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Replace AuthMeth, StartTLS, and DIGEST-MD5 I-Ds with RFC

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Network Working Group M. Wahl
INTERNET-DRAFT Innosoft International, Inc.
H. Alvestrand
MaXware AS
J. Hodges
Stanford University
RL "Bob" Morgan
Stanford University
Expires in six months from June 21, 1999
Authentication Methods for LDAP
<draft-ietf-ldapext-authmeth-04.txt>
1. Status of this Memo
This document is an Internet-Draft. 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."
To view the entire list of current Internet-Drafts, please check
the "1id-abstracts.txt" listing contained in the Internet-Drafts
Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net
(Northern Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au
(Pacific Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu
(US West Coast).
2. Abstract
This document specifies particular combinations of security
mechanisms which are required and recommended in LDAP [1]
implementations.
3. Introduction
LDAP version 3 is a powerful access protocol for directories.
It offers means of searching, fetching and manipulating directory
content, and ways to access a rich set of security functions.
In order to function for the best of the Internet, it is vital
that these security functions be interoperable; therefore there
has to be a minimum subset of security functions that is common to
all implementations that claim LDAPv3 conformance.
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Basic threats to an LDAP directory service include:
(1) Unauthorized access to data via data-fetching operations,
(2) Unauthorized access to reusable client authentication
information by monitoring others' access,
(3) Unauthorized access to data by monitoring others' access,
(4) Unauthorized modification of data,
(5) Unauthorized modification of configuration,
(6) Unauthorized or excessive use of resources (denial of
service), and
(7) Spoofing of directory: Tricking a client into believing
that information came from the directory when in fact it
did not, either by modifying data in transit or misdirecting
the client's connection.
Threats (1), (4), (5) and (6) are due to hostile clients. Threats
(2), (3) and (7) are due to hostile agents on the path between client
and server, or posing as a server.
The LDAP protocol suite can be protected with the following
security mechanisms:
(1) Client authentication by means of the SASL [2] mechanism set,
possibly backed by the TLS credentials exchange mechanism,
(2) Client authorization by means of access control based on
the requestor's authenticated identity,
(3) Data integrity protection by means of the TLS protocol or
data-integrity SASL mechanisms,
(4) Protection against snooping by means of the TLS protocol
or data-encrypting SASL mechanisms,
(5) Resource limitation by means of administrative limits on
service controls, and
(6) Server authentication by means of the TLS protocol or SASL
mechanism.
At the moment, imposition of access controls is done by means
outside the scope of the LDAP protocol.
In this document, the term "user" represents any application which
is an LDAP client using the directory to retrieve or store information.
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC 2119 [3].
4. Example deployment scenarios
The following scenarios are typical for LDAP directories on the
Internet, and have different security requirements. (In the
following, "sensitive" means data that will cause real damage to
the owner if revealed; there may be data that is protected but
not sensitive). This is not intended to be a comprehensive list,
other scenarios are possible, especially on physically protected
networks.
(1) A read-only directory, containing no sensitive data,
accessible to "anyone", and TCP connection hijacking
or IP spoofing is not a problem. This directory requires
no security functions except administrative service limits.
(2) A read-only directory containing no sensitive data; read
access is granted based on identity. TCP connection
hijacking is not currently a problem. This scenario requires
a secure authentication function.
(3) A read-only directory containing no sensitive data; and
the client needs to ensure that the directory data is
authenticated by the server and not modified while being
returned from the server.
(4) A read-write directory, containing no sensitive data; read
access is available to "anyone", update access to properly
authorized persons. TCP connection hijacking is not
currently a problem. This scenario requires a secure
authentication function.
(5) A directory containing sensitive data. This scenario
requires session confidentiality protection AND secure
authentication.
5. Authentication and Authorization: Definitions and Concepts
This section 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.
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5.1. Access Control Policy
An access control policy is a set of rules defining the protection
of resources, generally in terms of the capabilities of persons or
other entities accessing those resources. A common expression of an
access control policy is an access control list. Security objects
and mechanisms, such as those described here, enable the expression of
access control policies and their enforcement. Access control
policies are typically expressed in terms of access control attributes
as described below.
5.2. Access Control Factors
A request, when it is being processed by a server, may be associated
with a wide variety of security-related factors (section 4.2 of [1]).
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
connection via which the request is transmitted, others (e.g. time of
day) may be "environmental".
Access control policies are expressed in terms of access control
factors. E.g., a request having ACFs i,j,k can perform operation Y
on resource Z. The set of ACFs that a server makes available for such
expressions is implementation-specific.
5.3. Authentication, Credentials, Identity
Authentication credentials are the evidence supplied by one party to
another, asserting the identity of the supplying party (e.g. a user)
who is attempting to establish an association with the other party
(typically a server). Authentication is the process of generating,
transmitting, and verifying these credentials and thus the identity
they assert. An authentication identity is the name presented in a
credential.
There are many forms of authentication credentials -- the form used
depends upon the particular authentication mechanism negotiated by the
parties. For example: X.509 certificates, Kerberos tickets, simple
identity and password pairs. Note that an authentication mechanism may
constrain the form of authentication identities used with it.
5.4. 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; e.g., entity X can perform operation Y on
resource Z.
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The authorization identity bound to an association is often exactly the
same as the authentication identity presented by the client, but it may
be different. SASL allows clients to specify an authorization identity
distinct from the authentication identity asserted by the client's
credentials. This permits agents such as proxy servers to authenticate
using their own credentials, yet request the access privileges of the
identity for which they are proxying [2]. 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.
6. Required security mechanisms
It is clear that allowing any implementation, faced with the above
requirements, to pick and choose among the possible alternatives
is not a strategy that is likely to lead to interoperability. In
the absence of mandates, clients will be written that do not
support any security function supported by the server, or worse,
support only mechanisms like cleartext passwords that provide
clearly inadequate security.
Active intermediary attacks are the most difficult for an attacker
to perform, and for an implementation to protect against. Methods
that protect only against hostile client and passive eavesdropping
attacks are useful in situations where the cost of protection
against active intermediary attacks is not justified based on the
perceived risk of active intermediary attacks.
