Network Working Group R. Harrison, Ed.
Request for Comments: 4513 Novell, Inc.
Obsoletes: 2251, 2829, 2830 June 2006
Category: Standards Track
Lightweight Directory Access Protocol (LDAP):
Authentication Methods and Security Mechanisms
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright (C) The Internet Society (2006).
This document describes authentication methods and security
mechanisms of the Lightweight Directory Access Protocol (LDAP). 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 the specification's road map),
obsoletes RFC 2251, RFC 2829, and RFC 2830.
The Lightweight Directory Access Protocol (LDAP) [RFC4510] 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
(1) Unauthorized access to directory data via data-retrieval
(2) Unauthorized access to directory data by monitoring access of
(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
Threats (1), (4), (5), (6), (7), and (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
LDAP offers the following security mechanisms:
(1) Authentication by means of the Bind operation. The Bind
operation provides a simple method that 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
LDAP may also be protected by means outside the LDAP protocol, e.g.,
with IP layer 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][RFC4512], in string form [RFC4514], 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
This document, together with [RFC4510], [RFC4511], and [RFC4512],
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 [RFC4511]. The
remainder of RFC 2830 is obsoleted by this document. Appendix B.3
summarizes the substantive changes made to RFC 2830 by this document.
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 that 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
The term "SASL layer" refers to SASL services used in providing
security services, as well as associations established by these
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
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.
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 they 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
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 StartTLS operation (Section
3). (Other servers MUST support TLS per the second paragraph of this
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 [RFC4511] provides the ability to establish TLS
[RFC4346] in an LDAP session.
The goals of using the 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
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
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
- when there are outstanding responses for operation requests
previously issued on the session.
As described in [RFC4511], Section 4.14.1, a (detected) violation of
any of these requirements results in a return of the operationsError
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
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 220.127.116.11 - 18.104.22.168 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) [RFC4519] value in the leaf Relative
Distinguished Name (RDN) of the subjectName field of the server's
certificate. This comparison is performed using the rules for
comparison of DNS names in Section 22.214.171.124, 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
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
126.96.36.199. 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
* 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.
188.8.131.52. 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.
184.108.40.206. 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
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 [RFC4511], 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 that it
obtained prior to the initiation of the TLS negotiation and that it
did not obtain 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
- 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 they are
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) authentication
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 that
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 [RFC4511] 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.
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 ([RFC4511], 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
- 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
[RFC4514]) 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
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 [RFC4514] of non-zero length) and specifying
the simple authentication choice containing a password value of zero
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
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 [RFC4514] 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, that the password is not
valid for the DN, or that the server otherwise considers the
credentials invalid. A resultCode of success indicates that the
credentials are valid and that 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.
The name/password authentication mechanism of the simple Bind method
is not suitable for authentication in environments without
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 [RFC4422]. As LDAP
includes native anonymous and name/password (plain text)
authentication methods, the ANONYMOUS [RFC4505] 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 ([RFC4422], Section 4). This section explains how each of
these profiling requirements is met by LDAP.
220.127.116.11. 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.
18.104.22.168. SASL Authentication Initiation and Protocol Exchange
SASL authentication is initiated via a BindRequest message
([RFC4511], 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
[RFC4422], Sections 3 and 5).
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 that
are defined to have the server send additional data along with the
indication of successful completion.
22.214.171.124. 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 the 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.
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
include 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).
126.96.36.199. 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.
188.8.131.52. 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) ([RFC4512], 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 [RFC4422], 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.
184.108.40.206. Rules for Using SASL Layers
Upon installing a SASL layer, the client SHOULD discard or refresh
all information about the server that it obtained prior to the
initiation of the SASL negotiation and that it did not obtain through
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.
220.127.116.11. Support for Multiple Authentications
LDAP supports multiple SASL authentications as defined in [RFC4422],
18.104.22.168. SASL Authorization Identities
Some SASL mechanisms allow clients to request a desired authorization
identity for the LDAP session ([RFC4422], 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 Augmented
Backus-Naur Form (ABNF) [RFC4234] 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 [RFC4514]
and the UTF8 rule is defined in Section 1.4 of [RFC4512].
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 ([RFC4517], Section 4.2.15).
There is no requirement that the asserted distinguishedName value be
that of an entry in the directory.
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 ([RFC3454], Section 7) using the SASLprep [RFC4013] algorithm,
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 [RFC4520] for
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
For example, the SASL DIGEST-MD5 authentication mechanism
[DIGEST-MD5] utilizes an authentication identity and a realm that are
syntactically simple strings and semantically simple username
[RFC4013] and realm values. 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 ([RFC4422], Appendix A) 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.
