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INTERNET-DRAFT Editor: R. Harrison
draft-ietf-ldapbis-authmeth-19.txt Novell, Inc.
Obsoletes: 2251, 2829, 2830 February 2006
Intended Category: Standards Track
LDAP: Authentication Methods
and
Security Mechanisms
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
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 Revision 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
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet-Drafts as
reference material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.html
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Abstract
This document describes authentication methods and security
mechanisms of the Lightweight Directory Access Protocol (LDAP).
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This document details establishment of Transport Layer Security
(TLS) using the StartTLS operation.
This document details the simple Bind authentication method
including anonymous, unauthenticated, and name/password mechanisms
and the Simple Authentication and Security Layer (SASL) Bind
authentication method including the EXTERNAL mechanism.
This document discusses various authentication and authorization
states through which a session to an LDAP server may pass and the
actions that trigger these state changes.
This document, together with other documents in the LDAP Technical
Specification (see section 1 of [Roadmap]), obsoletes RFC 2251, RFC
2829 and RFC 2830.
Table of Contents
1. Introduction.....................................................3
1.1. Relationship to Other Documents................................5
1.2. Conventions....................................................6
2. Implementation Requirements......................................7
3. StartTLS Operation...............................................7
3.1. TLS Establishment Procedures...................................7
3.1.1. StartTLS Request Sequencing..................................8
3.1.2. Client Certificate...........................................8
3.1.3. Server Identity Check........................................8
3.1.3.1. Comparison of DNS Names...................................10
3.1.3.2. Comparison of IP Addresses................................10
3.1.3.3. Comparison of other subjectName types.....................10
3.1.4. Discovery of Resultant Security Level.......................10
3.1.5. Refresh of Server Capabilities Information..................11
3.2. Effect of TLS on Authorization State..........................11
3.3. TLS Ciphersuites..............................................11
4. Authorization State.............................................12
5. Bind Operation..................................................13
5.1. Simple Authentication Method..................................13
5.1.1. Anonymous Authentication Mechanism of Simple Bind...........13
5.1.2. Unauthenticated Authentication Mechanism of Simple Bind.....13
5.1.3. Name/Password Authentication Mechanism of Simple Bind.......14
5.2. SASL Authentication Method....................................15
5.2.1. SASL Protocol Profile.......................................15
5.2.1.1. SASL Service Name for LDAP................................15
5.2.1.2. SASL Authentication Initiation and Protocol Exchange......15
5.2.1.3. Optional Fields...........................................16
5.2.1.4. Octet Where Negotiated Security Layers Take Effect........17
5.2.1.5. Determination of Supported SASL Mechanisms................17
5.2.1.6. Rules for Using SASL Layers...............................17
5.2.1.7. Support for Multiple Authentications......................18
5.2.1.8. SASL Authorization Identities.............................18
5.2.2. SASL Semantics Within LDAP..................................19
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5.2.3. SASL EXTERNAL Authentication Mechanism......................19
5.2.3.1. Implicit Assertion........................................19
5.2.3.2. Explicit Assertion........................................20
6. Security Considerations.........................................20
6.1. General LDAP Security Considerations..........................20
6.2. StartTLS Security Considerations..............................21
6.3. Bind Operation Security Considerations........................21
6.3.1. Unauthenticated Mechanism Security Considerations...........21
6.3.2. Name/Password Mechanism Security Considerations.............22
6.3.3. Password-related Security Considerations....................22
6.3.4. Hashed Password Security Considerations.....................23
6.4.SASL Security Considerations...................................23
6.5. Related Security Considerations...............................23
7. IANA Considerations.............................................24
8. Acknowledgments.................................................24
9. Normative References............................................24
10. Informative References.........................................25
Author's Address...................................................26
Appendix A. Authentication and Authorization Concepts..............26
A.1. Access Control Policy.........................................26
A.2. Access Control Factors........................................26
A.3. Authentication, Credentials, Identity.........................27
A.4. Authorization Identity........................................27
Appendix B. Summary of Changes.....................................27
B.1. Changes Made to RFC 2251......................................28
B.1.1. Section 4.2.1 (Sequencing of the Bind Request)..............28
B.1.2. Section 4.2.2 (Authentication and Other Security Services)..28
B.2. Changes Made to RFC 2829......................................28
B.2.1. Section 4 (Required security mechanisms)....................29
B.2.2. Section 5.1 (Anonymous authentication procedure)............29
B.2.3. Section 6 (Password-based authentication)...................29
B.2.4. Section 6.1 (Digest authentication).........................29
B.2.5. Section 6.2 ("simple" authentication choice with TLS).......29
B.2.6. Section 6.3 (Other authentication choices with TLS).........29
B.2.7. Section 7.1 (Certificate-based authentication with TLS).....30
B.2.8. Section 8 (Other mechanisms)................................30
B.2.9. Section 9 (Authorization identity)..........................30
B.2.10. Section 10 (TLS Ciphersuites)..............................30
B.3. Changes Made to RFC 2830: ....................................30
B.3.1. Section 3.6 (Server Identity Check).........................30
B.3.2. Section 3.7 (Refresh of Server Capabilities Information)....31
B.3.3. Section 5.2 (Effects of TLS on Authorization Identity)......31
B.3.4. Section 5.1.1 (TLS Closure Effects).........................31
Appendix C. Changes for draft-ldapbis-authmeth-19..................31
Intellectual Property Rights.......................................32
Full Copyright Statement...........................................33
1. Introduction
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The Lightweight Directory Access Protocol (LDAP) [Roadmap] is a
powerful protocol for accessing directories. It offers means of
searching, retrieving 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 LDAP conformance.
Basic threats to an LDAP directory service include (but are not
limited to):
(1) Unauthorized access to directory data via data-retrieval
operations.
(2) Unauthorized access to directory data by monitoring access of
others.
(3) Unauthorized access to reusable client authentication
information by monitoring access of others.
(4) Unauthorized modification of directory data.
(5) Unauthorized modification of configuration information.
(6) Denial of Service: Use of resources (commonly in excess) in a
manner intended to deny service to others.
(7) Spoofing: Tricking a user or 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
transport connection. Tricking a user or client into sending
privileged information to a hostile entity that appears to be
the directory server but is not. Tricking a directory server
into believing that information came from a particular client
when in fact it came from a hostile entity.
(8) Hijacking: An attacker seizes control of an established protocol
session.
Threats (1), (4), (5), (6), (7) are (8) are active attacks. Threats
(2) and (3) are passive attacks.
Threats (1), (4), (5) and (6) are due to hostile clients. Threats
(2), (3), (7) and (8) are due to hostile agents on the path between
client and server or hostile agents posing as a server, e.g., IP
spoofing.