Given the presence of the Directory, there is a strong desire to
see mechanisms where identities take the form of a Distinguished
Name and authentication data can be stored in the directory; this
means that either this data is useless for faking authentication
(like the Unix "/etc/passwd" file format used to be), or its
content is never passed across the wire unprotected - that is,
it's either updated outside the protocol or it is only updated in
sessions well protected against snooping. It is also desirable
to allow authentication methods to carry authorization identities
based on existing forms of user identities for backwards compatibility
with non-LDAP-based authentication services.
Therefore, the following implementation conformance requirements
are in place:
(1) For a read-only, public directory, anonymous authentication,
described in section 7, can be used.
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(2) Implementations providing password-based authenticated access
MUST support authentication using Digest, as described in
section 8.1. This provides client authentication with
protection against passive eavesdropping attacks, but does
not provide protection against active intermediary attacks.
(3) For a directory needing session protection and
authentication, the Start TLS extended operation, and either
the simple authentication choice or the SASL EXTERNAL
mechanism, are to be used together. Implementations SHOULD
support authentication with a password as described in
section 8.2, and SHOULD support authentication with a
certificate as described in section 9.1. Together, these
can provide integrity and disclosure protection of
transmitted data, and authentication of client and server,
including protection against active intermediary attacks.
If TLS is negotated, the client MUST discard all information about
the server fetched prior to the TLS negotiation. In particular, the
value of supportedSASLMechanisms MAY be different after TLS has been
negotiated (specifically, the EXTERNAL mechanism or the proposed
PLAIN mechanism are likely to only be listed after a TLS negotiation
has been performed).
If a SASL security layer is negotiated, the client MUST discard all
information about the server fetched prior to SASL. In particular, if
the client is configured to support multiple SASL mechanisms, it SHOULD
fetch supportedSASLMechanisms both before and after the SASL security
layer is negotiated and verify that the value has not changed after the
SASL security layer was negotiated. This detects active attacks which
remove supported SASL mechanisms from the supportedSASLMechanisms list.
7. Anonymous authentication
Directory operations which modify entries or access protected
attributes or entries generally require client authentication.
Clients which do not intend to perform any of these operations
typically use anonymous authentication.
LDAP implementations MUST support anonymous authentication, as
defined in section 7.1.
LDAP implementations MAY support anonymous authentication with TLS,
as defined in section 7.2.
While there MAY be access control restrictions to prevent access to
directory entries, an LDAP server SHOULD allow an anonymously-bound
client to retrieve the supportedSASLMechanisms attribute of the root
DSE.
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An LDAP server MAY use other information about the client provided
by the lower layers or external means to grant or deny access even
to anonymously authenticated clients.
7.1. Anonymous authentication procedure
An LDAP client which has not successfully completed a bind operation
on a connection is anonymously authenticated.
An LDAP client MAY also specify anonymous authentication in a bind
request by using a zero-length OCTET STRING with the simple
authentication choice.
7.2. Anonymous authentication and TLS
An LDAP client MAY use the Start TLS operation [5] to negotiate the
use of TLS security [6]. If the client has not bound beforehand,
then until the client uses the EXTERNAL SASL mechanism to negotiate
the recognition of the client's certificate, the client is
anonymously authenticated.
Recommendations on TLS ciphersuites are given in section 12.
An LDAP server which requests that clients provide their certificate
during TLS negotiation MAY use a local security policy to determine
whether to successfully complete TLS negotiation if the client did not
present a certificate which could be validated.
8. Password-based authentication
LDAP implementations MUST support authentication with a password using
the DIGEST-MD5 mechanism for password protection, as defined in section
8.1.
LDAP implementations SHOULD support authentication with the "simple"
password choice when the connection is protected against eavesdropping
using TLS, as defined in section 8.2.
8.1. Digest authentication
An LDAP client MAY determine whether the server supports this
mechanism by performing a search request on the root DSE, requesting
the supportedSASLMechanisms attribute, and checking whether the
string "DIGEST-MD5" is present as a value of this attribute.
In the first stage of authentication, when the client is performing
an "initial authentication" as defined in section 2.1 of [4], the
client sends a bind request in which the version number is 3, the
authentication choice is sasl, the sasl mechanism name is "DIGEST-MD5",
and the credentials are absent. The client then waits for a response
from the server to this request.
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The server will respond with a bind response in which the resultCode
is saslBindInProgress, and the serverSaslCreds field is present. The
contents of this field is a string defined by "digest-challenge" in
section 2.1.1 of [4]. The server SHOULD include a realm indication and
MUST indicate support for UTF-8.
The client will send a bind request with a distinct mesage id, in which
the version number is 3, the authentication choice is sasl, the sasl
mechanism name is "DIGEST-MD5", and the credentials contain the string
defined by "digest-response" in section 2.1.2 of [4]. The serv-type
is "ldap".
The server will respond with a bind response in which the resultCode
is either success, or an error indication. If the authentication is
successful and the server does not support subsequent authentication,
then the credentials field is absent. If the authentication is
successful and the server supports subsequent authentication, then
the crendentials field contains the string defined by "response-auth"
in section 2.1.3 of [4]. Support for subsequent authentication is
OPTIONAL in clients and servers.
8.2. "simple" authentication choice under TLS encryption
A user who has a directory entry containing a userPassword attribute
MAY authenticate to the directory by performing a simple password
bind sequence following the negotiation of a TLS ciphersuite
providing connection confidentiality [6].
The client will use the Start TLS operation [5] to negotiate the
use of TLS security [6] on the connection to the LDAP server. The
client need not have bound to the directory beforehand.
For this authentication procedure to be successful, the client and
server MUST negotiate a ciphersuite which contains a bulk encryption
algorithm of appropriate strength. Recommendations on cipher
suites are given in section 12.
Following the successful completion of TLS negotiation, the client
MUST send an LDAP bind request with the version number of 3, the
name field containing the name of the user's entry, and the "simple"
authentication choice, containing a password.
