WDIFF

draft-ietf-ldapbis-authmeth-01.txt --> draft-ietf-ldapbis-authmeth-02.txt

Individual Submission R. Harrison, Editor
Internet Draft Novell, Inc.
Document: draft-ietf-ldapbis-authmeth-01.txt July 19, draft-ietf-ldapbis-authmeth-02.txt November 14, 2001
Intended Category: Draft Standard
Obsoletes: RFC 2829, RFC 2830

Authentication Methods
and
Connection Level Security Mechanisms
for LDAPv3

Status of this 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 Standard Track document.
Distribution of this memo is unlimited. Technical discussion of
this document will take place on the IETF LDAP Extension Working
Group mailing list <ietf-ldapbis@OpenLDAP.org>. Please send
editorial comments directly to the author
<roger_harrison@novell.com>.

Internet-Drafts are working documents of the Internet Engineering
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Abstract

This document describes LDAPv3 authentication methods and connection
level security mechanisms that are required of all conforming LDAPv3
server implementations and makes recommendations for combinations of
these mechanisms to be used in various deployment circumstances.

Among the mechanisms described are

- the LDAPv3 Bind operation used for authenticating LDAP clients
to LDAP servers.

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- the Start TLS operation used to initiate Transport Layer
Security on an established connection between an LDAP client and
server.

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- various forms of authentication including anonymous
authentication, password-based authentication, and certificate
based authentication.

1. Conventions Used in this Document

In
1.1. Glossary of Terms

The following terms are used in this document, document. To aid the term reader,
these terms are defined here.

- "user" represents any application which is an LDAP client using
the directory to retrieve or store information.

Several

- "LDAP association" is used to distinguish the LDAP-level
connection from any underlying TLS-level connection that may or
may not exist.

1.2. Security Terms and Concepts

In general, security terms in this document are used consistently
with the definitions provided in [RFC2828]. In addition, several
terms and concepts relating to authentication security, authentication, and
authorization are presented in Appendix B of this document. While
the formal definition of these terms and concepts is outside the
scope of this document, an understanding of them is prerequisite to
understanding much of the material in this document. Readers who are
unfamiliar with security-related concepts are encouraged to review
Appendix B before reading the remainder of this document.

1.3. Keywords

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119
[ReqsKeywords].

2. Introduction

LDAPv3 is a powerful access protocol for directories. It offers
means of searching, fetching and manipulating directory content, and
ways to access a rich set of security functions.

It is vital that these security functions be interoperable among all
LDAP clients and servers on the Internet; therefore there has to be
a minimum subset of security functions that is common to all
implementations that claim LDAPv3 conformance.

Basic threats to an LDAP directory service include:

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(1) Unauthorized access to directory data via data-fetching
operations,

(2) Unauthorized access to reusable client authentication
information by monitoring others' access,

(3) Unauthorized access to directory data by monitoring others'
access,

(4) Unauthorized modification of directory data,

(5) Unauthorized modification of configuration information,

(6) Unauthorized or excessive use of resources (denial of service),
and

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(7) Spoofing of directory: Tricking a client into believing that
information came from the directory when in fact it did not,
either by modifying data in transit or misdirecting the client's
connection.

Threats (1), (4), (5) and (6) are due to hostile clients. Threats
(2), (3) and (7) are due to hostile agents on the path between
client and server, server or hostile agents posing as a server.

The LDAP protocol suite can be protected with the following security
mechanisms:

(1) Client authentication by means of the SASL [SASL] mechanism set,
possibly backed by the TLS [TLS] credentials exchange mechanism,

(2) Client authorization by means of access control based on the
requestor's authenticated identity,

(3) Data integrity protection by means of the TLS protocol or data-
integrity SASL mechanisms,
mechanisms that provide data integrity services,

(4) Protection Data confidentiality protection against snooping by means of the
TLS protocol or
data-encrypting SASL mechanisms, mechanisms that provide data
confidentiality services,

(5) Resource Server resource usage limitation by means of administrative
service limits configured on
service controls, the server, and

(6) Server authentication by means of the TLS protocol or SASL
mechanism.

At the moment, imposition of access controls is done by means
outside the scope of the LDAP protocol.

3. Required Security Mechanisms

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It is clear that allowing any implementation, faced with the above
requirements, to pick and choose among the possible alternatives is
not a strategy that is likely to lead to interoperability. In the
absence of mandates, clients will be written that do not support any
security function supported by the server, or worse, support only
mechanisms like cleartext passwords that provide clearly inadequate
security.

Active intermediary attacks are the most difficult for an attacker
to perform, and for an implementation to protect against. Methods
that protect only against hostile client and passive eavesdropping
attacks are useful in situations where the cost of protection
against active intermediary attacks is not justified based on the
perceived risk of active intermediary attacks.

Given the presence of the Directory, there is a strong desire to see
mechanisms where identities take the form of an LDAP distinguished
name [LDAPDN] and authentication data can be stored in the
directory; this

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authentication (like the Unix "/etc/passwd" file format used to be),
or its content is never passed across the wire unprotected - that
is, it's either updated outside the protocol or it is only updated
in sessions well protected against snooping. It is also desirable to
allow authentication methods to carry authorization identities based
on existing forms of user identities for backwards compatibility
with non-LDAP-based authentication services.

Therefore, the following implementation conformance requirements are
in place:

(1) For a read-only, public directory, anonymous authentication,
described in section 5, 7, can be used.

(2) Implementations providing password-based authenticated access
MUST support authentication using the DIGEST-MD5 SASL mechanism
[4], as described in section 6.2. 8.2. This provides client
authentication with protection against passive eavesdropping
attacks, but does not provide protection against active
intermediary attacks.

(3) For a directory needing session protection data security (both data integrity and
data confidentiality) and authentication, the Start TLS
operation described in section 5, and either the simple
authentication choice or the SASL EXTERNAL mechanism, are to be
used together. Implementations SHOULD support authentication
with a password as described in section 7.2, 8.3, and SHOULD support
authentication with a certificate as described in section 8.1. 9.1.
Together, these can provide integrity and disclosure protection
of transmitted data, and authentication of client and server,
including protection against active intermediary attacks.

If TLS is negotiated, the client MUST discard all information about
the server fetched prior to the initiation of the TLS negotiation.

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In particular, the value of supportedSASLMechanisms MAY be different
after TLS has been negotiated (specifically, the EXTERNAL mechanism
or the proposed PLAIN mechanism are likely to only be listed after a
TLS negotiation has been performed).

If a SASL security layer is negotiated, the client MUST discard all
information about the server fetched prior to SASL. In particular,
if the initiation of the
SASL negotiation. If the client is configured to support multiple
SASL mechanisms, it SHOULD fetch supportedSASLMechanisms the supportedSASLmechanisms list
both before and after the SASL security layer is negotiated and verify that the value has not
changed after the SASL security layer was negotiated. This detects
allows the client to detect active attacks which that remove supported
SASL mechanisms from the supportedSASLMechanisms list, list and allows the
client to ensure that it is using the best mechanism supported by
both client and server
(additionally, this server. (This requirement is a SHOULD to allow for
environments where the
supported SASL mechanisms supportedSASLMechanisms list is provided to
the client through a different trusted source, e.g. as part of a
digitally signed
object).

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Appendix A contains example deployment scenarios that list the
mechanisms that might be used to achieve a reasonable level of
security in various circumstances.

4. Bind Operation

The Bind operation allows authentication information to be exchanged
between the client and server.

4.1

4.1. Bind Request

The Bind Request is defined in section 4.2 of [LDAPv3] as follows:

BindRequest ::= [APPLICATION 0] SEQUENCE {
version INTEGER (1 .. 127),
name LDAPDN,
authentication AuthenticationChoice }

AuthenticationChoice ::= CHOICE {
simple [0] OCTET STRING,
-- 1 and 2 reserved
sasl [ReqsKeywords] SaslCredentials }

SaslCredentials ::= SEQUENCE {
mechanism LDAPString,
credentials OCTET STRING OPTIONAL }

Parameters of the Bind Request are:

- version: A version number indicating the version of the protocol
to be used in this protocol session. This document describes
version 3 of the LDAP protocol. Note that there is no version
negotiation, and the client just sets this parameter to the
version it desires. If the client requests protocol version 2, a

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server that supports the version 2 protocol as described in
[RFC1777] will not return any v3-specific protocol fields. (Note
that not all LDAP servers will support protocol version 2, since
they may be unable to generate the attribute syntaxes associated
with version 2.)