LDAP offers the following security mechanisms:
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(1) Authentication by means of the Bind operation. The Bind
operation provides a simple method which supports anonymous,
unauthenticated, and name/password mechanisms, and the Simple
Authentication and Security Layer (SASL) method which supports a
wide variety of authentication mechanisms.
(2) Mechanisms to support vendor-specific access control facilities
(LDAP does not offer a standard access control facility).
(3) Data integrity service by means of security layers in Transport
Layer Security (TLS) or SASL mechanisms.
(4) Data confidentiality service by means of security layers in TLS
or SASL mechanisms.
(5) Server resource usage limitation by means of administrative
limits configured on the server.
(6) Server authentication by means of the TLS protocol or SASL
mechanisms.
LDAP may also be protected by means outside the LDAP protocol, e.g.,
with IP-level security [RFC4301].
Experience has shown that simply allowing implementations to pick
and choose the security mechanisms that will be implemented is not a
strategy that leads to interoperability. In the absence of
mandates, clients will continue to be written that do not support
any security function supported by the server, or worse, they will
only support mechanisms that provide inadequate security for most
circumstances.
It is desirable to allow clients to authenticate using a variety of
mechanisms including mechanisms where identities are represented as
distinguished names [X.501][Models] in string form [LDAPDN] or as
used in different systems (e.g., simple user names [RFC4013]).
Because some authentication mechanisms transmit credentials in plain
text form, and/or do not provide data security services and/or are
subject to passive attacks, it is necessary to ensure secure
interoperability by identifying a mandatory-to-implement mechanism
for establishing transport-layer security services.
The set of security mechanisms provided in LDAP and described in
this document is intended to meet the security needs for a wide
range of deployment scenarios and still provide a high degree of
interoperability among various LDAP implementations and deployments.
1.1. Relationship to Other Documents
This document is an integral part of the LDAP Technical
Specification [Roadmap].
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This document, together with [Roadmap], [Protocol], and [Models],
obsoletes RFC 2251 in its entirety. Sections 4.2.1 (portions), and
4.2.2 of RFC 2251 are obsoleted by this document. Appendix B.1
summarizes the substantive changes made to RFC 2251 by this document.
This document obsoletes RFC 2829 in its entirety. Appendix B.2
summarizes the substantive changes made to RFC 2829 by this document.
Sections 2 and 4 of RFC 2830 are obsoleted by [Protocol]. The
remainder of RFC 2830 is obsoleted by this document. Appendix B.3
summarizes the substantive changes made to RFC 2830 by this document.
1.2. Conventions
The key words "MUST", "MUST NOT", "SHALL", "SHOULD", "SHOULD NOT",
"MAY", and "OPTIONAL" in this document are to be interpreted as
described in RFC 2119 [RFC2119].
The term "user" represents any human or application entity which is
accessing the directory using a directory client. A directory
client (or client) is also known as a directory user agent (DUA).
The term "transport connection" refers to the underlying transport
services used to carry the protocol exchange, as well as
associations established by these services.
The term "TLS layer" refers to TLS services used in providing
security services, as well as associations established by these
services.
The term "SASL layer" refers to SASL services used in providing
security services, as well as associations established by these
services.
The term "LDAP message layer" refers to the LDAP Message (PDU)
services used in providing directory services, as well as
associations established by these services.
The term "LDAP session" refers to combined services (transport
connection, TLS layer, SASL layer, LDAP message layer) and their
associations.
In general, security terms in this document are used consistently
with the definitions provided in [RFC2828]. In addition, several
terms and concepts relating to security, authentication, and
authorization are presented in Appendix A 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 A before reading the remainder of this document.
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2. Implementation Requirements
LDAP server implementations MUST support the anonymous
authentication mechanism of the simple Bind method (section 5.1.1).
LDAP implementations that support any authentication mechanism other
than the anonymous authentication mechanism of the simple Bind
method MUST support the name/password authentication mechanism of
the simple Bind method (section 5.1.3) and MUST be capable of
protecting this name/password authentication using TLS as
established by the StartTLS operation (section 3).
Implementations SHOULD disallow the use of the name/password
authentication mechanism by default when suitable data security
services are not in place, and MAY provide other suitable data
security services for use with this authentication mechanism.
Implementations MAY support additional authentication mechanisms.
Some of these mechanisms are discussed below.
LDAP server implementations SHOULD support client assertion of
authorization identity via the SASL EXTERNAL mechanism (section
5.2.3).
LDAP server implementations that support no authentication mechanism
other than the anonymous mechanism of the simple bind method SHOULD
support use of TLS as established by the the StartTLS operation
(section 3). (Other servers MUST support TLS per the second
paragraph of this section.)
Implementations supporting TLS MUST support the
TLS_RSA_WITH_3DES_EDE_CBC_SHA ciphersuite and SHOULD support the
TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA ciphersuite. Support for the
latter ciphersuite is recommended to encourage interoperability with
implementations conforming to earlier LDAP StartTLS specifications.
3. StartTLS Operation
The Start Transport Layer Security (StartTLS) operation defined in
section 4.14 of [Protocol] provides the ability to establish TLS
[TLS] in an LDAP session.
The goals of using the TLS [TLS] protocol with LDAP are to ensure
data confidentiality and integrity, and to optionally provide for
authentication. TLS expressly provides these capabilities, although
the authentication services of TLS are available to LDAP only in
combination with the SASL EXTERNAL authentication method (see
section 5.2.3), and then only if the SASL EXTERNAL implementation
chooses to make use of the TLS credentials.
3.1. TLS Establishment Procedures
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This section describes the overall procedures clients and servers
must follow for TLS establishment. These procedures take into
consideration various aspects of the TLS layer including discovery
of resultant security level and assertion of the client's
authorization identity.
3.1.1. StartTLS Request Sequencing
A client may send the StartTLS extended request at any time after
establishing an LDAP session, except:
- when TLS is currently established on the session,
- when a multi-stage SASL negotiation is in progress on the
session, or
- when there are outstanding responses for operation requests
previously issued on the session.
As described in [Protocol] Section 4.14.1, a (detected) violation of
any of these requirements results in a return of the operationsError
resultCode.
Client implementers should ensure that they strictly follow these
operation sequencing requirements to prevent interoperability
issues. Operational experience has shown that violating these
requirements causes interoperability issues because there are race
conditions that prevent servers from detecting some violations of
these requirements due to factors such as server hardware speed and
network latencies.
There is no general requirement that the client have or have not
already performed a Bind operation (section 5) before sending a
StartTLS operation request, however where a client intends to
perform both a Bind operation and a StartTLS operation, it SHOULD
first perform the StartTLS operation so that the Bind request and
response messages are protected by the data security services
established by the StartTLS operation.
3.1.2. Client Certificate
If an LDAP server requests or demands that a client provide a user
certificate during TLS negotiation and the client does not present a
suitable user certificate (e.g., one that can be validated), the
server may use a local security policy to determine whether to
successfully complete TLS negotiation.