The server will, for each value of the userPassword attribute in
the named user's entry, compare these for case-sensitive equality
with the client's presented password. If there is a match, then the
server will respond with resultCode success, otherwise the server will
respond with resultCode invalidCredentials.
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8.3. Other authentication choices with TLS
It is also possible to perform a SASL authentication exchange of
passwords following the negotiation of TLS. In this case the
client and server need not negotiate a ciphersuite which provides
confidentiality if the only service required is data integrity.
9. Certificate-based authentication
LDAP implementations SHOULD support authentication via a client
certificate in TLS, as defined in section 9.1.
9.1. Certificate-based authentication with TLS
A user who has a public/private key pair in which the public key has
been signed by a Certification Authority may use this key pair to
authenticate to the directory server if the user's certificate is
requested by the server. The user's certificate subject field
SHOULD be the name of the user's directory entry, and the
Certification Authority must be sufficiently trusted by the
directory server to have issued the certificate in order that the
server can process the certificate. The means by which servers
validate certificate paths is outside the scope of this document.
A server MAY support mappings for certificates in which the subject
field name is different from the name of the user's directory entry.
A server which supports mappings of names MUST be capable of being
configured to support certificates for which no mapping is required.
The client will use the Start TLS operation [5] to negotiate the
use of TLS security [6] on the connection to the LDAP server. The
client need not have bound to the directory beforehand.
In the TLS negotiation, the server MUST request a certificate. The
client will provide its certificate to the server, and MUST perform
a private key-based encryption, proving it has it private key
associated with the certificate.
As deployments will require protection of sensitive data in transit,
the client and server MUST negotiate a ciphersuite which contains a
bulk encryption algorithm of appropriate strength. Recommendations
of cipher suites are given in section 12.
The server MUST verify that the client's certificate is valid.
The server will normally check that the certificate is issued by a
known CA, and that none of the certificates on the client's
certificate chain are invalid or revoked. There are several
procedures by which the server can perform these checks.
Following the successful completion of TLS negotiation, the client
will send an LDAP bind request with the SASL "EXTERNAL" mechanism.
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10. Other mechanisms
The LDAP "simple" authentication choice is not suitable for
authentication on the Internet where there is no network or transport
layer confidentiality.
As LDAP includes a native anonymous and plaintext authentication
methods, the "ANONYMOUS" and "PLAIN" SASL mechanisms are not used
with LDAP. If an authorization identity of a form different from
a DN is requested by the client, a mechanism that protects the
password in transit SHOULD be used.
The following SASL-based mechanisms are not considered in this
document: KERBEROS_V4, GSSAPI and SKEY.
The "EXTERNAL" SASL mechanism can be used to request the LDAP server
make use of security credentials exchanged by a lower layer. If a
TLS session has not been established between the client and server
prior to making the SASL EXTERNAL Bind request and there is no other
external source of authentication credentials (e.g. IP-level
security [8]), or if, during the process of establishing the
TLS session, the server did not request the client's authentication
credentials, the SASL EXTERNAL bind MUST fail with a result code of
inappropriateAuthentication. Any authentication identity and
authorization identity, as well as TLS connection, which were in
effect prior to making the Bind request, MUST remain in force.
11. Authorization Identity
The authorization identity is carried as part of the SASL credentials
field in the LDAP Bind request and response.
When the "EXTERNAL" mechanism is being negotiated, if the
credentials field is present, it contains an authorization identity
of the authzId form described below.
Other mechanisms define the location of the authorization
identity in the credentials field.
The authorization identity is a string in the UTF-8 character set,
corresponding to the following ABNF [7]:
; Specific predefined authorization (authz) id schemes are
; defined below -- new schemes may be defined in the future.
authzId = dnAuthzId / uAuthzId
; distinguished-name-based authz id.
dnAuthzId = "dn:" dn
dn = utf8string ; with syntax defined in RFC 2253
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; unspecified userid, UTF-8 encoded.
uAuthzId = "u:" userid
userid = utf8string ; syntax unspecified
A utf8string is defined to be the UTF-8 encoding of one or more
ISO 10646 characters.
All servers which support the storage of authentication credentials,
such as passwords or certificates, in the directory MUST support the
dnAuthzId choice.
The uAuthzId choice allows for compatibility with client applications
which wish to authenticate to a local directory but do not know their
own Distinguished Name or have a directory entry. The format of the
string is defined as only a sequence of UTF-8 encoded ISO 10646
characters, and further interpretation is subject to prior agreement
between the client and server.
For example, the userid could identify a user of a specific directory
service, or be a login name or the local-part of an RFC 822 email
address. In general a uAuthzId MUST NOT be assumed to be globally
unique.
Additional authorization identity schemes MAY be defined in future
versions of this document.
12. TLS Ciphersuites
The following ciphersuites defined in [6] MUST NOT be used for
confidentiality protection of passwords or data:
TLS_NULL_WITH_NULL_NULL
TLS_RSA_WITH_NULL_MD5
TLS_RSA_WITH_NULL_SHA
The following ciphersuites defined in [6] can be cracked easily
(less than a week of CPU time on a standard CPU in 1997). The
client and server SHOULD carefully consider the value of the
password or data being protected before using these ciphersuites:
TLS_RSA_EXPORT_WITH_RC4_40_MD5
TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5
TLS_RSA_EXPORT_WITH_DES40_CBC_SHA
TLS_DH_DSS_EXPORT_WITH_DES40_CBC_SHA
TLS_DH_RSA_EXPORT_WITH_DES40_CBC_SHA
TLS_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA
TLS_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA
TLS_DH_anon_EXPORT_WITH_RC4_40_MD5
TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA
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The following ciphersuites are vulnerable to man-in-the-middle
attacks, and 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 tolerable:
TLS_DH_anon_EXPORT_WITH_RC4_40_MD5
TLS_DH_anon_WITH_RC4_128_MD5
TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA
TLS_DH_anon_WITH_DES_CBC_SHA
TLS_DH_anon_WITH_3DES_EDE_CBC_SHA
A client or server that supports TLS MUST support at least
TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA.