- name: The name of the directory object that the client wishes to
bind as. This field may take on a null value (a zero length
string) for the purposes of anonymous binds, when authentication
has been performed at a lower layer, or when using SASL
credentials with a mechanism that includes the name in the
credentials. Server behavior is undefined when the name is a
null value, simple authentication is used, and a password is
specified. Note that the server SHOULD NOT perform any alias
dereferencing in determining the object to bind as.

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- authentication: information used to authenticate the name, if
any, provided in the Bind Request.

Upon receipt of a Bind Request, a protocol server will authenticate
the requesting client, if necessary. The server will then return a
Bind Response to the client indicating the status of the
authentication.

Authorization is the use of this authentication information when
performing operations. Authorization MAY be affected by factors
outside of the LDAP Bind request, such as lower layer security
services.

4.2. Bind Response

The Bind Response is defined in section 4.2 of [LDAPv3] as follows.

BindResponse ::= [APPLICATION 1] SEQUENCE {
COMPONENTS OF LDAPResult,
serverSaslCreds [ABNF] OCTET STRING OPTIONAL }

BindResponse consists simply of an indication from the server of the
status of the client's request for authentication.

If the bind was successful, the resultCode will be success.
Otherwise it will be one of:

- operationsError: server encountered an internal error.

- protocolError: unrecognized version number or incorrect PDU
structure.

- authMethodNotSupported: unrecognized SASL mechanism name.

- strongAuthRequired: the server requires authentication be
performed with a SASL mechanism.

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- referral: this server cannot accept this bind and the client
should try another.

- saslBindInProgress: the server requires the client to send a new
bind request, with the same sasl mechanism, to continue the
authentication process.

- inappropriateAuthentication: the server requires the client
which had attempted to bind anonymously or without supplying
credentials to provide some form of credentials.

- invalidCredentials: the wrong password was supplied or the SASL
credentials could not be processed.

- unavailable: the server is shutting down.

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If the server does not support the client's requested protocol
version it MUST set the resultCode to protocolError.

If the client receives a BindResponse response where the resultCode
was protocolError it MUST close the connection as the server will be
unwilling to accept further operations. (This is for compatibility
with earlier versions of LDAP, in which the bind was always the
first operation and there was no negotiation.)

The serverSaslCreds are used as part of a SASL-defined bind
mechanism to allow the client to authenticate the server to which it
is communicating, or to perform "challenge-response" authentication.
If the client bound with the password choice, or the SASL mechanism
does not require the server to return information to the client,
then this field is not to be included in the result.

4.3. Sequencing of the Bind Operation

4.3.1. Effect of Multiple Bind Requests

Subsequent to sending a bind request, A client MAY send a bind
request to change its identity. Such a bind request has the effect
of abandoning all operations outstanding on the connection. (This
simplifies server implementation.) Authentication from earlier binds
are subsequently ignored, and so if the bind fails, the connection
will be treated as anonymous (see section 4.3.3). If a SASL transfer
encryption or integrity mechanism has been negotiated, and that
mechanism does not support the changing of credentials from one
identity to another, then the client MUST instead establish a new
connection.

For some SASL authentication mechanisms, it may be necessary for the
client to invoke the BindRequest multiple times. If at any stage the
client wishes to abort the bind process it MAY unbind and then drop
the underlying connection. Clients MUST NOT invoke operations
between two Bind requests made as part of a multi-stage bind.

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4.3.2. Aborting SASL Bind Negotiation

A client may abort a SASL bind negotiation by sending a BindRequest
with a different value in the mechanism field of SaslCredentials, or
an AuthenticationChoice other than sasl.

If the client sends a BindRequest with the sasl mechanism field as
an empty string, the server MUST return a BindResponse with
authMethodNotSupported as the resultCode. This will allow clients to
abort a negotiation if it wishes to try again with the same SASL
mechanism.

4.3.3. Unbound Connection Treated as Anonymous

Unlike LDAP v2, the client need not send a Bind Request in the first
PDU of the connection. The client may request any operations and the

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server MUST treat these as anonymous. If the server requires that
the client bind before browsing or modifying the directory, the
server MAY reject a request other than binding, unbinding or an
extended request with the "operationsError" result.

If the client did not bind before sending a request and receives an
operationsError, it may then send a Bind Request. If this also fails
or the client chooses not to bind on the existing connection, it
will close the connection, reopen it and begin again by first
sending a PDU with a Bind Request. This will aid in interoperating
with servers implementing other versions of LDAP.

4.4. Using SASL for Other Security Services

The simple authentication option provides minimal authentication
facilities, with the contents of the authentication field consisting
only of a cleartext password. Note that the use of cleartext
passwords is not recommended over open networks when the underlying
transport service cannot guarantee confidentiality; see the
"Security Considerations" section.

The sasl choice allows for any mechanism defined for use with SASL
[RFC2222].
[RFC2222] not specifically prohibited by this document (see section
4.4.1). The mechanism field contains the name of the mechanism. The
credentials field contains the arbitrary data used for
authentication, inside an OCTET STRING wrapper. Note that unlike
some Internet application protocols where SASL is used, LDAP is not
text-based, thus no base64 Base64 transformations are performed on the
credentials.

If any SASL-based integrity or confidentiality services are enabled,
they take effect following the transmission by the server and
reception by the client of the final BindResponse with resultCode
success.

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The client can request that the server use authentication
information from a lower layer protocol by using the SASL EXTERNAL
mechanism.
mechanism (see section 5.5.1.2).

4.4.1. Use of ANONYMOUS and PLAIN SASL Mechanisms

As LDAP includes native anonymous and plaintext authentication
methods, the "ANONYMOUS" and "PLAIN" SASL mechanisms are not used
with LDAP. If an authorization identity of a form different from a
DN is requested by the client, a mechanism that protects the
password in transit SHOULD be used.

4.4.2. Use of EXTERNAL SASL Mechanism

The "EXTERNAL" SASL mechanism can be used to request the LDAP server
make use of security credentials exchanged by a lower layer. If a
TLS session has not been established between the client and server
prior to making the SASL EXTERNAL Bind request and there is no other
external source of authentication credentials (e.g. IP-level

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security [RFC2401]), or if, during the process of establishing the
TLS session, the server did not request the client's authentication
credentials, the SASL EXTERNAL bind MUST fail with a result code of
inappropriateAuthentication. Any client authentication and
authorization state of the LDAP association is lost, so the LDAP
association is in an anonymous state after the failure.

4.4.3. SASL Mechanisms not Considered in this Document

The following SASL-based mechanisms are not considered in this
document: KERBEROS_V4, GSSAPI and SKEY.

4.5. SASL Authorization Identity

The authorization identity is carried as part of the SASL
credentials field in the LDAP Bind request and response.

When the "EXTERNAL" SASL mechanism is being negotiated, if the
credentials field is present, it contains an authorization identity
of the authzId form described below.

Other mechanisms define the location of the authorization identity
in the credentials field.

4.5.1. Authorization Identity Syntax

The authorization identity is a string in the UTF-8 character set,
corresponding to the following ABNF grammar [ABNF]:

; Specific predefined authorization (authz) id schemes are
; defined below -- new schemes may be defined in the future.

authzId = dnAuthzId / uAuthzId

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; distinguished-name-based authz id.
dnAuthzId = "dn:" dn
dn = utf8string ; with syntax defined in RFC 2253 [LDAPDN] section 3.

; unspecified authorization id, UTF-8 encoded.
uAuthzId = "u:" userid
userid = utf8string ; syntax unspecified

4.5.1.1. DN-based Authorization Identity

All servers that support the storage of authentication credentials,
such as passwords or certificates, in the directory MUST support the
dnAuthzId choice. The format for distinguishedName is defined in
Section
section 3 of draft-zeilenga-ldapbis-rfc2253-01.txt. [LDAPDN].

4.5.1.2. Unspecified Authorization Identity

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The uAuthzId choice allows for compatibility with client
applications that wish to authenticate to a local directory but do
not know their own distinguished name or that do not have a
directory entry. The format of utf8string is defined as only a
sequence of UTF-8 encoded ISO 10646 characters, and further
interpretation is subject to prior agreement between the client and
server.