22.214.171.124. Implicit Assertion
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
126.96.36.199. 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 188.8.131.52.
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
6.1. General LDAP Security Considerations
LDAP itself provides no security or protection from accessing or
updating the directory by means other than through the LDAP protocol,
e.g., from inspection of server database files by database
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.
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
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 [RFC4346].
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
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
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
[DIGEST-MD5], 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 [RFC4422]
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 that:
A TLS layer has been successfully installed.
Some other data confidentiality mechanism that protects the
password value from eavesdropping has been provided.
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
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
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 [RFC4422], [RFC4013], [RFC3454], and
7. IANA Considerations
The IANA has updated the LDAP Protocol Mechanism registry to indicate
that this document and [RFC4511] provide the definitive technical
specification for the StartTLS (184.108.40.206.4.1.1466.20037) extended
The IANA has updated the LDAP LDAPMessage types registry to indicate
that this document and [RFC4511] provide the definitive technical
specification for the bindRequest (0) and bindResponse (1) message
The IANA has updated the LDAP Bind Authentication Method registry to
indicate that this document and [RFC4511] provide the definitive
technical specification for the simple (0) and sasl (3) bind
The IANA has updated the LDAP authzid prefixes registry to indicate
that this document provides the definitive technical specification
for the dnAuthzId (dn:) and uAuthzId (u:) authzid prefixes.
This document combines information originally contained in RFC 2251,
RFC 2829, and RFC 2830. RFC 2251 was a product of the Access,
Searching, and Indexing of Directories (ASID) Working Group. RFC
2829 and RFC 2830 were products of the LDAP Extensions (LDAPEXT)
This document is a product of the IETF LDAP Revision (LDAPBIS)
9. Normative References
[RFC791] Postel, J., "Internet Protocol", STD 5, RFC 791,
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454,
[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications
(IDNA)", RFC 3490, March 2003.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User
Names and Passwords", RFC 4013, February 2005.
[RFC4234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[RFC4346] Dierks, T. and E. Rescorla, "The TLS Protocol Version
1.1", RFC 4346, March 2006.
[RFC4422] Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
Authentication and Security Layer (SASL)", RFC 4422,
[RFC4510] Zeilenga, K., Ed., "Lightweight Directory Access
Protocol (LDAP): Technical Specification Road Map", RFC
4510, June 2006.
[RFC4511] Sermersheim, J., Ed., "Lightweight Directory Access
Protocol (LDAP): The Protocol", RFC 4511, June 2006.
[RFC4512] Zeilenga, K., "Lightweight Directory Access Protocol
(LDAP): Directory Information Models", RFC 4512, June
[RFC4514] Zeilenga, K., Ed., "Lightweight Directory Access
Protocol (LDAP): String Representation of Distinguished
Names", RFC 4514, June 2006.
[RFC4517] Legg, S., Ed., "Lightweight Directory Access Protocol
(LDAP): Syntaxes and Matching Rules", RFC 4517, June
[RFC4519] Sciberras, A., Ed., "Lightweight Directory Access
Protocol (LDAP): Schema for User Applications", RFC
4519, June 2006.
[RFC4520] Zeilenga, K., "Internet Assigned Numbers Authority
(IANA) Considerations for the Lightweight Directory
Access Protocol (LDAP)", BCP 64, RFC 4520, June 2006.
[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"
[X.501] ITU-T Rec. X.501, "The Directory: Models", 1993.
10. Informative References
[DIGEST-MD5] Leach, P., Newman, C., and A. Melnikov, "Using Digest
Authentication as a SASL Mechanism", Work in Progress,
[PLAIN] Zeilenga, K., "The Plain SASL Mechanism", Work in
Progress, March 2005.
[RFC2828] Shirey, R., "Internet Security Glossary", FYI 36, RFC
2828, May 2000.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4505] Zeilenga, K., "The Anonymous SASL Mechanism", RFC 4505,
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. 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; and others (e.g.,
time of day) may be "environmental".
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 [RFC4422].
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, thus 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.
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 2251Sections 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
- 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
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
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
B.2. Changes Made to RFC 2829
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 [DIGEST-MD5].
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
B.2.10. Section 10 ("TLS Ciphersuites")
- TLS ciphersuite recommendations are no longer included in this
specification. Implementations must now support the
TLS_RSA_WITH_3DES_EDE_CBC_SHA ciphersuite and should continue to
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 [RFC4511] for summaries of
changes to other sections.
B.3.1. Section 3.6 ("Server Identity Check")
- 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.
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