13. SASL service name for LDAP
For use with SASL [2], a protocol must specify a service name to be
used with various SASL mechanisms, such as GSSAPI. For LDAP, the
service name is "ldap", which has been registered with the IANA
as a GSSAPI service name.
14. Security Considerations
Security issues are discussed throughout this memo; the
(unsurprising) conclusion is that mandatory security is important,
and that session encryption is required when snooping is a
problem.
Servers are encouraged to prevent modifications by anonymous
users. Servers may also wish to minimize denial of service attacks
by timing out idle connections, and returning the unwillingToPerform
result code rather than performing computationally expensive
operations requested by unauthorized clients.
A connection on which the client has not performed the Start TLS
operation or negotiated a suitable SASL mechanism for connection
integrity and encryption services is subject to man-in-the-middle
attacks to view and modify information in transit.
Additional security considerations relating to the EXTERNAL
EXTERNAL mechanism to negotiate TLS can be found in [2], [5]
and [6].
15. Acknowledgements
This document is a product of the LDAPEXT Working Group of the
IETF. The contributions of its members is greatly appreciated.
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16. Bibliography
[1] M. Wahl, T. Howes, S. Kille, "Lightweight Directory Access
Protocol (v3)", Dec. 1997, RFC 2251.
[2] J. Myers, "Simple Authentication and Security Layer (SASL)",
Oct. 1997, RFC 2222.
[3] S. Bradner, "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119.
[4] P. Leach, C. Newman, "Using Digest Authentication as a SASL
Mechanism", INTERNET DRAFT <draft-leach-digest-sasl-00.txt>.
[5] J. Hodges, RL Morgan, M. Wahl, "LDAPv3 Extension for Transport
Layer Security", Oct. 1998, INTERNET DRAFT
<draft-ietf-ldapext-ldapv3-tls-03.txt>.
[6] T. Diers, C. Allen, "The TLS Protocol Version 1.0", Jan. 1999,
RFC 2246.
[7] D. Crocker, Ed., P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234.
[8] S. Kent, R. Atkinson, "Security Architecture for the Internet
Protocol", Nov. 1998, RFC 2401.
17. Authors Address
Mark Wahl
Innosoft International, Inc.
8911 Capital of Texas Hwy, Suite 4140
Austin, TX 78759
USA
Phone: +1 512 231 1600
EMail: Mark.Wahl@innosoft.com
Harald Tveit Alvestrand
EMail: Harald.Alvestrand@maxware.no
Jeff Hodges
Computing & Communication Services
Stanford University
Pine Hall
241 Panama Street
Stanford, CA 94305-4122
USA
Phone: +1-650-723-2452
EMail: Jeff.Hodges@Stanford.edu
Wahl, Alvestrand, Hodges, Morgan Page 13
INTERNET-DRAFT Authentication Methods for LDAP June 1999
RL "Bob" Morgan
Computing & Communication Services
Stanford University
Pine Hall
241 Panama Street
Stanford, CA 94305-4122
USA
Phone: +1-650-723-9711
EMail: Bob.Morgan@Stanford.edu
Full Copyright Statement
Copyright (C) The Internet Society (1998). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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LDAPEXT Working Group Jeff Hodges, Stanford
INTERNET-DRAFT RL "Bob" Morgan, Univ of Washington
Intended Category: Mark Wahl, Innosoft
Standards Track June, 1999
Lightweight Directory Access Protocol (v3):
Extension for Transport Layer Security
<draft-ietf-ldapext-ldapv3-tls-05.txt>
Status of this Document
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.
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.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Comments and suggestions on this document are encouraged. Comments on
this document should be sent to the LDAPEXT working group discussion
list:
ietf-ldapext@netscape.com
This document expires in December 1999.
1. Abstract
This document defines the "Start Transport Layer Security (TLS) Opera-
tion" for LDAP [LDAPv3, TLS]. This operation provides for TLS establish-
ment in an LDAP association and is defined in terms of an LDAP extended
request.
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2. Conventions Used in this Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [ReqsKeywords].
3. The Start TLS Request
This section describes the Start TLS extended request and extended
response themselves: how to form the request, the form of the response,
and enumerates the various result codes the client MUST be prepared to
handle.
The section following this one then describes how to sequence an overall
Start TLS Operation.
3.1. Requesting TLS Establishment
A client may perform a Start TLS operation by transmitting an LDAP PDU
containing an ExtendedRequest [LDAPv3] specifying the OID for the Start
TLS operation:
1.3.6.1.4.1.1466.20037
An LDAP ExtendedRequest is defined as follows:
ExtendedRequest ::= [APPLICATION 23] SEQUENCE {
requestName [0] LDAPOID,
requestValue [1] OCTET STRING OPTIONAL }
A Start TLS extended request is formed by setting the requestName field
to the OID string given above. The requestValue field is absent. The
client MUST NOT send any PDUs on this connection following this request
until it receives a Start TLS extended response.
When a Start TLS extended request is made, the server MUST return an
LDAP PDU containing a Start TLS extended response. An LDAP Exten-
dedResponse is defined as follows:
ExtendedResponse ::= [APPLICATION 24] SEQUENCE {
COMPONENTS OF LDAPResult,
responseName [10] LDAPOID OPTIONAL,
response [11] OCTET STRING OPTIONAL }
A Start TLS extended response MUST contain a responseName field which
MUST be set to the same string as that in the responseName field present
in the Start TLS extended request. The response field is absent. The
server MUST set the resultCode field to either success or one of the
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other values outlined in section 3.3.
3.2. "Success" Response
If the ExtendedResponse contains a resultCode of success, this indicates
that the server is willing and able to negotiate TLS. Refer to section
4, below, for details.
3.3. Response other than "success"
If the ExtendedResponse contains a resultCode other than success, this
indicates that the server is unwilling or unable to negotiate TLS.