For example, the userid could identify a user of a specific
directory service, or be a login name or the local-part of an RFC
822 email address. In general a uAuthzId MUST NOT be assumed to be
globally unique.

Additional authorization identity schemes MAY be defined in future
versions of this document.

4.6. SASL Service Name for LDAP

For use with SASL [SASL], a protocol must specify a service name to
be used with various SASL mechanisms, such as GSSAPI. For LDAP, the
service name is "ldap", which has been registered with the IANA as a
GSSAPI service name.

4.7. SASL Integrity and Privacy Protections

Any negotiated SASL integrity and privacy protections SHALL start on
the first octet of the first LDAP PDU following successful
completion of the SASL bind operation. If lower level security layer
is negotiated, such as TLS, any SASL security services SHALL be
layered on top of such security layers regardless of the order of
their negotiation.

5. Start TLS Operation

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The Start Transport Layer Security (StartTLS) operation provides the
ability to establish Transport Layer Security [TLS] on an LDAP
association.

5.1. Start TLS Request

A client requests TLS establishment by transmitting a Start TLS
request PDU to the server. The Start TLS request is defined in terms
of the [LDAPv3] ExtendedRequest as follows:

ExtendedRequest ::= [APPLICATION 23] SEQUENCE {
requestName [0] LDAPOID,
requestValue [LDAPv3] OCTET STRING OPTIONAL }

The requestName portion of the Start TLS request MUST be the OID
"1.3.6.1.4.1.1466.20037".

The requestValue field is absent.

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The client MUST NOT send any PDUs on this connection following this
request until it receives a Start TLS extended response.

5.2. Start TLS Response

When a Start TLS request is made, the server MUST return a Start TLS
response PDU to the requestor. The Start TLS response id defined in
terms of the [LDAPv3] ExtendedResponse as follows:

ExtendedResponse ::= [APPLICATION 24] SEQUENCE {
COMPONENTS OF LDAPResult,
responseName [10] LDAPOID OPTIONAL,
response [11] OCTET STRING OPTIONAL }

The responseName portion of the Start TLS response MUST be the OID
"1.3.6.1.4.1.1466.20037". (Note that this OID is the same OID value
used in the requestName of the Start TLS request.)

The response field is absent.

The server MUST set the resultCode field to either success or one of
the other values outlined in section 5.2.2.

5.2.1. "Success" Response

If the ExtendedResponse contains a resultCode of success, this
indicates that the server is willing and able to negotiate TLS.
Refer to section 3, 5.3, below, for details.

5.2.2. Response other than "success"

If the ExtendedResponse contains a resultCode other than success,
this indicates that the server is unwilling or unable to negotiate
TLS.

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If the Start TLS extended request was not successful, the resultCode
will be one of:

operationsError (operations sequencing incorrect; e.g. TLS already
established)

protocolError (TLS not supported or incorrect PDU structure)

referral (this server doesn't do TLS, try this one)

unavailable (e.g. some major problem with TLS, or server is
shutting down)

The server MUST return operationsError if the client violates any of
the Start TLS extended operation sequencing requirements described
in section 5.3, below.

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If the server does not support TLS (whether by design or by current
configuration), it MUST set the resultCode to protocolError (see
section 4.1.1 of [LDAPv3]), or to referral. The server MUST include
an actual referral value in the LDAP Result if it returns a
resultCode of referral. The client's current session is unaffected
if the server does not support TLS. The client MAY proceed with any
LDAP operation, or it MAY close the connection.

The server MUST return unavailable if it supports TLS but cannot
establish a TLS connection for some reason, e.g. the certificate
server not responding, it cannot contact its TLS implementation, or
if the server is in process of shutting down. The client MAY retry
the StartTLS operation, or it MAY proceed with any other LDAP
operation, or it MAY close the connection.

5.3. Sequencing of the Start TLS Operation

This section describes the overall procedures clients and servers
MUST follow for TLS establishment. These procedures take into
consideration various aspects of the overall security of the LDAP
association including discovery of resultant security level and
assertion of the client's authorization identity.

Note that the precise effects, on a client's authorization identity,
of establishing TLS on an LDAP association are described in detail
in section 5.5.

5.3.1. Requesting to Start TLS on an LDAP Association

The client MAY send the Start TLS extended request at any time after
establishing an LDAP association, except that in the following cases
the client MUST NOT send a Start TLS extended request:

- if TLS is currently established on the connection, or
- during a multi-stage SASL negotiation, or

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- if there are any LDAP operations outstanding on the
connection.

The result of violating any of these requirements is a resultCode of
operationsError, as described above in section 2.3. 5.2.2.

The client MAY have already performed a Bind operation when it sends
a Start TLS request, or the client might have not yet bound.

If the client did not establish a TLS connection before sending any
other requests, and the server requires the client to establish a
TLS connection before performing a particular request, the server
MUST reject that request with a confidentialityRequired or
strongAuthRequired result. The client MAY send a Start TLS extended
request, or it MAY choose to close the connection.

5.3.2. Starting TLS

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The server will return an extended response with the resultCode of
success if it is willing and able to negotiate TLS. It will return
other resultCodes, documented above, if it is unable.

In the successful case, the client, which has ceased to transfer
LDAP requests on the connection, MUST either begin a TLS negotiation
or close the connection. The client will send PDUs in the TLS Record
Protocol directly over the underlying transport connection to the
server to initiate TLS negotiation [TLS].

5.3.3. TLS Version Negotiation

Negotiating the version of TLS or SSL to be used is a part of the
TLS Handshake Protocol, as documented in [TLS]. Please refer to that
document for details.

5.3.4. Discovery of Resultant Security Level

After a TLS connection is established on an LDAP association, both
parties MUST individually decide whether or not to continue based on
the privacy level achieved. Ascertaining the TLS connection's
privacy level is implementation dependent, and accomplished by
communicating with one's respective local TLS implementation.

If the client or server decides that the level of authentication or
privacy is not high enough for it to continue, it SHOULD gracefully
close the TLS connection immediately after the TLS negotiation has
completed (see sections 5.4.1 and 5.5.2 below). If the client
decides to continue, it MAY attempt to Start TLS again, it MAY send
an unbind request, or it MAY send any other LDAP request.

5.3.5. Assertion of Client's Authorization Identity

The client MAY, upon receipt of a Start TLS response indicating
success, assert that a specific authorization identity be utilized

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in determining the client's authorization status. The client
accomplishes this via an LDAP Bind request specifying a SASL
mechanism of "EXTERNAL" [SASL] (see section 5.5.1.2 below).

5.3.6. Server Identity Check

The client MUST check its understanding of the server's hostname
against the server's identity as presented in the server's
Certificate message, in order to prevent man-in-the-middle attacks.

Matching is performed according to these rules:

- The client MUST use the server hostname it used to open the LDAP
connection as the value to compare against the server name as
expressed in the server's certificate. The client MUST NOT use
the server's canonical DNS name or any other derived form of
name.

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- If a subjectAltName extension of type dNSName is present in the
certificate, it SHOULD be used as the source of the server's
identity.

- Matching is case-insensitive.

- The "*" wildcard character is allowed. If present, it applies
only to the left-most name component.

E.g.

For example, *.bar.com would match a.bar.com, a.bar.com and b.bar.com, etc. but it
would not match a.x.bar.com nor would it match bar.com. If more
than one identity of a given type is present in the certificate
(e.g. more than one dNSName name), a match in any one of the set is
considered acceptable.

If the hostname does not match the dNSName-based identity in the
certificate per the above check, user-oriented clients SHOULD either
notify the user (clients MAY give the user the opportunity to
continue with the connection in any case) or terminate the
connection and indicate that the server's identity is suspect.
Automated clients SHOULD close the connection, returning and/or
logging an error indicating that the server's identity is suspect.

Beyond the server identity checks described in this section, clients
SHOULD be prepared to do further checking to ensure that the server
is authorized to provide the service it is observed to provide. The
client MAY need to make use of local policy information.

5.3.7. Refresh of Server Capabilities Information

The client MUST refresh any cached server capabilities information
(e.g. from the server's root DSE; see section 3.4 of [LDAPv3]) upon
TLS session establishment. This is necessary to protect against
active-intermediary attacks that may have altered any server

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capabilities information retrieved prior to TLS establishment. The
server MAY advertise different capabilities after TLS establishment.