If a client that has provided a suitable certificate subsequently
performs a Bind operation using the SASL EXTERNAL authentication
mechanism (section 5.2.3), information in the certificate may be
used by the server to identify and authenticate the client.
3.1.3. Server Identity Check
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In order to prevent man-in-the-middle attacks the client MUST verify
the server's identity (as presented in the server's Certificate
message). In this section, the client's understanding of the
server's identity (typically the identity used to establish the
transport connection) is called the "reference identity".
The client determines the type (e.g., DNS name or IP address) of the
reference identity and performs a comparison between the reference
identity and each subjectAltName value of the corresponding type
until a match is produced. Once a match is produced, the server's
identity has been verified and the server identity check is
complete. Different subjectAltName types are matched in different
ways. Sections 3.1.3.1 - 3.1.3.3 explain how to compare values of
various subjectAltName types.
The client may map the reference identity to a different type prior
to performing a comparison. Mappings may be performed for all
available subjectAltName types to which the reference identity can
be mapped, however the reference identity should only be mapped to
types for which the mapping is either inherently secure (e.g.,
extracting the DNS name from a URI to compare with a subjectAltName
of type dNSName) or for which the mapping is performed in a secure
manner (e.g., using DNSSec, or using user- (or admin-) configured
host-to-address/address-to-host lookup tables).
The server's identity may also be verified by comparing the
reference identity to the Common Name (CN) [Schema] value in the
leaf RDN of the subjectName field of the server's certificate. This
comparison is performed using the rules for comparison of DNS names
in section 3.1.3.1 below, with the exception that no wildcard
matching is allowed. Although the use of the Common Name value is
existing practice, it is deprecated and Certification Authorities
are encouraged to provide subjectAltName values instead. Note that
the TLS implementation may represent DNs in certificates according
to X.500 or other conventions. For example, some X.500
implementations order the RDNs in a DN using a left-to-right (most
significant to least significant) convention instead of LDAP's
right-to-left convention.
If the server identity check fails, user-oriented clients SHOULD
either notify the user (clients may give the user the opportunity to
continue with the LDAP session in this case) or close the transport
connection and indicate that the server's identity is suspect.
Automated clients SHOULD close the transport connection and then
return or log an error indicating that the server's identity is
suspect or both.
Beyond the server identity check 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 requested to provide.
The client may need to make use of local policy information in
making this determination.
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3.1.3.1. Comparison of DNS Names
If the reference identity is an internationalized domain name,
conforming implementations MUST convert it to the ASCII Compatible
Encoding (ACE) format as specified in section 4 of RFC 3490
[RFC3490] before comparison with subjectAltName values of type
dNSName. Specifically, conforming implementations MUST perform the
conversion operation specified in section 4 of RFC 3490 as follows:
* in step 1, the domain name SHALL be considered a "stored
string";
* in step 3, set the flag called "UseSTD3ASCIIRules";
* in step 4, process each label with the "ToASCII"
operation; and
* in step 5, change all label separators to U+002E (full
stop).
After performing the "to-ASCII" conversion, the DNS labels and names
MUST be compared for equality according to the rules specified in
section 3 of RFC3490.
The '*' (ASCII 42) wildcard character is allowed in subjectAltName
values of type dNSName and then only as the left-most (least
significant) DNS label in that value. This wildcard matches any
left-most DNS label in the server name. That is, the subject
*.example.com matches the server names a.example.com and
b.example.com but does not match example.com or a.b.example.com.
3.1.3.2. Comparison of IP Addresses
When the reference identity is an IP address, the identity MUST be
converted to the "network byte order" octet string representation
[RFC791][RFC2460]. For IP Version 4, as specified in RFC 791, the
octet string will contain exactly four octets. For IP Version 6, as
specified in RFC 2460, the octet string will contain exactly sixteen
octets. This octet string is then compared against subjectAltName
values of type iPAddress. A match occurs if the reference identity
octet string and value octet strings are identical.
3.1.3.3. Comparison of other subjectName types
Client implementations MAY support matching against subjectAltName
values of other types as described in other documents.
3.1.4. Discovery of Resultant Security Level
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After a TLS layer is established in an LDAP session, both parties
are to each independently decide whether or not to continue based on
local policy and the security level achieved. If either party
decides that the security level is inadequate for it to continue, it
SHOULD remove the TLS layer immediately after the TLS
(re)negotiation has completed (see [Protocol] section 4.14.3 and
section 3.2 below). Implementations may reevaluate the security
level at any time and, upon finding it inadequate, should remove the
TLS layer.
3.1.5. Refresh of Server Capabilities Information
After a TLS layer is established in an LDAP session, the client
SHOULD discard or refresh all information about the server it
obtained prior to the initiation of the TLS negotiation and not
obtained through secure mechanisms. This protects against man-in-
the-middle attacks that may have altered any server capabilities
information retrieved prior to TLS layer installation.
The server may advertise different capabilities after installing a
TLS layer. In particular, the value of 'supportedSASLMechanisms'
may be different after a TLS layer has been installed (specifically,
the EXTERNAL and PLAIN [PLAIN] mechanisms are likely to be listed
only after a TLS layer has been installed).
3.2. Effect of TLS on Authorization State
The establishment, change, and/or closure of TLS may cause the
authorization state to move to a new state. This is discussed
further in Section 4.
3.3. TLS Ciphersuites
Several issues should be considered when selecting TLS ciphersuites
that are appropriate for use in a given circumstance. These issues
include the following:
- The ciphersuite's ability to provide adequate confidentiality
protection for passwords and other data sent over the transport
connection. Client and server implementers should recognize
that some TLS ciphersuites provide no confidentiality protection
while other ciphersuites that do provide confidentiality
protection may be vulnerable to being cracked using brute force
methods, especially in light of ever-increasing CPU speeds that
reduce the time needed to successfully mount such attacks.
- Client and server implementers should carefully consider the
value of the password or data being protected versus the level
of confidentiality protection provided by the ciphersuite to
ensure that the level of protection afforded by the ciphersuite
is appropriate.
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- The ciphersuite's vulnerability (or lack thereof) to man-in-the-
middle attacks. Ciphersuites vulnerable to man-in-the-middle
attacks SHOULD NOT be used to protect passwords or sensitive
data, unless the network configuration is such that the danger
of a man-in-the-middle attack is negligible.
- After a TLS negotiation (either initial or subsequent) is
completed, both protocol peers should independently verify that
the security services provided by the negotiated ciphersuite are
adequate for the intended use of the LDAP session. If not, the
TLS layer should be closed.
4. Authorization State
Every LDAP session has an associated authorization state. This
state is comprised of numerous factors such as what (if any)
authorization state has been established, how it was established,
and what security services are in place. Some factors may be
determined and/or affected by protocol events (e.g., Bind, StartTLS,
or TLS closure), and some factors may be determined by external
events (e.g., time of day or server load).