If the Start TLS extended request was not successful, the resultCode
will be one of:
operationsError (operations sequencing incorrect; e.g. TLS already
established)
protocolError (TLS not supported or incorrect PDU structure)
referral (this server doesn't do TLS, try this one)
unavailable (e.g. some major problem with TLS, or server is
shutting down)
The server MUST return operationsError if the client violates any of the
Start TLS extended operation sequencing requirements described in sec-
tion 4, below.
If the server does not support TLS (whether by design or by current con-
figuration), it MUST set the resultCode to protocolError (see section
4.1.1 of [LDAPv3]), or to referral. The server MUST include an actual
referral value in the LDAP Result if it returns a resultCode of refer-
ral. The client's current session is unaffected if the server does not
support TLS. The client MAY proceed with any LDAP operation, or it MAY
close the connection.
The server MUST return unavailable if it supports TLS but cannot estab-
lish a TLS connection for some reason, e.g. the certificate server not
responding, it cannot contact its TLS implementation, or if the server
is in process of shutting down. The client MAY retry the StartTLS opera-
tion, or it MAY proceed with any other LDAP operation, or it MAY close
the connection.
4. Sequencing of the Start TLS Operation
This section describes the overall procedures clients and servers MUST
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follow for TLS establishment. These procedures take into consideration
various aspects of the overall security of the LDAP association includ-
ing discovery of resultant security level and assertion of the client's
authorization identity.
Note that the precise effects, on a client's authorzation identity, of
establishing TLS on an LDAP association are described in detail in sec-
tion 7.
4.1. Requesting to Start TLS on an LDAP Association
The client MAY send the Start TLS extended request at any time after
establishing an LDAP association, except that in the following cases the
client MUST NOT send a Start TLS extended request:
- if TLS is currently established on the connection, or
- during a multi-stage SASL negotiation, or
- if there are any LDAP operations outstanding on the connection.
The result of violating any of these requirements is a resultCode of
operationsError, as described above in section 3.3.
The client MAY have already perfomed a Bind operation when it sends a
Start TLS request, or the client might have not yet bound.
If the client did not establish a TLS connection before sending any
other requests, and the server requires the client to establish a TLS
connection before performing a particular request, the server MUST
reject that request with a confidentialityRequired or strongAuthRequired
result. The client MAY send a Start TLS extended request, or it MAY
choose to close the connection.
4.2. Starting TLS
The server will return an extended response with the resultCode of suc-
cess if it is willing and able to negotiate TLS. It will return other
resultCodes, documented above, if it is unable.
In the successful case, the client, which has ceased to transfer LDAP
requests on the connection, MUST either begin a TLS negotiation or close
the connection. The client will send PDUs in the TLS Record Protocol
directly over the underlying transport connection to the server to ini-
tiate TLS negotiation [TLS].
4.3. TLS Version Negotiation
Negotiating the version of TLS or SSL to be used is a part of the TLS
Handshake Protocol, as documented in [TLS]. Please refer to that
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document for details.
4.4. Discovery of Resultant Security Level
After a TLS connection is established on an LDAP association, both par-
ties MUST individually decide whether or not to continue based on the
privacy level achieved. Ascertaining the TLS connection's privacy level
is implementation dependent, and accomplished by communicating with
one's respective local TLS implementation.
If the client or server decides that the level of authentication or
privacy is not high enough for it to continue, it SHOULD gracefully
close the TLS connection immediately after the TLS negotiation has com-
pleted (see sections 5 and 7.2, below).
The client MAY attempt to Start TLS again, or MAY send an unbind
request, or send any other LDAP request.
4.5. Assertion of Client's Authorization Identity
The client MAY, upon receipt of a Start TLS extended response indicating
success, assert that a specific authorization identity be utilized in
determining the client's authorization status. The client accomplishes
this via an LDAP Bind request specifying a SASL mechanism of "EXTERNAL"
[SASL]. See section 7, below.
4.6. Server Identity Check
The client MUST check its understanding of the server's hostname against
the server's identity as presented in the server's Certificate message,
in order to prevent man-in-the-middle attacks.
If a subjectAltName extension of type dNSName is present, it SHOULD be
used as the source of the server's identity.
Matching is performed according to these rules:
- The client MUST use the server hostname it used to open
the LDAP connection as the value to compare against the
server name as expressed in the server's certificate.
The client MUST NOT use the server's canonical DNS name or
any other derived form of name.
- If a subjectAltName extension of type dNSName is present
in the certificate, it SHOULD be used as the source of the
server's identity.
- Matching is case-insensitive.
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- The "*" wildcard character is allowed.
- If present, it applies only to the left-most name component.
E.g. *.bar.com would match a.bar.com, b.bar.com, etc. but not bar.com.
If more than one identity of a given type is present in the certificate
(e.g. more than one dNSName name), a match in any one of the set is con-
sidered acceptable.
If the hostname does not match the dNSName-based identity in the certi-
ficate per the above check, user-oriented clients SHOULD either notify
the user (clients MAY give the user the opportunity to continue with the
connection in any case) or terminate the connection and indicate that
the server's identity is suspect. Automated clients SHOULD close the
connection, returning and/or logging an error indicating hat the
server's identity is suspect.
4.7. Refresh of Server Capabilities Information
The client MUST refresh any cached server capabilities information (e.g.
from the server's root DSE; see section 3.4 of [LDAPv3]) upon TLS ses-
sion establishment. This is necessary to protect against active-
intermediary attacks which may have altered any server capabilities
information retrieved prior to TLS establishment. The server MAY adver-
tise different capabilities after TLS establishment.
5. Closing a TLS Connection
5.1. Graceful Closure
Either the client or server MAY terminate the TLS connection on an LDAP
association by sending a TLS closure alert. This will leave the LDAP
association intact.
Before closing a TLS connection, the client MUST either wait for any
outstanding LDAP operations to complete, or explicitly abandon them
[LDAPv3].
After the initiator of a close has sent a closure alert, it MUST discard
any TLS messages until it has received an alert from the other party.
It will cease to send TLS Record Protocol PDUs, and following the
reciept of the alert, MAY send and receive LDAP PDUs.