5.4. Closing a TLS Connection

Two forms of TLS connection closure--graceful and abrupt--are
supported.

5.4.1. Graceful Closure

Either the client or server MAY terminate the TLS connection on an
LDAP association by sending a TLS closure alert. This will leave the
LDAP association intact.

Before closing a TLS connection, the client MUST [RGH18]either [RGH10]either wait
for any outstanding LDAP operations to complete, or explicitly
abandon them [LDAPv3].

After the initiator of a close has sent a TLS closure alert, it MUST
discard any TLS messages until it has received a TLS closure alert

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from the other party. It will cease to send TLS Record Protocol
PDUs, and following the receipt of the alert, MAY send and receive
LDAP PDUs.

The other party, if it receives a TLS closure alert, MUST
immediately transmit a TLS closure alert. It will subsequently
cease to send TLS Record Protocol PDUs, and MAY send and receive
LDAP PDUs.

5.4.2. Abrupt Closure

Either the client or server MAY abruptly close the entire LDAP
association and any TLS connection established on it by dropping the
underlying TCP connection. In this circumstance, a server MAY send
the client a Notice of Disconnection [LDAPv3] before dropping the
TCP connection.

5.5. Effects of TLS on a Client's Authorization Identity

This section describes the effects on a client's authorization
identity brought about by establishing TLS on an LDAP association.
The default effects are described first, and next the facilities for
client assertion of authorization identity are discussed including
error conditions. Lastly, the effects of closing the TLS connection
are described.

Authorization identities and related concepts are described in
Appendix B.

5.5.1. TLS Connection Establishment Effects

5.5.1.1. Default Effects

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Upon establishment of the TLS connection onto the LDAP association,
any previously established authentication and authorization
identities MUST remain in force, including anonymous state. This
holds even in the case where the server requests client
authentication via TLS -- e.g. requests the client to supply its
certificate during TLS negotiation (see [TLS]).

5.5.1.2. Client Assertion of Authorization Identity

A client MAY either implicitly request that its LDAP authorization
identity be derived from its authenticated TLS credentials or it MAY
explicitly provide an authorization identity and assert that it be
used in combination with its authenticated TLS credentials. The
former is known as an implicit assertion, and the latter as an
explicit assertion.

5.5.1.2.1. Implicit Assertion

An implicit authorization identity assertion is accomplished after
TLS establishment by invoking a Bind request of the SASL form using

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the "EXTERNAL" mechanism name [SASL, LDAPv3] that SHALL NOT include
the optional credentials octet string (found within the
SaslCredentials sequence in the Bind Request). The server will
derive the client's authorization identity from the authentication
identity supplied in the client's TLS credentials (typically a
public key certificate) according to local policy. The underlying
mechanics of how this is accomplished are implementation specific.

5.5.1.2.2. Explicit Assertion

An explicit authorization identity assertion is accomplished after
TLS establishment by invoking a Bind request of the SASL form using
the "EXTERNAL" mechanism name [SASL, LDAPv3] that SHALL include the
credentials octet string. This string MUST be constructed as
documented in section 9 of [AuthMeth]. 4.5.

5.5.1.2.3. Error Conditions

For either form of assertion, the server MUST verify that the
client's authentication identity as supplied in its TLS credentials
is permitted to be mapped to the asserted authorization identity.
The server MUST reject the Bind operation with an invalidCredentials
resultCode in the Bind response if the client is not so authorized.

Additionally, with either form of assertion, if a TLS session has
not been established between the client and server prior to making
the SASL EXTERNAL Bind request and there is no other external source
of authentication credentials (e.g. IP-level security [IPSEC]), or
if, during the process of establishing the TLS session, the server
did not request the client's authentication credentials, the SASL
EXTERNAL bind MUST fail with a result code of
inappropriateAuthentication.

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After the above Bind operation failures, any client authentication
and authorization state of the LDAP association is lost, so the LDAP
association is in an anonymous state after the failure. TLS
connection state is unaffected, though a server MAY end the TLS
connection, via a TLS close_notify message, based on the Bind
failure (as it MAY at any time).

5.5.2. TLS Connection Closure Effects

Closure of the TLS connection MUST cause the LDAP association to
move to an anonymous authentication and authorization state
regardless of the state established over TLS and regardless of the
authentication and authorization state prior to TLS connection
establishment.

6. Anonymous Authentication

Directory operations that modify entries or access protected
attributes or entries generally require client authentication.
Clients LDAP Association State Transition Tables

To comprehensively diagram the various authentication and TLS states
through which an LDAP association may pass, this section provides a
state transition table to represent a state diagram for the various
states through which an LDAP association may pass during the course
of its existence and the actions that do not intend cause these changes in state.

6.1. LDAP Association States

The following table lists the valid LDAP association states and
provides a description of each state. The ID for each state is used
in the state transition table in section 6.4.

ID State Description
-- --------------------------------------------------------------
S1 no Auth ID
no AuthZ ID
TLS: no Creds OFF
S2 no Auth ID
no AuthZ ID
TLS: no Creds ON
S3 no Auth ID
no AuthZ ID
TLS: Creds Auth ID "I", ON.
S4 Auth ID = Xn
AuthZ ID= Yn
[TLS: no Creds, Off]
S5 Auth ID = Xn
AuthZ ID= Yn
[TLS: no Creds, On]
S6 Auth ID = Xn
AuthZ ID= Yn
[TLS: Creds Auth ID "I", On]
S7 Auth ID = I
AuthZ ID= Jn

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[TLS: Creds Auth ID "I", On]
S8 Auth ID = I
AuthZ ID= is based on "I"
[TLS: Creds Auth ID "I", On]

6.2. Actions that Affect LDAP Association State

The following table lists the actions that can affect the state of
an LDAP association. The ID for each action is used in the state
transition table in section 6.4.

ID Action
-- ------------------------------------------------
A1 Client binds anonymously
A2 Error: Inappropriate authentication
A3 Client or Server: close TLS connection (section 5.5.2)
A4 Client StartTLS
Server: client auth NOT required

A5 Client: StartTLS
Server: client creds requested
Client: {TLS creds: Auth ID "I"]
A6 Client: Bind w/simple password or SASL mechanism (e.g. DIGEST-
MD5 password, Kerberos, etc. � except EXTERNAL [Auth ID "X"
maps to AuthZ ID "Y"]
A7 Client Binds SASL EXTERNAL w/ credentials: AuthZ ID "J"
[Explicit Assertion (section 5.5.1.2.2)]
A8 Client Bind SASL EXTERNAL w/saslcredentials: NULL [Implicit
Assertion (section 5.5.1.2.1)]

6.3. Decisions Used in Making LDAP Association State Changes

Certain changes in the state of an LDAP association are only allowed
if the server can affirmatively answer a question. These questions
are applied as part of the criteria for allowing or disallowing a
state change in the state transition table in section 6.4.

ID Decision Question
-- --------------------------------------------------------------
D1 Can TLS Credentials Auth ID "I" be mapped to perform AuthZ ID "J"?
D2 Can a valid AuthZ ID "J" be derived from TLS Credentials Auth
ID "I"?

6.4. LDAP Association State Transition Table

The LDAP Association table below lists the valid states for an LDAP
association and the actions that could affect them. For any given
row in the table, the Current State column gives the state of these operations an
LDAP association, the Action column gives an action that could
affect the state of an LDAP assocation, and the Next State column
gives the resulting state of an LDAP association after the action
occurs.

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The initial state for the state machine described in this table is
S1.