While it is often convenient to view authorization state in
simplistic terms (as we often do in this technical specification)
such as "an anonymous state", it is noted that authorization systems
in LDAP implementations commonly involve many factors which
interrelate in complex manners.
Authorization in LDAP is a local matter. One of the key factors in
making authorization decisions is authorization identity. The Bind
operation defined in section 4.2 of [Protocol] and discussed further
in section 5 below allows information to be exchanged between the
client and server to establish an authorization identity for the
LDAP session. The Bind operation may also be used to move the LDAP
session to an anonymous authorization state (see section 5.1.1).
Upon initial establishment of the LDAP session, the session has an
anonymous authorization identity. Among other things this implies
that the client need not send a BindRequest in the first PDU of the
LDAP message layer. The client may send any operation request prior
to performing a Bind operation, and the server MUST treat it as if
it had been performed after an anonymous Bind operation (section
5.1.1).
Upon receipt of a Bind request, the server immediately moves the
session to an anonymous authorization state. If the Bind request is
successful, the session is moved to the requested authentication
state with its associated authorization state. Otherwise, the
session remains in an anonymous state.
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It is noted that other events both internal and external to LDAP may
result in the authentication and authorization states being moved to
an anonymous one. For instance, the establishment, change or
closure of data security services may result in a move to an
anonymous state, or the user's credential information (e.g.,
certificate) may have expired. The former is an example of an event
internal to LDAP whereas the latter is an example of an event
external to LDAP.
5. Bind Operation
The Bind operation ([Protocol] section 4.2) allows authentication
information to be exchanged between the client and server to
establish a new authorization state.
The Bind request typically specifies the desired authentication
identity. Some Bind mechanisms also allow the client to specify the
authorization identity. If the authorization identity is not
specified, the server derives it from the authentication identity in
an implementation-specific manner.
If the authorization identity is specified, the server MUST verify
that the client's authentication identity is permitted to assume
(e.g., proxy for) 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.
5.1. Simple Authentication Method
The simple authentication method of the Bind Operation provides
three authentication mechanisms:
- An anonymous authentication mechanism (section 5.1.1).
- An unauthenticated authentication mechanism (section 5.1.2).
- A name/password authentication mechanism using credentials
consisting of a name (in the form of an LDAP distinguished name
[LDAPDN]) and a password (section 5.1.3).
5.1.1. Anonymous Authentication Mechanism of Simple Bind
An LDAP client may use the anonymous authentication mechanism of the
simple Bind method to explicitly establish an anonymous
authorization state by sending a Bind request with a name value of
zero length and specifying the simple authentication choice
containing a password value of zero length.
5.1.2. Unauthenticated Authentication Mechanism of Simple Bind
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An LDAP client may use the unauthenticated authentication mechanism
of the simple Bind method to establish an anonymous authorization
state by sending a Bind request with a name value (a distinguished
name in LDAP string form [LDAPDN] of non-zero length) and specifying
the simple authentication choice containing a password value of zero
length.
The distinguished name value provided by the client is intended to
be used for trace (e.g., logging) purposes only. The value is not
to be authenticated or otherwise validated (including verification
that the DN refers to an existing directory object). The value is
not to be used (directly or indirectly) for authorization purposes.
Unauthenticated Bind operations can have significant security issues
(see section 6.3.1). In particular, users intending to perform
Name/Password Authentication may inadvertently provide an empty
password and thus cause poorly implemented clients to request
Unauthenticated access. Clients SHOULD be implemented to require
user selection of the Unauthenticated Authentication Mechanism by
means other than user input of an empty password. Clients SHOULD
disallow an empty password input to a Name/Password Authentication
user interface. Additionally, Servers SHOULD by default fail
Unauthenticated Bind requests with a resultCode of
unwillingToPerform.
5.1.3. Name/Password Authentication Mechanism of Simple Bind
An LDAP client may use the name/password authentication mechanism of
the simple Bind method to establish an authenticated authorization
state by sending a Bind request with a name value (a distinguished
name in LDAP string form [LDAPDN] of non-zero length) and specifying
the simple authentication choice containing an OCTET STRING password
value of non-zero length.
Servers that map the DN sent in the Bind request to a directory
entry with an associated set of one or more passwords used with this
mechanism will compare the presented password to that set of
passwords. The presented password is considered valid if it matches
any member of this set.
A resultCode of invalidDNSyntax indicates that the DN sent in the
name value is syntactically invalid. A resultCode of
invalidCredentials indicates that the DN is syntactically correct
but not valid for purposes of authentication, or the password is not
valid for the DN or the server otherwise considers the credentials
to be invalid. A resultCode of success indicates that the
credentials are valid and the server is willing to provide service
to the entity these credentials identify.
Server behavior is undefined for Bind requests specifying the
name/password authentication mechanism with a zero-length name value
and a password value of non-zero length.
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The name/password authentication mechanism of the simple Bind method
is not suitable for authentication in environments without
confidentiality protection.
5.2. SASL Authentication Method
The sasl authentication method of the Bind Operation provides
facilities for using any SASL mechanism including authentication
mechanisms and other services (e.g., data security services).
5.2.1. SASL Protocol Profile
LDAP allows authentication via any SASL mechanism [SASL]. As LDAP
includes native anonymous and name/password (plain text)
authentication methods, the ANONYMOUS [ANONYMOUS] and PLAIN [PLAIN]
SASL mechanisms are typically not used with LDAP.
Each protocol that utilizes SASL services is required to supply
certain information profiling the way they are exposed through the
protocol ([SASL] section 4). This section explains how each of
these profiling requirements are met by LDAP.
5.2.1.1. SASL Service Name for LDAP
The SASL service name for LDAP is "ldap", which has been registered
with the IANA as a SASL service name.
5.2.1.2. SASL Authentication Initiation and Protocol Exchange
SASL authentication is initiated via a BindRequest message
([Protocol] section 4.2) with the following parameters:
- The version is 3.
- The AuthenticationChoice is sasl.
- The mechanism element of the SaslCredentials sequence contains
the value of the desired SASL mechanism.
- The optional credentials field of the SaslCredentials sequence
MAY be used to provide an initial client response for
mechanisms that are defined to have the client send data first
(see [SASL] sections 3 and 5 ).
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In general, a SASL authentication protocol exchange consists of a
series of server challenges and client responses, the contents of
which are specific to and defined by the SASL mechanism. Thus for
some SASL authentication mechanisms, it may be necessary for the
client to respond to one or more server challenges by sending
BindRequest messages multiple times. A challenge is indicated by
the server sending a BindResponse message with the resultCode set to
saslBindInProgress. This indicates that the server requires the
client to send a new BindRequest message with the same SASL
mechanism to continue the authentication process.