The other party, if it receives a closure alert, MUST immediately
transmit a TLS closure alert. It will subequently cease to send TLS
Record Protocol PDUs, and MAY send and receive LDAP PDUs.
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5.2. Abrupt Closure
Either the client or server MAY abruptly close the entire LDAP associa-
tion and any TLS connection established on it by dropping the underlying
TCP connection. A server MAY beforehand send the client a Notice of
Disconnection [LDAPv3] in this case.
6. Authentication and Authorization: Definitions and Concepts
This section 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.
6.1. Access Control Policy
An access control policy is a set of rules defining the protection of
resources, generally in terms of the capabilities of persons or other
entities accessing those resources. A common expression of an access
control policy is an access control list. Security objects and mechan-
isms, such as those described here, enable the expression of access con-
trol policies and their enforcement. Access control policies are typi-
cally expressed in terms of access control attributes as described
below.
7. Effects of TLS on a Client's Authorization Identity
This section describes the effects on a client's authorization identity
brought about by establishing TLS on an LDAP association. The default
effects are described first, and next the facilities for client asser-
tion of authorization identity are discussed including error conditions.
Lastly, the effects of closing the TLS connection are described.
Authorization identities and related concepts are defined in [AuthMeth].
7.1. TLS Connection Establishment Effects
7.1.1. Default Effects
Upon establishment of the TLS connection onto the LDAP association, any
previously established authentication and authorization identities MUST
remain in force, including anonymous state. This holds even in the case
where the server requests client authentication via TLS -- e.g. requests
the client to supply its certificate during TLS negotiation (see [TLS]).
7.1.2. Client Assertion of Authorization Identity
A client MAY either implicitly request that its LDAP authorization
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identity be derived from its authenticated TLS credentials or it MAY
explicitly provide an authorization identity and assert that it be used
in combination with its authenticated TLS credentials. The former is
known as an implicit assertion, and the latter as an explicit assertion.
7.1.2.1. Implicit Assertion
An implicit authorization identity assertion is accomplished after TLS
establishment by invoking a Bind request of the SASL form using the
"EXTERNAL" mechanism name [SASL, LDAPv3] that SHALL NOT include the
optional credentials octet string (found within the SaslCredentials
sequence in the Bind Request). The server will derive the client's
authorization identity from the authentication identity supplied in the
client's TLS credentials (typically a public key certificate) according
to local policy. The underlying mechanics of how this is accomplished
are implementation specific.
7.1.2.2. Explicit Assertion
An explicit authorization identity assertion is accomplished after TLS
establishment by invoking a Bind request of the SASL form using the
"EXTERNAL" mechanism name [SASL, LDAPv3] that SHALL include the creden-
tials octet string. This string MUST be constructed as documented in
section 11 of [AuthMeth].
7.1.2.3. Error Conditions
For either form of assertion, the server MUST verify that the client's
authentication identity as supplied in its TLS credentials is permitted
to be mapped to the asserted authorization identity. The server MUST
reject the Bind operation with an invalidCredentials resultCode in the
Bind response if the client is not so authorized. The LDAP association's
authentication identity and authorization identity (if any) which were
in effect after TLS establishment but prior to making the Bind request,
MUST remain in force.
Additionally, with either form of assertion, if a TLS session has not
been established between the client and server prior to making the SASL
EXTERNAL Bind request and there is no other external source of authenti-
cation credentials (e.g. IP-level security RFC 1825), or if, during the
process of establishing the TLS session, the server did not request the
client's authentication credentials, the SASL EXTERNAL bind MUST fail
with a result code of inappropriateAuthentication. Any authentication
identity and authorization identity, as well as TLS connection, which
were in effect prior to making the Bind request, MUST remain in force.
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7.2. TLS Connection Closure Effects
Closure of the TLS connection MUST cause the LDAP association to move to
an anonymous authentication and authorization state regardless of the
state established over TLS and regardless of the authentication and
authorization state prior to TLS connection establishment.
8. Security Considerations
The goals of using the TLS protocol with LDAP are to ensure connection
confidentiality and integrity, and to optionally provide for authentica-
tion. TLS expressly provides these capabilities, as described in [TLS].
All security gained via use of the Start TLS operation is gained by the
use of TLS itself. The Start TLS operation, on its own, does not provide
any additional security.
The use of TLS does not provide or ensure for confidentiality and/or
non-repudiation of the data housed by an LDAP-based directory server.
Nor does it secure the data from inspection by the server administra-
tors. Once established, TLS only provides for and ensures confidential-
ity and integrity of the operations and data in transit over the LDAP
association, and only if the implementations on the client and server
support and negotiate it.
The level of security provided though the use of TLS depends directly on
both the quality of the TLS implementation used and the style of usage
of that implementation. Additionally, an active-intermediary attacker
can remove the Start TLS extended operation from the supportedExtension
attribute of the root DSE. Therefore, both parties SHOULD independently
ascertain and consent to the security level achieved once TLS is esta-
blished and before begining use of the TLS connection. For example, the
security level of the TLS connection might have been negotiated down to
plaintext.
Clients SHOULD either warn the user when the security level achieved
does not provide confidentiality and/or integrity protection, or be con-
figurable to refuse to proceed without an acceptable level of security.
Client and server implementors SHOULD take measures to ensure proper
protection of credentials and other confidential data where such meas-
ures are not otherwise provided by the TLS implementation.
Server implementors SHOULD allow for server administrators to elect
whether and when connection confidentiality and/or integrity is
required, as well as elect whether and when client authentication via
TLS is required.
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9. Acknowledgements
The authors thank Tim Howes, Paul Hoffman, John Kristian, Shirish Rai,
Jonathan Trostle, and Harald Alvestrand for their contributions to this
document.
10. References
[AuthMeth] M. Wahl, H. Alvestrand, J. Hodges, R. Morgan. "Authenti-
cation Methods for LDAP". INTERNET-DRAFT, Work In Pro-
gress. draft-ietf-ldapext-authmeth-04.txt
[LDAPv3] M. Wahl, S. Kille and T. Howes. "Lightweight Directory
Access Protocol (v3)". RFC 2251.