Current Next
State Action State Comment
------- ------------- ----- -----------------------------------
S1 A1 S1
S1 A2 S1 Error: Inappropriate authentication
S1 A4 S2
S1 A5 S3
S1 A6 S4
S2 A1 S2
S2 A2 S2 Error: Inappropriate authentication
S2 A3 S1
S2 A6 S5
S3 A1 S3
S3 A3 S1
S3 A6 S6
S3 A7 and D1=NO S3 Error: InvalidCredentials
S3 A7 and D1=YES S7
S3 A8 and D2=NO S3 Error: InvalidCredentials
S3 A8 and D2=YES S8
S4 A6 S4
S4 A2 S4 Error: Inappropriate Authentication
S4 A3 S1
S4 A4 S5
S4 A5 S6
S5 A6 S5
S5 A2 S5 Error: Inappropriate Authentication
S5 A3 S1
S5 A1 S2
S6 A6 S6
S6 A3 S1
S6 A1 S3
S6 A7 and D1=NO S6 Error: InvalidCredentials
S6 A7 and D1=YES S10
S6 A8 and D2=NO S6 Error: InvalidCredentials
S6 A8 and D2=YES S8
S7 A1 S3
S7 A3 S1
S7 A6 S6
S7 A7 S7
S7 A8 and D2=NO S3 Error: InvalidCredentials
S7 A8 and D2=YES S8
S8 A1 S3
S8 A3 S1
S8 A8 S8
S8 A7 and D1=NO S6 Error: InvalidCredentials
S8 A7 and D1=YES S7
S8 A6 S6

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7. Anonymous Authentication

Directory operations that modify entries or access protected
attributes or entries generally require client authentication.
Clients that do not intend to perform any of these operations
typically use anonymous authentication. Servers SHOULD NOT allow
clients with anonymous authentication to modify directory entries or
access sensitive information in directory entries.

LDAP implementations MUST support anonymous authentication, as
defined in section 6.1. 7.1.

LDAP implementations MAY support anonymous authentication with TLS,
as defined in section 6.2. 7.2.

While there MAY be access control restrictions to prevent access to
directory entries, an LDAP server SHOULD allow an anonymously-bound
client to retrieve the supportedSASLMechanisms attribute of the root
DSE.

An LDAP server MAY use other information about the client provided
by the lower layers or external means to grant or deny access even
to anonymously authenticated clients.

6.1.

7.1. Anonymous Authentication Procedure

An LDAPv3 client that has not successfully completed a bind
operation on a connection is anonymously authenticated. See section
4.3.3.

An LDAP client MAY also choose to explicitly bind anonymously. A
client that wishes to do so MUST choose the simple authentication
option in the Bind Request (see section XXX) 4.1) and set the password to
be of zero length. (This is often done by LDAPv2 clients.) Typically
the name is also of zero length.

6.2.

7.2. Anonymous Authentication and TLS

An LDAP client MAY use the Start TLS operation (section 4) 5) to
negotiate the use of TLS security [TLS]. If the client has not bound
beforehand, then until the client uses the EXTERNAL SASL mechanism
to negotiate the recognition of the client's certificate, the client
is anonymously authenticated.

Recommendations on TLS ciphersuites are given in section 10. 11.

An LDAP server which requests that clients provide their certificate
during TLS negotiation MAY use a local security policy to determine
whether to successfully complete TLS negotiation if the client did
not present a certificate which could be validated.

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8. Password-based authentication

7.1.

8.1. Simple authentication

The LDAP "simple" authentication choice is not suitable for
authentication in environments where there is no network or
transport layer confidentiality. LDAP implementations SHOULD support

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authentication with the "simple" authentication choice when the
connection is protected against eavesdropping using TLS, as defined
in section 6.3. 7.3. LDAP implementations SHOULD NOT support
authentication with the "simple" authentication choice unless the
data on the connection is protected using TLS or other privacy and
data-integrity protection.

7.2.

8.2. Digest Authentication

LDAP implementations MUST support authentication with a password
using the DIGEST-MD5 SASL mechanism for password protection.

An LDAP client MAY determine whether the server supports this
mechanism by performing a search request on the root DSE, requesting
the supportedSASLMechanisms attribute, and checking whether the
string "DIGEST-MD5" is present as a value of this attribute.

In the first stage of authentication, when the client is perfCorming performing
an "initial authentication" as defined in section 2.1 of [RFC2831],
the client sends a bind request in which the version number is 3,
the authentication choice is sasl, the sasl mechanism name is
"DIGEST- MD5", and the credentials are absent. The client then waits
for a response from the server to this request.

The server will respond with a bind response in which the resultCode
is saslBindInProgress, and the serverSaslCreds field is present. The
contents of this field is a string defined by "digest-challenge" in
section 2.1.1 of [RFC2831]. The server SHOULD include a realm
indication and MUST indicate support for UTF-8.

The client will send a bind request with a distinct message id, in
which the version number is 3, the authentication choice is sasl,
the sasl mechanism name is "DIGEST-MD5", and the credentials contain
the string defined by "digest-response" in section 2.1.2 of
[RFC2831]. The serv-type is "ldap".

The server will respond with a bind response in which the resultCode
is either success, or an error indication. If the authentication is
successful and the server does not support subsequent
authentication, then the credentials field is absent. If the
authentication is successful and the server supports subsequent
authentication, then the credentials field contains the string
defined by "response-auth" in section 2.1.3 of [4]. Support for
subsequent authentication is OPTIONAL in clients and servers.

7.3.

8.3. "simple" authentication choice under TLS encryption

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Following the negotiation of an appropriate TLS ciphersuite
providing connection confidentiality [6], a client MAY authenticate
to a directory that supports the simple authentication choice by
performing a simple bind operation.

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The client will use the Start TLS operation [5] to negotiate the use
of TLS security [6] on the connection to the LDAP server. The client
need not have bound to the directory beforehand.

For this authentication procedure to be successful, the client and
server MUST negotiate a ciphersuite which contains a bulk encryption
algorithm of appropriate strength. Recommendations on cipher suites
are given in section 10. 11.

Following the successful completion of TLS negotiation, the client
MUST send an LDAP bind request with the version number of 3, the
name field containing a DN , and the "simple" authentication choice,
containing a password.

7.3.1

8.3.1. "simple" Authentication Choice

DSAs that map the DN sent in the bind request to a directory entry
with a userPassword attribute will, for each value of the
userPassword attribute in the named user's entry, compare these for
case-sensitive equality with the client's presented password. If
there is a match, then the server will respond with resultCode
success, otherwise the server will respond with resultCode
invalidCredentials.

7.4.

8.4. Other authentication choices with TLS

It is also possible, following the negotiation of TLS, to perform a
SASL authentication that does not involve the exchange of plaintext
reusable passwords. In this case the client and server need not
negotiate a ciphersuite which that provides confidentiality if the only
service required is data integrity.

9. Certificate-based authentication

LDAP implementations SHOULD support authentication via a client
certificate in TLS, as defined in section 7.1. 8.1.

9.1. Certificate-based authentication with TLS

A user who has a public/private key pair in which the public key has
been signed by a Certification Authority may use this key pair to
authenticate to the directory server if the user's certificate is
requested by the server. The user's certificate subject field SHOULD
be the name of the user's directory entry, and the Certification
Authority that issued the user�s certificate must be sufficiently
trusted by the directory server in order for the server to process

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the certificate. The means by which servers validate certificate
paths is outside the scope of this document.

A server MAY support mappings for certificates in which the subject
field name is different from the name of the user's directory entry.
A server which supports mappings of names MUST be capable of being
configured to support certificates for which no mapping is required.

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The client will use the Start TLS operation [5] to negotiate the use
of TLS security [6] on the connection to the LDAP server. The client
need not have bound to the directory beforehand.

In the TLS negotiation, the server MUST request a certificate. The
client will provide its certificate to the server, and the server
MUST perform a private key-based encryption, proving it has the
private key associated with the certificate.

In deployments that require protection of sensitive data in transit,
the client and server MUST negotiate a ciphersuite which that contains a
bulk encryption algorithm of appropriate strength. Recommendations
of cipher suites are given in section 10. 11.

The server MUST verify that the client's certificate is valid. The
server will normally check that the certificate is issued by a known
CA, and that none of the certificates on the client's certificate
chain are invalid or revoked. There are several procedures by which
the server can perform these checks.

Following the successful completion of TLS negotiation, the client
will send an LDAP bind request with the SASL "EXTERNAL" mechanism.