To the LDAP message layer, these challenges and responses are opaque
binary tokens of arbitrary length. LDAP servers use the
serverSaslCreds field (an OCTET STRING) in a BindResponse message to
transmit each challenge. LDAP clients use the credentials field
(an OCTET STRING) in the SaslCredentials sequence of a BindRequest
message to transmit each response. Note that unlike some Internet
protocols where SASL is used, LDAP is not text based and does not
Base64-transform these challenge and response values.
Clients sending a BindRequest message with the sasl choice selected
SHOULD send a zero-length value in the name field. Servers
receiving a BindRequest message with the sasl choice selected SHALL
ignore any value in the name field.
A client may abort a SASL Bind negotiation by sending a BindRequest
message with a different value in the mechanism field of
SaslCredentials or with 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 a
resultCode of authMethodNotSupported. This will allow the client to
abort a negotiation if it wishes to try again with the same SASL
mechanism.
The server indicates completion of the SASL challenge-response
exchange by responding with a BindResponse in which the resultCode
value is not saslBindInProgress.
The serverSaslCreds field in the BindResponse can be used to include
an optional challenge with a success notification for mechanisms
which are defined to have the server send additional data along with
the indication of successful completion.
5.2.1.3. Optional Fields
As discussed above, LDAP provides an optional field for carrying an
initial response in the message initiating the SASL exchange and
provides an optional field for carrying additional data in the
message indicating outcome of the authentication exchange. As the
mechanism-specific content in these fields may be zero-length, SASL
requires protocol specifications to detail how an empty field is
distinguished from an absent field.
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Zero-length initial response data is distinguished from no initial
response data in the initiating message, a BindRequest PDU, by the
presence of the SaslCredentials.credentials OCTET STRING (of length
zero) in that PDU. If the client does not intend to send an
initial response with the BindRequest initiating the SASL exchange,
it MUST omit the SaslCredentials.credentials OCTET STRING (rather
than including an zero-length OCTET STRING).
Zero-length additional data is distinguished from no additional
response data in the outcome message, a BindResponse PDU, by the
presence of the serverSaslCreds OCTET STRING (of length zero) in
that PDU. If a server does not intend to send additional data in
the BindResponse message indicating outcome of the exchange, the
server SHALL omit the serverSaslCreds OCTET STRING (rather than
including a zero-length OCTET STRING).
5.2.1.4. Octet Where Negotiated Security Layers Take Effect
SASL layers take effect following the transmission by the server and
reception by the client of the final BindResponse in the SASL
exchange with a resultCode of success.
Once a SASL layer providing data integrity or confidentiality
services takes effect, the layer remains in effect until a new layer
is installed (i.e. at the first octet following the final
BindResponse of the Bind operation that caused the new layer to take
effect). Thus, an established SASL layer is not affected by a
failed or non-SASL Bind.
5.2.1.5. Determination of Supported SASL Mechanisms
Clients may determine the SASL mechanisms a server supports by
reading the 'supportedSASLMechanisms' attribute from the root DSE
(DSA-Specific Entry) ([Models] section 5.1). The values of this
attribute, if any, list the mechanisms the server supports in the
current LDAP session state. LDAP servers SHOULD allow all clients--
even those with an anonymous authorization--to retrieve the
'supportedSASLMechanisms' attribute of the root DSE both before and
after the SASL authentication exchange. The purpose of the latter
is to allow the client to detect possible downgrade attacks (see
section 6.4 and [SASL] section 6.1.2).
Because SASL mechanisms provide critical security functions, clients
and servers should be configurable to specify what mechanisms are
acceptable and allow only those mechanisms to be used. Both clients
and servers must confirm that the negotiated security level meets
their requirements before proceeding to use the session.
5.2.1.6. Rules for Using SASL Layers
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Upon installing a SASL layer, the client SHOULD discard or refresh
all information about the server it obtained prior to the initiation
of the SASL negotiation and not obtained through secure mechanisms.
If a lower level security layer (such as TLS) is installed, any SASL
layer SHALL be layered on top of such security layers regardless of
the order of their negotiation. In all other respects, the SASL
layer and other security layers act independently, e.g., if both a
TLS layer and a SASL layer are in effect then removing the TLS layer
does not affect the continuing service of the SASL layer.
5.2.1.7. Support for Multiple Authentications
LDAP supports multiple SASL authentications as defined in [SASL]
section 4.
5.2.1.8. SASL Authorization Identities
Some SASL mechanisms allow clients to request a desired
authorization identity for the LDAP session ([SASL] section 3.4.
The decision to allow or disallow the current authentication
identity to have access to the requested authorization identity is a
matter of local policy. The authorization identity is a string of
UTF-8 [RFC3629] encoded [Unicode] characters corresponding to the
following ABNF [RFC2234bis] grammar:
authzId = dnAuthzId / uAuthzId
; distinguished-name-based authz id
dnAuthzId = "dn:" distinguishedName
; unspecified authorization id, UTF-8 encoded
uAuthzId = "u:" userid
userid = *UTF8 ; syntax unspecified
where the distinguishedName rule is defined in section 3 of [LDAPDN]
and the UTF8 rule is defined in section 1.4 of [Models].
The dnAuthzId choice is used to assert authorization identities in
the form of a distinguished name to be matched in accordance with
the distinguishedNameMatch matching rule [Syntaxes]. There is no
requirement that the asserted distinguishedName value be that of an
entry in the directory.
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The uAuthzId choice allows clients to assert an authorization
identity that is not in distinguished name form. The format of
userid is defined only as a sequence of UTF-8 [RFC3629] encoded
[Unicode] characters, and any further interpretation is a local
matter. For example, the userid could identify a user of a specific
directory service, be a login name or be an email address. A
uAuthzId SHOULD NOT be assumed to be globally unique. To compare
uAuthzID values, each uAuthzID value MUST be prepared as a "query"
string using [RFC4013] and then the two values are compared octet-
wise.
The above grammar is extensible. The authzId production may be
extended to support additional forms of identities. Each form is
distinguished by its unique prefix (see section 3.12 of [LDAPIANA]
for registration requirements).
5.2.2. SASL Semantics Within LDAP
Implementers must take care to maintain the semantics of SASL
specifications when handling data that has different semantics in
the LDAP protocol.
For example, the SASL DIGEST-MD5 authentication mechanism [RFC2829]
utilizes an authentication identity and a realm which are
syntactically simple strings and semantically simple username and
realm values ([DIGEST-MD5] section 2.1). These values are not LDAP
DNs, and there is no requirement that they be represented or treated
as such.
5.2.3. SASL EXTERNAL Authentication Mechanism
A client can use the SASL EXTERNAL [SASL] mechanism to request the
LDAP server to authenticate and establish a resulting authorization
identity using security credentials exchanged by a lower security
layer (such as by TLS authentication). If the client's
authentication credentials have not been established at a lower
security layer, the SASL EXTERNAL Bind MUST fail with a resultCode
of inappropriateAuthentication. Although this situation has the
effect of leaving the LDAP session in an anonymous state (section
4), the state of any installed security layer is unaffected.