[ReqsKeywords] Scott Bradner. "Key Words for use in RFCs to Indicate
Requirement Levels". RFC 2119.
[SASL] J. Myers. "Simple Authentication and Security Layer
(SASL)". RFC 2222.
[TLS] Tim Dierks, C. Allen. "The TLS Protocol Version 1.0". RFC
2246.
11. Authors' Addresses
Jeff Hodges
Computing & Communication Services
Stanford University
Pine Hall
241 Panama Street
Stanford, CA 94305-4122
USA
Phone: +1-650-723-2452
EMail: Jeff.Hodges@Stanford.edu
RL "Bob" Morgan
Networks and Distributed Computing
University of Washington
Seattle, WA
USA
Phone: +1-206-221-3307
EMail: rlmorgan@cac.washington.edu
Hodges, Morgan, Wahl [Page 10]
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Mark Wahl
Innosoft International, Inc.
8911 Capital of Texas Hwy, Suite 4140
Austin, TX 78759
USA
Phone: +1 626 919 3600
EMail: Mark.Wahl@innosoft.com
-----------------------------------
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lectual property or other rights that might be claimed to pertain to
the implementation or use of the technology described in this document
or the extent to which any license under such rights might or might not
be available; neither does it represent that it has made any effort to
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The IETF invites any interested party to bring to its attention any
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dard. Please address the information to the IETF Executive Director.
13. Copyright Notice and Disclaimer
Copyright (C) The Internet Society (1998). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
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ing Internet standards in which case the procedures for copyrights
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The limited permissions granted above are perpetual and will not be
Hodges, Morgan, Wahl [Page 11]
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revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
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Network Working Group M. Wahl
Request for Comments: 2829 Sun Microsystems, Inc.
Category: Standards Track H. Alvestrand
EDB Maxware
J. Hodges
Oblix, Inc.
R. Morgan
University of Washington
May 2000
Authentication Methods for LDAP
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 Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
This document specifies particular combinations of security
mechanisms which are required and recommended in LDAP [1]
implementations.
1. Introduction
LDAP version 3 is a powerful access protocol for directories.
It offers means of searching, fetching and manipulating directory
content, and ways to access a rich set of security functions.
In order to function for the best of the Internet, it is vital that
these security functions be interoperable; therefore there has to be
a minimum subset of security functions that is common to all
implementations that claim LDAPv3 conformance.
Basic threats to an LDAP directory service include:
(1) Unauthorized access to data via data-fetching operations,
Wahl, et al. Standards Track [Page 1]
RFC 2829 Authentication Methods for LDAP May 2000
(2) Unauthorized access to reusable client authentication
information by monitoring others' access,
(3) Unauthorized access to data by monitoring others' access,
(4) Unauthorized modification of data,
(5) Unauthorized modification of configuration,
(6) Unauthorized or excessive use of resources (denial of
service), and
(7) Spoofing of directory: Tricking a client into believing that
information came from the directory when in fact it did not,
either by modifying data in transit or misdirecting the
client's connection.
Threats (1), (4), (5) and (6) are due to hostile clients. Threats
(2), (3) and (7) are due to hostile agents on the path between client
and server, or posing as a server.
The LDAP protocol suite can be protected with the following security
mechanisms:
(1) Client authentication by means of the SASL [2] mechanism
set, possibly backed by the TLS credentials exchange
mechanism,
(2) Client authorization by means of access control based on the
requestor's authenticated identity,
(3) Data integrity protection by means of the TLS protocol or
data-integrity SASL mechanisms,
(4) Protection against snooping by means of the TLS protocol or
data-encrypting SASL mechanisms,
(5) Resource limitation by means of administrative limits on
service controls, and
(6) Server authentication by means of the TLS protocol or SASL
mechanism.
At the moment, imposition of access controls is done by means outside
the scope of the LDAP protocol.
In this document, the term "user" represents any application which is
an LDAP client using the directory to retrieve or store information.
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RFC 2829 Authentication Methods for LDAP May 2000
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [3].
2. Example deployment scenarios
The following scenarios are typical for LDAP directories on the
Internet, and have different security requirements. (In the
following, "sensitive" means data that will cause real damage to the
owner if revealed; there may be data that is protected but not
sensitive). This is not intended to be a comprehensive list, other
scenarios are possible, especially on physically protected networks.
(1) A read-only directory, containing no sensitive data,
accessible to "anyone", and TCP connection hijacking or IP
spoofing is not a problem. This directory requires no
security functions except administrative service limits.
(2) A read-only directory containing no sensitive data; read
access is granted based on identity. TCP connection
hijacking is not currently a problem. This scenario requires
a secure authentication function.
(3) A read-only directory containing no sensitive data; and the
client needs to ensure that the directory data is
authenticated by the server and not modified while being
returned from the server.
(4) A read-write directory, containing no sensitive data; read
access is available to "anyone", update access to properly
authorized persons. TCP connection hijacking is not
currently a problem. This scenario requires a secure
authentication function.
(5) A directory containing sensitive data. This scenario
requires session confidentiality protection AND secure
authentication.
3. Authentication and Authorization: Definitions and Concepts
This section 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.
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RFC 2829 Authentication Methods for LDAP May 2000
3.1. Access Control Policy
An access control policy is a set of rules defining the protection of
resources, generally in terms of the capabilities of persons or other
entities accessing those resources. A common expression of an access
control policy is an access control list. Security objects and
mechanisms, such as those described here, enable the expression of
access control policies and their enforcement. Access control
policies are typically expressed in terms of access control
attributes as described below.
3.2. Access Control Factors
A request, when it is being processed by a server, may be associated
with a wide variety of security-related factors (section 4.2 of [1]).
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
connection via which the request is transmitted, others (e.g. time of
day) may be "environmental".
Access control policies are expressed in terms of access control
factors. E.g., a request having ACFs i,j,k can perform operation Y
on resource Z. The set of ACFs that a server makes available for such
expressions is implementation-specific.