10. TLS Ciphersuites

The following ciphersuites defined in [6] MUST NOT be used for
confidentiality protection of passwords or data:

TLS_NULL_WITH_NULL_NULL
TLS_RSA_WITH_NULL_MD5
TLS_RSA_WITH_NULL_SHA

The following ciphersuites defined in [6] can be cracked easily
(less than a day of CPU time on a standard CPU in 2000). These
ciphersuites are NOT RECOMMENDED for use in confidentiality
protection of passwords or data. Client and server implementers
SHOULD carefully consider the value of the password or data being
protected before using these ciphersuites:

TLS_RSA_EXPORT_WITH_RC4_40_MD5
TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5
TLS_RSA_EXPORT_WITH_DES40_CBC_SHA
TLS_DH_DSS_EXPORT_WITH_DES40_CBC_SHA
TLS_DH_RSA_EXPORT_WITH_DES40_CBC_SHA
TLS_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA

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TLS_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA
TLS_DH_anon_EXPORT_WITH_RC4_40_MD5
TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA

The following ciphersuites are vulnerable to man-in-the-middle
attacks, and SHOULD NOT be used to protect passwords or sensitive

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data, unless the network configuration is such that the danger of a
man-in-the-middle attack is tolerable:

TLS_DH_anon_EXPORT_WITH_RC4_40_MD5
TLS_DH_anon_WITH_RC4_128_MD5
TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA
TLS_DH_anon_WITH_DES_CBC_SHA
TLS_DH_anon_WITH_3DES_EDE_CBC_SHA

A client or server that supports TLS MUST support
TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA and MAY support other ciphersuites
offering equivalent or better protection.

10.

11. Security Considerations

Security issues are discussed throughout this memo; the
(unsurprising) conclusion is that mandatory security is important,
and that session encryption is required when snooping is a problem.

Servers are encouraged to prevent modifications by anonymous users.
Servers may also wish to minimize denial of service attacks by
timing out idle connections, and returning the unwillingToPerform
result code rather than performing computationally expensive
operations requested by unauthorized clients.

A connection on which the client has not performed the Start TLS
operation or negotiated a suitable SASL mechanism for connection
integrity and encryption services is subject to man-in-the-middle
attacks to view and modify information in transit.

10.1.

11.1. Start TLS Security Considerations

The goals of using the TLS protocol with LDAP are to ensure
connection confidentiality and integrity, and to optionally provide
for authentication. TLS expressly provides these capabilities, as
described in [TLS].

All security gained via use of the Start TLS operation is gained by
the use of TLS itself. The Start TLS operation, on its own, does not
provide any additional security.

The use of TLS does not provide or ensure for confidentiality and/or
non-repudiation of the data housed by an LDAP-based directory
server. Nor does it secure the data from inspection by the server
administrators. Once established, TLS only provides for and ensures
confidentiality and integrity of the operations and data in transit

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over the LDAP association, and only if the implementations on the
client and server support and negotiate it.

The level of security provided though the use of TLS depends
directly on both the quality of the TLS implementation used and the
style of usage of that implementation. Additionally, an active-

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intermediary attacker can remove the Start TLS extended operation
from the supportedExtension attribute of the root DSE. Therefore,
both parties SHOULD independently ascertain and consent to the
security level achieved once TLS is established and before beginning
use of the TLS connection. For example, the security level of the
TLS connection might have been negotiated down to plaintext.

Clients SHOULD either warn the user when the security level achieved
does not provide confidentiality and/or integrity protection, or be
configurable to refuse to proceed without an acceptable level of
security.

Client and server implementors SHOULD take measures to ensure proper
protection of credentials and other confidential data where such
measures are not otherwise provided by the TLS implementation.

Server implementors SHOULD allow for server administrators to elect
whether and when connection confidentiality and/or integrity is
required, as well as elect whether and when client authentication
via TLS is required.

Additional security considerations relating to the EXTERNAL
mechanism to negotiate TLS can be found in [SASL] and [6].

11.

12. Acknowledgements

This document combines information originally contained in RFC 2829,
RFC 2830 and portions of RFC 2251. The author acknowledges the work
of Harald Tveit Alvestrand, Jeff Hodges, Tim Howes, Steve Kille, RL
"Bob" Morgan , and Mark Wahl, each of whom authored one or more of
these documents. RFC 2829 and RFC 2830 were products of the IETF
LDAPEXT Working Group. RFC 2251 was a product of the ASID Working
Group.

This document is based upon input of the IETF LDAP Revision working
group. The contributions of its members is greatly appreciated.

12.

13. Bibliography

[ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.

[IPSEC] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.

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[LDAPDN] Zeilenga, Kurt D., "Lightweight Directory Access Protocol
(v3): UTF-8 String Representation of Distinguished Names",
draft-ietf-ldapbis-dn-06.txt, July, 2001.

[LDAPv3] Wahl, M., Kille S. and T. Howes, "Lightweight Directory
Access Protocol (v3)", RFC 2251, December 1997.

[RFC2828] Shirey, R., "Internet Security Glossary", RFC 2828, May
2000.

[RFC2401] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.

[RFC2831] Leach, P. and C. Newman, "Using Digest Authentication as a
SASL Mechanism", RFC 2831, May 2000.

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[ReqsKeywords] 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.

[TLS] Dierks, T. and C. Allen. "The TLS Protocol Version 1.0", RFC
2246, January 1999.

13.

14. Author's Address

Roger Harrison
Novell, Inc.
1800 S. Novell Place
Provo, UT 84606
+1 801 861 2642
roger_harrison@novell.com

14.

15. Full Copyright Statement

This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
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.

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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.

Appendix A. Example Deployment Scenarios

The following scenarios are typical for LDAP directories on the
Internet, and have different security requirements. (In the
following, "sensitive" means data that will cause real damage
following discussion, "sensitive data" refers to information whose
disclosure, alteration, destruction, or loss would adversely affect
the

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Authentication Methods for LDAPv3 July 19, 2001 interests or business of its owner if revealed; or user. Also note that there
may be data that is protected but not
sensitive). sensitive.) This is not
intended to be a comprehensive list, list; other scenarios are possible,
especially on physically protected networks.

(1) A read-only directory, containing no sensitive data, accessible
to "anyone", and TCP connection hijacking or IP spoofing is not
a problem. This directory requires no security functions except
administrative service limits.

(2) A read-only directory containing no sensitive data; read access
is granted based on identity. TCP connection hijacking is not
currently a problem. This scenario requires a secure data confidentiality
for sensitive authentication information AND data integrity for
all authentication function. information.

(3) A read-only directory containing no sensitive data; and the
client needs to ensure the identity of the directory server and
that the directory data is authenticated
by the server and not modified while being returned
from the server. A data origin authentication service AND data
integrity service are required.

(4) A read-write directory, containing no sensitive data; read
access is available to "anyone", update access to properly
authorized persons. TCP connection hijacking is not currently a
problem. This scenario requires a secure data confidentiality for
sensitive authentication information AND data integrity for all
authentication
function. information.

(5) A directory containing sensitive data. This scenario requires
session
data confidentiality protection AND secure authentication.

Appendix B. Authentication and Authorization: Definitions and Concepts

This appendix defines basic terms, concepts, and interrelationships
regarding authentication, authorization, credentials, and identity.

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These concepts are used in describing how various security
approaches are utilized in client authentication and authorization.

B.1. Access Control Policy

An access control policy is a set of rules defining the protection
of resources, generally in terms of the capabilities of persons or
other entities accessing those resources. A common expression of an
access control policy is an access control list. Security objects
and mechanisms, such as those described here, enable the expression
of access control policies and their enforcement. Access control
policies are typically expressed in terms of access control
attributes as described below.

B.2. Access Control Factors

A request, when it is being processed by a server, may be associated
with a wide variety of security-related factors (section 4.2 of
[LDAPv3]). The server uses these factors to determine whether and
how to process the request. These are called access control factors
(ACFs). They might include source IP address, encryption strength,

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the type of operation being requested, time of day, etc. Some
factors may be specific to the request itself, others may be
associated with the connection via which the request is transmitted,
others (e.g. time of day) may be "environmental".

Access control policies are expressed in terms of access control
factors. E.g., a request having ACFs i,j,k can perform operation Y
on resource Z. The set of ACFs that a server makes available for
such expressions is implementation-specific.

B.3. Authentication, Credentials, Identity

Authentication credentials are the evidence supplied by one party to
another, asserting the identity of the supplying party (e.g. a user)
who is attempting to establish an association with the other party
(typically a server). Authentication is the process of generating,
transmitting, and verifying these credentials and thus the identity
they assert. An authentication identity is the name presented in a
credential.

There are many forms of authentication credentials -- the form used
depends upon the particular authentication mechanism negotiated by
the parties. For example: X.509 certificates, Kerberos tickets,
simple identity and password pairs. Note that an authentication
mechanism may constrain the form of authentication identities used
with it.

B.4. Authorization Identity

An authorization identity is one kind of access control factor. It
is the name of the user or other entity that requests that
operations be performed. Access control policies are often expressed

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in terms of authorization identities; e.g., entity X can perform
operation Y on resource Z.