A client may either request that its authorization identity be
automatically derived from its authentication credentials exchanged
at a lower security layer or it may explicitly provide a desired
authorization identity. The former is known as an implicit
assertion, and the latter as an explicit assertion.
5.2.3.1. Implicit Assertion
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An implicit authorization identity assertion is performed by
invoking a Bind request of the SASL form using the EXTERNAL
mechanism name that does not include the optional credentials field
(found within the SaslCredentials sequence in the BindRequest). The
server will derive the client's authorization identity from the
authentication identity supplied by a security layer (e.g., a public
key certificate used during TLS layer installation) according to
local policy. The underlying mechanics of how this is accomplished
are implementation specific.
5.2.3.2. Explicit Assertion
An explicit authorization identity assertion is performed by
invoking a Bind request of the SASL form using the EXTERNAL
mechanism name that includes the credentials field (found within the
SaslCredentials sequence in the BindRequest). The value of the
credentials field (an OCTET STRING) is the asserted authorization
identity and MUST be constructed as documented in section 5.2.1.8.
6. Security Considerations
Security issues are discussed throughout this document. The
unsurprising conclusion is that security is an integral and
necessary part of LDAP. This section discusses a number of LDAP-
related security considerations.
6.1. General LDAP Security Considerations
LDAP itself provides no security or protection from accessing or
updating the directory by other means than through the LDAP
protocol, e.g., from inspection of server database files by database
administrators.
Sensitive data may be carried in almost any LDAP message and its
disclosure may be subject to privacy laws or other legal regulation
in many countries. Implementers should take appropriate measures to
protect sensitive data from disclosure to unauthorized entities.
A session on which the client has not established data integrity and
privacy services (e.g., via StartTLS, IPsec or a suitable SASL
mechanism) is subject to man-in-the-middle attacks to view and
modify information in transit. Client and server implementers
SHOULD take measures to protect sensitive data in the LDAP session
from these attacks by using data protection services as discussed in
this document. Clients and servers should provide the ability to be
configured to require these protections. A resultCode of
confidentialityRequired indicates that the server requires
establishment of (stronger) data confidentiality protection in order
to perform the requested operation.
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Access control should always be applied when reading sensitive
information or updating directory information.
Various security factors, including authentication and authorization
information and data security services may change during the course
of the LDAP session, or even during the performance of a particular
operation. Implementations should be robust in the handling of
changing security factors.
6.2. StartTLS Security Considerations
All security gained via use of the StartTLS operation is gained by
the use of TLS itself. The StartTLS operation, on its own, does not
provide any additional security.
The level of security provided through the use of TLS depends
directly on both the quality of the TLS implementation used and the
style of usage of that implementation. Additionally, a man-in-the-
middle attacker can remove the StartTLS extended operation from the
'supportedExtension' attribute of the root DSE. Both parties SHOULD
independently ascertain and consent to the security level achieved
once TLS is established and before beginning use of the TLS-
protected session. For example, the security level of the TLS layer
might have been negotiated down to plaintext.
Clients MUST either warn the user when the security level achieved
does not provide an acceptable level of data confidentiality and/or
data integrity protection, or be configurable to refuse to proceed
without an acceptable level of security.
As stated in section 3.1.2, a server may use a local security policy
to determine whether to successfully complete TLS negotiation.
Information in the user's certificate that is originated or verified
by the certification authority should be used by the policy
administrator when configuring the identification and authorization
policy.
Server implementers SHOULD allow server administrators to elect
whether and when data confidentiality and integrity are required, as
well as elect whether authentication of the client during the TLS
handshake is required.
Implementers should be aware of and understand TLS security
considerations as discussed in the TLS specification [TLS].
6.3. Bind Operation Security Considerations
This section discusses several security considerations relevant to
LDAP authentication via the Bind operation.
6.3.1. Unauthenticated Mechanism Security Considerations
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Operational experience shows that clients can (and frequently do)
misuse the unauthenticated authentication mechanism of the simple
Bind method (see section 5.1.2). For example, a client program
might make a decision to grant access to non-directory information
on the basis of successfully completing a Bind operation. LDAP
server implementations may return a success response to an
unauthenticated Bind request. This may erroneously leave the client
with the impression that the server has successfully authenticated
the identity represented by the distinguished name when in reality,
an anonymous authorization state has been established. Clients that
use the results from a simple Bind operation to make authorization
decisions should actively detect unauthenticated Bind requests (by
verifying that the supplied password is not empty) and react
appropriately.
6.3.2. Name/Password Mechanism Security Considerations
The name/password authentication mechanism of the simple Bind method
discloses the password to the server, which is an inherent security
risk. There are other mechanisms such as SASL DIGEST-MD5 [RFC2829]
that do not disclose the password to the server.
6.3.3. Password-related Security Considerations
LDAP allows multi-valued password attributes. In systems where
entries are expected to have one and only one password,
administrative controls should be provided to enforce this behavior.
The use of clear text passwords and other unprotected authentication
credentials is strongly discouraged over open networks when the
underlying transport service cannot guarantee confidentiality. LDAP
implementations SHOULD NOT by default support authentication methods
using clear text passwords and other unprotected authentication
credentials unless the data on the session is protected using TLS or
other data confidentiality and data integrity protection.
The transmission of passwords in the clear--typically for
authentication or modification--poses a significant security risk.
This risk can be avoided by using SASL authentication [SASL]
mechanisms that do not transmit passwords in the clear or by
negotiating transport or session layer data confidentiality services
before transmitting password values.
To mitigate the security risks associated with the transfer of
passwords, a server implementation that supports any password-based
authentication mechanism that transmits passwords in the clear MUST
support a policy mechanism that at the time of authentication or
password modification, requires:
A TLS layer has been successfully installed.
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OR
Some other data confidentiality mechanism that protects the
password value from eavesdropping has been provided.
OR
The server returns a resultCode of confidentialityRequired for
the operation (i.e. name/password Bind with password value,
SASL Bind transmitting a password value in the clear, add or
modify including a userPassword value, etc.), even if the
password value is correct.
Server implementations may also want to provide policy mechanisms to
invalidate or otherwise protect accounts in situations where a
server detects that a password for an account has been transmitted
in the clear.
6.3.4. Hashed Password Security Considerations
Some authentication mechanisms (e.g., DIGEST-MD5) transmit a hash of
the password value that may be vulnerable to offline dictionary
attacks. Implementers should take care to protect such hashed
password values during transmission using TLS or other
confidentiality mechanisms.
