL mechanism must decide where SASLPrep is
taking place.
Added 4 new examples for initial response and additional data with
success.
Added text on checking the list of available SASL mechanisms after
negotiating a security layer.
Added definition of "integrity protection" and "confidentiality
protection".
Added text about preventing password cracking/username harvesting
attacks.
Added warning about negotiating no layer once a security layer is
negotiated.
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Status of this Memo .......................................... i
1. Abstract ............................................... 2
2. Organization of this document .......................... 2
2.1. How to read this document .............................. 2
2.2. Conventions used in this document ...................... 2
3. Overview ............................................... 3
4. Authentication mechanisms .............................. 3
4.1. Authentication protocol exchange ....................... 4
4.2. Authorization and authentication identities ............ 4
4.2.1. Authorization identities and proxy authentication .... 5
4.2.2. Authorization Identity Format ........................ 6
4.3. Security layers ........................................ 6
4.4. Character string issues ................................ 7
5. Protocol profile requirements .......................... 7
6. Specific issues ........................................ 9
6.1. Client sends data first ................................ 9
6.1.1. Examples ............................................. 9
6.2. Server returns success with additional data ........... 10
6.2.1. Examples ............................................ 10
6.3. Multiple authentications .............................. 12
6.3.1. Description of Multiple Authentication attack ....... 13
7. The EXTERNAL mechanism ................................ 14
7.1. Formal syntax ......................................... 15
7.2. Example ............................................... 15
8. IANA Considerations ................................... 15
8.1. Guidelines for IANA ................................... 16
8.2. Registration procedure ................................ 16
8.3. Comments on SASL mechanism registrations .............. 16
8.4. Change control ........................................ 17
8.5. Registration template ................................. 17
8.6. The EXTERNAL mechanism registration ................... 18
9. Security considerations ................................ 18
10. References ........................................... 20
10.1. Normative References ................................. 20
10.2. Informative References ............................... 21
11. Editor's Address ...................................... 21
12. Acknowledgments ....................................... 22
13. Full Copyright Statement .............................. 22
Appendix A. Relation of SASL to transport security .......... 23
Appendix B. Changes since RFC 2222 .......................... 24
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