The authorization identity bound to an association is often exactly
the same as the authentication identity presented by the client, but
it may be different. SASL allows clients to specify an authorization
identity distinct from the authentication identity asserted by 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 [SASL]. Also,
the form of authentication identity supplied by a service like TLS
may not correspond to the authorization identities used to express a
server's access control policy, requiring a server-specific mapping
to be done. The method by which a server composes and validates an
authorization identity from the authentication credentials supplied
by a client is implementation-specific.

Appendix C. RFC 2829 Change History

This appendix lists the changes made to the text of RFC 2829 in
preparing this document.

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C.0. General Editorial Changes
Version -00

- Changed other instances of the term LDAP to LDAPv3 where v3 of
the protocol is implied. Also made all references to LDAPv3 use
the same wording.

- Miscellaneous grammatical changes to improve readability.

- Made capitalization in section headings consistent.

Version -01

- Changed title to reflect inclusion of material from RFC 2830 and
2251.

C.1. Changes to Section 1

Version -01

- Moved conventions used in document to a separate section.

C.2. Changes to Section 2

Version -01

- Moved section to an appendix.

C.3. Changes to Section 3

Version -01

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- Moved section to an appendix.

C.4 Changes to Section 4

Version -00

- Changed "Distinguished Name" to "LDAP distinguished name".

C.5. Changes to Section 5

Version -00

- Added the following sentence: "Servers SHOULD NOT allow clients
with anonymous authentication to modify directory entries or
access sensitive information in directory entries."

C.5.1. Changes to Section 5.1

Version -00

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- Replaced the text describing the procedure for performing an
anonymous bind (protocol) with a reference to section 4.2 of RFC
2251 (the protocol spec).

Version -01

- Brought text describing procedure for performing an anonymous
bind from section 4.2 of RFC 2251 bis. This text will be
removed from the draft standard version of that document.

C.6. Changes to Section 6.

Version -00

Reorganized text in section 6.1 as follows:

1. Added a new section (6.1) titled "Simple Authentication" and
moved one of two introductory paragraphs for section 6 into
section 6.1. Added sentences to the paragraph indicating:

a. simple authentication is not suitable for environments where
confidentiality is not available.

b. LDAP implementations SHOULD NOT support simple
authentication unless confidentiality and data integrity
mechanisms are in force.

2. Moved first paragraph of section 6 (beginning with "LDAP
implementations MUST support authentication with a password�") to
section on Digest Authentication (Now section 6.2).

C.6.1. Changes to Section 6.1.

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Version -00 Renamed section to 6.2

- Added sentence from original section 6 indicating that the
DIGEST-MD5 SASL mechanism is required for all conforming LDAPv3
implementations

C.6.2

C.6.2. Changes to Section 6.2

Version -00

- Renamed section to 6.3

- Reworded first paragraph to remove reference to user and the
userPassword password attribute Made the first paragraph more
general by simply saying that if a directory supports simple
authentication that the simple bind operation MAY performed
following negotiation of a TLS ciphersuite that supports
confidentiality.

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- Replaced "the name of the user's entry" with "a DN" since not
all bind operations are performed on behalf of a "user."

- Added Section 6.3.1 heading just prior to paragraph 5.

- Paragraph 5: replaced "The server" with "DSAs that map the DN
sent in the bind request to a directory entry with a
userPassword attribute."

C.6.3. Changes to section 6.3.

Version -00

- Renamed to section 6.4.

C.7. Changes to section 7.

none

C.7.1. Changes to section 7.1.

Version -00

- Clarified the entity issuing a certificate by moving the phrase
"to have issued the certificate" immediately after
"Certification Authority."

C.8. Changes to section 8.

Version -00

- Removed the first paragraph because simple authentication is
covered explicitly in section 6.

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- Added section 8.1. heading just prior to second paragraph.

- Added section 8.2. heading just prior to third paragraph.

- Added section 8.3. heading just prior to fourth paragraph.

Version -01

- Moved entire section 8 of RFC 2829 into section 3.4 (Using SASL
for Other Security Services) to bring material on SASL
mechanisms together into one location.

C.9. Changes to section 9.

Version -00

- Paragraph 2: changed "EXTERNAL mechanism" to "EXTERNAL SASL
mechanism."

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- Added section 9.1. heading.

- Modified a comment in the ABNF from "unspecified userid" to
"unspecified authz id".

- Deleted sentence, "A utf8string is defined to be the UTF-8
encoding of one or more ISO 10646 characters," because it is
redundant.

- Added section 9.1.1. heading.

- Added section 9.1.2. heading.

Version -01

- Moved entire section 9 to become section 3.5 so that it would be
with other SASL material.

C.10. Changes to Section 10.

Version -00

- Updated reference to cracking from a week of CPU time in 1997 to
be a day of CPU time in 2000.

- Added text: "These ciphersuites are NOT RECOMMENDED for use...
and server implementers SHOULD" to sentence just prior the
second list of ciphersuites.

- Added text: "and MAY support other ciphersuites offering
equivalent or better protection," to the last paragraph of the
section.

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C.11. Changes to Section 11.

Version -01

- Moved to section 3.6 to be with other SASL material.

C.12. Changes to Section 12.

Version -00

- Inserted new section 12 that specifies when SASL protections
begin following SASL negotiation, etc. The original section 12
is renumbered to become section 13.

Version -01

- Moved to section 3.7 to be with other SASL material.

C.13

C.13. Changes to Section 13 (original section 12).

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None

Appendix D. RFC 2830 Change History

This appendix lists the changes made to the text of RFC 2830 in
preparing this document.

D.0. General Editorial Changes

- Material showing the PDUs for the Start TLS response was broken
out into a new section.

- The wording of the definition of the Start TLS request and Start
TLS response was changed to make them parallel. NO changes were
made to the ASN.1 definition or the associated values of the
parameters.

- A separate section heading for graceful TLS closure was added
for parallelism with section on abrupt TLS closure.

Appendix E. RFC 2251 Change History

This appendix lists the changes made to the text of RFC 2251 in
preparing this document.

E.0. General Editorial Changes

- All material from section 4.2 of RFC 2251 was moved into this
document.

- A new section was created for the Bind Request

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- Section 4.2.1 of RFC 2251 (Sequencing Bind Request) was moved
after the section on the Bind Response for parallelism with the
presentation of the Start TLS operations. The section was also
subdivided to explicitly call out the various effects being
described within it.

- All SASL profile information from RFC 2829 was brought within
the discussion of the Bind operation (primarily sections 4.4 -
4.7).

Appendix F. Change History to Combined Document

F.1. Changes for draft-ldap-bis-authmeth-02.doc

Section 1.
- Added glossary of terms and added sub-section headings

Section 2.
- Clarified security mechanisms 3, 4, & 5 and brought language in
line with IETF security glossary.

Section 3.
- Brought language in requirement (3) in line with security
glossary.

- Clarified that information fetched prior to initiation of TLS
negotiation must be discarded

-Clarified that information fetched prior to initiation of SASL
negotiation must be discarded

- Rewrote paragraph on SASL negotiation requirements to clarify
intent

Section 4.4.
- Added stipulation that sasl choice allows for any SASL mechanism
not prohibited by this document. (Resolved conflict between this
statement and one that prohibited use of ANONYMOUS and PLAIN
SASL mechanisms.)

Section 5.3.6
- Added a.x.bar.com to wildcard matching example on hostname
check.

Section 6
- Added LDAP Association State Transition Tables to show the
various states through which an LDAP association may pass along
with the actions and decisions required to traverse from state
to state.

Appendix A
- Brought security terminology in line with IETF security glossary
throughout the appendix.

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General
- Added references to other LDAP standard documents, to sections
within the document, and fixed broken references.

- General editorial changes�punctuation, spelling, formatting,
etc.

Appendix G. Issues to be Resolved

This appendix lists open questions and issues that need to be
resolved before work on this document is deemed complete.

F.1.

G.1.

Section 1 lists 6 security mechanisms that can be used by LDAP
servers. I'm not sure what mechanism 5, "Resource limitation by
means of administrative limits on service controls" means.

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F.2.

Status: resolved. Changed wording to "administrative service limits"
to clarify meaning.

G.2.