6.4.SASL Security Considerations
Until data integrity service is installed on an LDAP session, an
attacker can modify the transmitted values of the
'supportedSASLMechanisms' attribute response and thus downgrade the
list of available SASL mechanisms to include only the least secure
mechanism. To detect this type of attack, the client may retrieve
the SASL mechanisms the server makes available both before and after
data integrity service is installed on an LDAP session. If the
client finds that the integrity-protected list (the list obtained
after data integrity service was installed) contains a stronger
mechanism than those in the previously obtained list, the client
should assume the previously obtained list was modified by an
attacker. In this circumstance it is recommended that the client
close the underlying transport connection and then reconnect to
reestablish the session.
6.5. Related Security Considerations
Additional security considerations relating to the various
authentication methods and mechanisms discussed in this document
apply and can be found in [SASL], [RFC4013], [StringPrep] and
[RFC3629].
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7. IANA Considerations
It is requested that the IANA update the LDAP Protocol Mechanism
registry to indicate that this document and [Protocol] provide the
definitive technical specification for the StartTLS
(1.3.6.1.4.1.1466.20037) extended operation.
8. Acknowledgments
This document combines information originally contained in RFC 2251,
RFC 2829 and RFC 2830 which are products of the LDAP Extensions
(LDAPEXT) Working Group.
This document is a product of the IETF LDAP Revision (LDAPBIS)
working group.
9. Normative References
[[Note to the RFC Editor: please replace the citation tags used in
referencing Internet-Drafts with tags of the form RFCnnnn.]]
[LDAPDN] Zeilenga, Kurt D. (editor), "LDAP: String
Representation of Distinguished Names", draft-ietf-
ldapbis-dn-xx.txt, a work in progress.
[LDAPIANA] Zeilenga, K., "IANA Considerations for LDAP", draft-
ietf-ldapbis-bcp64-xx.txt, (a work in progress).
[Matching] Hoffman, Paul and Steve Hanna, "Matching Text Strings
in PKIX Certificates", draft-hoffman-pkix-stringmatch-
xx.txt, a work in progress.
[Models] Zeilenga, Kurt D. (editor), "LDAP: Directory
Information Models", draft-ietf-ldapbis-models-xx.txt,
a work in progress.
[Protocol] Sermersheim, J., "LDAP: The Protocol", draft-ietf-
ldapbis-protocol-xx.txt, a work in progress.
[RFC791] Information Sciences Institute, "INTERNET PROTOCOL
DARPA INTERNET PROGRAM PROTOCOL SPECIFICATION", RFC
791, September 1981.
[RFC2119] Bradner, S., "Key Words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2234bis] Crocker, D., Ed. and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", draft-crocker-abnf-
rfc2234bis-xx, a work in progress.
[RFC2460] Deering, S., R. Hinden, "Internet Protocol, Version 6
(IPv6)", RFC 2460, December 1998.
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[RFC3490] Falstrom, P., P. Hoffman, and A. Costello,
"Internationalizing Domain Names In Applications
(IDNA)", RFC 3490, March 2003.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 3629, STD 63, November 2003.
[RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User
Names and Passwords", RFC 4013, February 2005.
[Roadmap] K. Zeilenga, "LDAP: Technical Specification Road Map",
draft-ietf-ldapbis-roadmap-xx.txt, a work in progress.
[SASL] Melnikov, A. (editor), "Simple Authentication and
Security Layer (SASL)", draft-ietf-sasl-rfc2222bis-
xx.txt, a work in progress.
[Schema] Dally, K. (editor), "LDAP: User Schema", draft-ietf-
ldapbis-user-schema-xx.txt, a work in progress.
[StringPrep] M. Blanchet, "Preparation of Internationalized Strings
('stringprep')", draft-hoffman-rfc3454bis-xx.txt, a
work in progress.
[Syntaxes] Legg, S. (editor), "LDAP: Syntaxes and Matching Rules",
draft-ietf-ldapbis-syntaxes-xx.txt, a work in progress.
[TLS] Dierks, T. and C. Allen. "The TLS Protocol Version
1.1", draft-ietf-tls-rfc2246-bis-xx.txt, a work in
progress.
[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/).
10. Informative References
[[Note to the RFC Editor: please replace the citation tags used in
referencing Internet-Drafts with tags of the form RFCnnnn.]]
[ANONYMOUS] Zeilenga, K., "Anonymous SASL Mechanism", draft-
zeilenga-sasl-anon-xx.txt, a work in progress.
[DIGEST-MD5] Leach, P. C. Newman, and A. Melnikov, "Using Digest
Authentication as a SASL Mechanism", draft-ietf-sasl-
rfc2831bis-xx.txt, a work in progress.
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[PLAIN] Zeilenga, K.,"Plain SASL Mechanism", draft-zeilenga-
sasl-plain-xx.txt, a work in progress.
[RFC2828] Shirey, R., "Internet Security Glossary", RFC 2828, May
2000.
[RFC2829] Wahl, M. et al, "Authentication Methods for LDAP", RFC
2829, May 2000.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
Author's Address
Roger Harrison
Novell, Inc.
1800 S. Novell Place
Provo, UT 84606
USA
+1 801 861 2642
roger_harrison@novell.com
Appendix A. Authentication and Authorization Concepts
This appendix is non-normative.
This appendix defines basic terms, concepts, and interrelationships
regarding authentication, authorization, credentials, and identity.
These concepts are used in describing how various security
approaches are utilized in client authentication and authorization.
A.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. Security objects and
mechanisms, such as those described here, enable the expression of
access control policies and their enforcement.
A.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 ([Protocol] section
4.2). 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,
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 transport connection via which the request is
transmitted, others (e.g., time of day) may be "environmental".
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Access control policies are expressed in terms of access control
factors. For example, "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.
A.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 a new authorization state 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. X.509 certificates, Kerberos tickets, and simple
identity and password pairs are all examples of authentication
credential forms. Note that an authentication mechanism may
constrain the form of authentication identities used with it.
A.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 in terms of authorization identities; for example, "entity
X can perform operation Y on resource Z."
The authorization identity of an LDAP session is often semantically
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 B. Summary of Changes
This appendix is non-normative.
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This appendix summarizes substantive changes made to RFC 2251, RFC
2829 and RFC 2830. In addition to the specific changes detailed
below, the reader of this document should be aware that numerous
general editorial changes have been made to the original content
from the source documents. These changes include the following:
- The material originally found in RFC 2251 sections 4.2.1 and
4.2.2, RFC 2829 (all sections except sections 2 and 4) and RFC
2830 was combined into a single document
- The combined material was substantially reorganized and edited
to group related subjects, improve the document flow and clarify
intent.
- Changes were made throughout the text to align with definitions
of LDAP protocol layers and IETF security terminology.
- Substantial updates and additions were made to security
considerations from both documents based on current operational
experience.