Section 2 paragraph 1 defines the term, "sensitive." Do we want to
bring this term and other security-related terms in alignment with
usage with the IETF security glossary (RFC 2828)?

F.3.

Status: resolved. WG input at IETF 51 was that we should do this, so
the appropriate changes have been made.

G.3.

Section 2, deployment scenario 2: What is meant by the term "secure term "secure
authentication function?"

Status: resolved. Based on the idea that a "secure authentication
function" could be provided by TLS, I changed the wording to require
data confidentiality for sensitive authentication function?"

F.4. information and
data integrity for all authentication information.

G.4.

Section 3, deployment scenario 3: What is meant by the phrase,
"directory data is authenticated by the server?"

F.5.

Status: resolved. I interpreted this to mean the ability to ensure
the identity of the directory server and the integrity of the data
sent from that server to the client, and explictly stated such.

G.5.

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Section 4 paragraph 3: What is meant by the phrase, "this means that
either this data is useless for faking authentication (like the Unix
"/etc/passwd" file format used to be)?"

F.6.

G.6.

Section 4 paragraph 7 begins: "For a directory needing session
protection..." Is this referring to data confidentiality or data
integrity or both?

F.7.

Status: resolved. Changed wording to say, "For a directory needing
data security (both data integrity and data confidentiality)..."

G.7.

Section 4 paragraph 8 indicates that "information about the server
fetched fetched prior to the TLS negotiation" must be discarded. Do
we want to explicitly state that this applies to information fetched
prior to the *completion* of the TLS negotiation or is this going
too far?

F.8.

Status: resolved. Based on comments in the IETF 51 LDAPBIS WG
meeting, this has been changed to explicitly state, "fetched prior
to the initiation of the TLS negotiation..."

G.8.

Section 4 paragraph 9 indicates that clients SHOULD check the
supportedSASLMechanisms list both before and after a SASL security
layer is negotiated to ensure that they are using the best available
security mechanism supported mutually by the client and server. A
note at the end of the paragraph indicates that this is a SHOULD
since there are environments where the client might get a list of
supported SASL mechanisms from a different trusted source.

I wonder if the intent of this could be restated more plainly using
one of these two approaches (I've paraphrased for the sake of
brevity):

Approach 1: Clients SHOULD check the supportedSASLMechanisms
list both before and after SASL negotiation or clients SHOULD
use a

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SASL mechanisms.

Approach 2: Clients MUST check the supportedSASLMechanisms list
both before and after SASL negotiation UNLESS they use a
different trusted source to determine available supported SASL
mechanisms.

F.9.

Status: Resolved. WG input at IETF 51 was that Approach 1 was
probably best. I ended up keeping the basic structure similar to the
original to meet this intent.

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G.9.

Section 6.3.1 states: "DSAs that map the DN sent in the bind request
to a directory entry with a userPassword attribute will... compare
[each value in the named user's entry]... with the presented
password." This implies that this this applies only to user entries
with userPassword attributes. What about other types of entries
that might allow passwords and might store in the password
information in other attributes? Do we want to make this text more
general?

F.10

G.10 userPassword and simple bind

We need to be sure that we don't require userPassword to be the only
attribute used for authenticating via simple bind. (See 2251 sec 4.2
and authmeth 6.3.1. Work with Jim Sermersheim on resolution to this.
On publication state something like: "This is the specific
implementation of what we discussed in our general reorg
conversation on the list." (Source: Kurt Zeilenga)

F.11

G.11. Meaning of LDAP Association

The original RFC 2830 uses the term "LDAP association" in describing
a connection between an LDAP client and server regardless of the
state of TLS on that connection. This term needs to be defined or
possibly changed.

F.12.

Status: Resolved. at IETF 51 Bob Morgan indicated that the term
"LDAP association" was intended to distinguish the LDAP-level
connection from the TLS-level connection. This still needs to be
clarified somewhere in the draft. Added "LDAP association" to a
glossary in section 1.

G.12. Is DIGEST-MD5 mandatory for all implementations?

Reading 2829bis I think DIGEST-MD5 is mandatory ONLY IF your server
supports password based authentication...but the following makes it
sound mandatory to provide BOTH password authentication AND DIGEST-
MD5:

"6.2. Digest authentication

LDAP implementations MUST support authentication with a password
using the DIGEST-MD5 SASL mechanism for password protection, as
defined in section 6.1."

The thing is for acl it would be nice (though not critical) to be
able to default the required authentication level for a subject to a
single "fairly secure" mechanism--if there is no such mandatory
authentication scheme then you cannot do that. (Source: Rob Byrne)

G.13. Ordering of authentication levels requested

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F.13. Ordering of authentication levels requested

Again on the subject of authentication level, is it possible to
define an ordering on authentication levels which defines their
relative "strengths" ? This would be useful in acl as you could say
things like"a given aci grants access to a given subject at this
authentication level AND ABOVE". David Chadwick raised this before
in the context of denying access to a subject at a given
authentication level, in which case he wanted to express "deny
access to this subject at this authentication level AND TO ALL
IDENTITIES AUTHENTICATED BELOW THAT LEVEL". (Source: Rob Byrne)

F.14.

G.14. Document vulnerabilities of various mechanisms

While I'm here...in 2829, I think it would be good to have some
comments or explicit reference to a place where the security
properties of the particular mandatory authentication schemes are
outlined. When I say "security properties" I mean stuff like "This
scheme is vulnerable to such and such attacks, is only safe if the
key size is > 50, this hash is widely considered the best, etc...".
I think an LDAP implementor is likely to be interested in that
information, without having to wade through the security RFCs.
(Source: Rob Byrne)

F.15.

G.15. Include a StartTLS state transition table

The pictoral representation it is nominally based on is here (URL
possibly folded):

http://www.stanford.edu/~hodges/doc/LDAPAssociationStateDiagram-
1999-12-14.html

(Source: Jeff Hodges)

F.16.

Status: Table provided. Final review of content for accuracy is
still needed.

G.16. Empty sasl credentials question

I spent some more time looking microscopically at ldap-auth-methods
and ldap-ext-tls drafts. The drafts say that the credential must
have the form dn:xxx or u:xxx or be absent, and although they don't
say what to do in the case of an empty octet string I would say that
we could send protocolError (claim it is a bad PDU).

There is still the question of what to do if the credential is 'dn:'
(or 'u:') followed by the empty string. (Source: ariel@columbia.edu
via Jeff Hodges)

F.17.

G.17. Hostname check from MUST to SHOULD?

I am uneasy about the hostname check. My experience from PKI with
HTTP probably is a contributing factor; we have people using the
short hostname to get to a server which naturally has the FQDN in
the certificate, no end of problems. I have a certificate on my

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laptop which has the FQDN for the casse when the system is on our

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Columbia network with a fixed IP; when I dial in however, I have
some horrible dialup name, and using the local https server becomes
annoying. Issuing a certificate in the name 'localhost' is not a
solution! Wildcard match does not solve this problem. For these
reasons I am inclined to argue for 'SHOULD' instead of
'MUST' in paragraph...

Also, The hostname check against the name in the certificate is a
very weak means of preventing man-in-the-middle attacks; the proper
solution is not here yet (SecureDNS or some equivalent). Faking out
DNS is not so hard, and we see this sort of thing in the press on a
pretty regular basis, where site A hijacks the DNS server for site B
and gets all their requests. Some mention of this should be made in
the draft. (Source: ariel@columbia.edu via Jeff Hodges)

F.18.

G.18. Must SASL DN exist in the directory?

If the 'dn:' form of sasl creds is used, is it the intention of the
draft(ers) that this DN must exist in the directory and the client
will have the privileges associated with that entry, or can the
server map the sasl DN to perhaps some other DN in the directory,
in an implementation-dependent fashion?

We already know that if *no* sasl credentials are presented, the DN
or altname in the client certificate may be mapped to a DN in an
implementation-dependent fashion, or indeed to something not in the
directory at all. (Right?) (Source: ariel@columbia.edu via Jeff
Hodges)

G.19. DN used in conjunction with SASL mechanism

We need to specify whether the DN field in Bind operation can/cannot
be used when SASL mechanism is specified. (source: RL Bob)

G.20. Bind states

Differences between unauthenticated and anonymous. four states you
can get into. One is completely undefined (this is now explicitly
called out in document). This text needs to be moved from
RFC2251bis to this draft. (source: Jim Sermersheim)

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