B.1. Changes Made to RFC 2251
This section summarizes the substantive changes made to sections
4.2.1 and 4.2.2 of RFC 2251 by this document. Additional
substantive changes to section 4.2.1 of RFC 2251 are also documented
in [Protocol].
B.1.1. Section 4.2.1 (Sequencing of the Bind Request)
- Paragraph 1: Removed the sentence, "If at any stage the client
wishes to abort the bind process it MAY unbind and then drop the
underlying connection." The Unbind operation still permits this
behavior, but it is not documented explicitly.
- Clarified that the session is moved to an anonymous state upon
receipt of the BindRequest PDU and that it is only moved to a
non-anonymous state if and when the Bind request is successful.
B.1.2. Section 4.2.2 (Authentication and Other Security Services)
- RFC 2251 states that anonymous authentication MUST be performed
using the simple bind method. This specification defines the
anonymous authentication mechanism of the simple bind method and
requires all conforming implementations to support it. Other
authentication mechanisms producing anonymous authentication and
authorization state may also be implemented and used by
conforming implementations.
B.2. Changes Made to RFC 2829
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This section summarizes the substantive changes made to RFC 2829.
B.2.1. Section 4 (Required security mechanisms)
- The name/password authentication mechanism (see section B.2.5
below) protected by TLS replaces the SASL DIGEST-MD5 mechanism
as LDAP's mandatory-to-implement password-based authentication
mechanism. Implementations are encouraged to continue
supporting SASL DIGEST-MD5 [RFC2829].
B.2.2. Section 5.1 (Anonymous authentication procedure)
- Clarified that anonymous authentication involves a name value of
zero length and a password value of zero length. The
unauthenticated authentication mechanism was added to handle
simple Bind requests involving a name value with a non-zero
length and a password value of zero length.
B.2.3. Section 6 (Password-based authentication)
- See section B.2.1.
B.2.4. Section 6.1 (Digest authentication)
- As the SASL-DIGEST-MD5 mechanism is no longer mandatory to
implement, this section is now historical and was not included
in this document. RFC 2829 section 6.1 continues to document the
SASL DIGEST-MD5 authentication mechanism.
B.2.5. Section 6.2 ("simple" authentication choice with TLS)
- Renamed the "simple" authentication mechanism to the
name/password authentication mechanism to better describe it.
- The use of TLS was generalized to align with definitions of LDAP
protocol layers. TLS establishment is now discussed as an
independent subject and is generalized for use with all
authentication mechanisms and other security layers.
- Removed the implication that the userPassword attribute is the
sole location for storage of password values to be used in
authentication. There is no longer any implied requirement for
how or where passwords are stored at the server for use in
authentication.
B.2.6. Section 6.3 (Other authentication choices with TLS)
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Internet-Draft LDAP Authentication Methods February 2006
- See section B.2.5.
B.2.7. Section 7.1 (Certificate-based authentication with TLS)
- See section B.2.5.
B.2.8. Section 8 (Other mechanisms)
- All SASL authentication mechanisms are explicitly allowed within
LDAP. Specifically, this means the SASL ANONYMOUS and SASL PLAIN
mechanisms are no longer precluded from use within LDAP.
B.2.9. Section 9 (Authorization identity)
- Specified matching rules for dnAuthzID and uAuthzID values. In
particular, the DN value in the dnAuthzID form must be matched
using DN matching rules and the uAuthzID value MUST be prepared
using SASLprep rules before being compared octet-wise.
- Clarified that uAuthzID values should not be assumed to be
globally unique.
B.2.10. Section 10 (TLS Ciphersuites)
- TLS Ciphersuite recommendations are no longer included in this
specification. Implementations must still support the
TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA ciphersuite.
- Clarified that anonymous authentication involves a name value of
zero length and a password value of zero length. The
unauthenticated authentication mechanism was added to handle
simple Bind requests involving a name value with a non-zero
length and a password value of zero length.
B.3. Changes Made to RFC 2830:
This section summarizes the substantive changes made to sections 3
and 5 of RFC 2830. Readers should consult [Protocol] for summaries
of changes to other sections.
B.3.1. Section 3.6 (Server Identity Check)
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- Substantially updated the server identity check algorithm to
ensure that it is complete and robust. In particular, the use
of all relevant values in the subjectAltName and the subjectName
fields are covered by the algorithm and matching rules are
specified for each type of value. Mapped (derived) forms of the
server identity may now be used when the mapping is performed in
a secure fashion.
B.3.2. Section 3.7 (Refresh of Server Capabilities Information)
- Clients are no longer required to always refresh information
about server capabilities following TLS establishment to allow
for situations where this information was obtained through a
secure mechanism.
B.3.3. Section 5.2 (Effects of TLS on Authorization Identity)
- Establishing a TLS layer on an LDAP session may now cause the
authorization state of the LDAP session to change.
B.3.4. Section 5.1.1 (TLS Closure Effects)
- Closing a TLS layer on an LDAP session changes the
authentication and authorization state of the LDAP session based
on local policy. Specifically, this means that implementations
are not required to to change the authentication and
authorization states to anonymous upon TLS closure.
Appendix C. Changes for draft-ldapbis-authmeth-19
[[Note to RFC Editor: Please remove this appendix upon publication
of this Internet-Draft as an RFC.]]
This appendix is non-normative.
This appendix summarizes changes made in this revision of the
document.
General
- This draft has addressed all issues raised for the -18 version.
- Several minor edits for clarity and to remove typos based on
feedback from WG, IETF and IESG reviews.
Abstract
- Added statement regarding RFCs obsoleted by this document.
Section 2
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- Changed mandatory-to-implement TLS ciphersuite from
TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA to
TLS_RSA_WITH_3DES_EDE_CBC_SHA based on IESG recommendation and
WG consensus.
Section 5.2.1.5
- Clarified that 'supportedSASLMechanisms' should be retrievable
by all clients both before and after SASL negotiation to allow
detection of mechanism downgrade attacks.
Section 5.2.1.6
- Changed wording to reflect the fact that SASL layers cannot be
uninstalled from the session.
Section 5.2.3
- Removed reference to IPsec as a source of authorization identity.
Section 6.2
- When TLS layer does not provide an acceptable level of security
client MUST warn the user or refuse to proceed. (This was
changed from SHOULD based on IESG recommendation and WG
consensus.)
- Added a security consideration regarding the proper usage of
information found in the client certificate.
Section 6.4
- Added a new section on SASL security considerations that
discusses SASL mechanism downgrade attacks.
Section 10
- Replaced references to RFC 2401 with RFC 4301.
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specification can be obtained from the IETF on-line IPR repository
at http://www.ietf.org/ipr.
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The IETF invites any interested party to bring to its attention any
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Full Copyright Statement
Copyright (C) The Internet Society (2006).
This document is subject to the rights, licenses and restrictions
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This document and the information contained herein are provided on
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