3.3. Authentication, Credentials, Identity
Authentication credentials are the evidence supplied by one party to
another, asserting the identity of the supplying party (e.g. a user)
who is attempting to establish an association with the other party
(typically a server). Authentication is the process of generating,
transmitting, and verifying these credentials and thus the identity
they assert. An authentication identity is the name presented in a
credential.
There are many forms of authentication credentials -- the form used
depends upon the particular authentication mechanism negotiated by
the parties. For example: X.509 certificates, Kerberos tickets,
simple identity and password pairs. Note that an authentication
mechanism may constrain the form of authentication identities used
with it.
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RFC 2829 Authentication Methods for LDAP May 2000
3.4. 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; e.g., entity X can perform operation Y
on resource Z.
The authorization identity bound to an association is often exactly
the same as the authentication identity presented by the client, but
it may be different. SASL allows clients to specify an authorization
identity distinct from the authentication identity asserted by the
client's credentials. This permits agents such as proxy servers to
authenticate using their own credentials, yet request the access
privileges of the identity for which they are proxying [2]. 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.
4. Required security mechanisms
It is clear that allowing any implementation, faced with the above
requirements, to pick and choose among the possible alternatives is
not a strategy that is likely to lead to interoperability. In the
absence of mandates, clients will be written that do not support any
security function supported by the server, or worse, support only
mechanisms like cleartext passwords that provide clearly inadequate
security.
Active intermediary attacks are the most difficult for an attacker to
perform, and for an implementation to protect against. Methods that
protect only against hostile client and passive eavesdropping attacks
are useful in situations where the cost of protection against active
intermediary attacks is not justified based on the perceived risk of
active intermediary attacks.
Given the presence of the Directory, there is a strong desire to see
mechanisms where identities take the form of a Distinguished Name and
authentication data can be stored in the directory; this means that
either this data is useless for faking authentication (like the Unix
"/etc/passwd" file format used to be), or its content is never passed
across the wire unprotected - that is, it's either updated outside
the protocol or it is only updated in sessions well protected against
snooping. It is also desirable to allow authentication methods to
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RFC 2829 Authentication Methods for LDAP May 2000
carry authorization identities based on existing forms of user
identities for backwards compatibility with non-LDAP-based
authentication services.
Therefore, the following implementation conformance requirements are
in place:
(1) For a read-only, public directory, anonymous authentication,
described in section 5, can be used.
(2) Implementations providing password-based authenticated
access MUST support authentication using the DIGEST-MD5 SASL
mechanism [4], as described in section 6.1. This provides
client authentication with protection against passive
eavesdropping attacks, but does not provide protection
against active intermediary attacks.
(3) For a directory needing session protection and
authentication, the Start TLS extended operation [5], and
either the simple authentication choice or the SASL EXTERNAL
mechanism, are to be used together. Implementations SHOULD
support authentication with a password as described in
section 6.2, and SHOULD support authentication with a
certificate as described in section 7.1. Together, these
can provide integrity and disclosure protection of
transmitted data, and authentication of client and server,
including protection against active intermediary attacks.
If TLS is negotiated, the client MUST discard all information about
the server fetched prior to the TLS negotiation. In particular, the
value of supportedSASLMechanisms MAY be different after TLS has been
negotiated (specifically, the EXTERNAL mechanism or the proposed
PLAIN mechanism are likely to only be listed after a TLS negotiation
has been performed).
If a SASL security layer is negotiated, the client MUST discard all
information about the server fetched prior to SASL. In particular,
if the client is configured to support multiple SASL mechanisms, it
SHOULD fetch supportedSASLMechanisms both before and after the SASL
security layer is negotiated and verify that the value has not
changed after the SASL security layer was negotiated. This detects
active attacks which remove supported SASL mechanisms from the
supportedSASLMechanisms list, and allows the client to ensure that it
is using the best mechanism supported by both client and server
(additionally, this is a SHOULD to allow for environments where the
supported SASL mechanisms list is provided to the client through a
different trusted source, e.g. as part of a digitally signed object).
Wahl, et al. Standards Track [Page 6]
RFC 2829 Authentication Methods for LDAP May 2000
5. Anonymous authentication
Directory operations which modify entries or access protected
attributes or entries generally require client authentication.
Clients which do not intend to perform any of these operations
typically use anonymous authentication.
LDAP implementations MUST support anonymous authentication, as
defined in section 5.1.
LDAP implementations MAY support anonymous authentication with TLS,
as defined in section 5.2.
While there MAY be access control restrictions to prevent access to
directory entries, an LDAP server SHOULD allow an anonymously-bound
client to retrieve the supportedSASLMechanisms attribute of the root
DSE.
An LDAP server MAY use other information about the client provided by
the lower layers or external means to grant or deny access even to
anonymously authenticated clients.
5.1. Anonymous authentication procedure
An LDAP client which has not successfully completed a bind operation
on a connection is anonymously authenticated.
An LDAP client MAY also specify anonymous authentication in a bind
request by using a zero-length OCTET STRING with the simple
authentication choice.
5.2. Anonymous authentication and TLS
An LDAP client MAY use the Start TLS operation [5] to negotiate the
use of TLS security [6]. If the client has not bound beforehand,
then until the client uses the EXTERNAL SASL mechanism to negotiate
the recognition of the client's certificate, the client is
anonymously authenticated.
Recommendations on TLS ciphersuites are given in section 10.
An LDAP server which requests that clients provide their certificate
during TLS negotiation MAY use a local security policy to determine
whether to successfully complete TLS negotiation if the client did
not present a certificate which could be validated.
Wahl, et al. Standards Track [Page 7]
RFC 2829 Authentication Methods for LDAP May 2000
6. Password-based authentication
LDAP implementations MUST support authentication with a password
using the DIGEST-MD5 SASL mechanism for password protection, as
defined in section 6.1.
LDAP implementations SHOULD support authentication with the "simple"
password choice when the connection is protected against
eavesdropping using TLS, as defined in section 6.2.
6.1. Digest authentication
An LDAP client MAY determine whether the server supports this
mechanism by performing a search request on the root DSE, requesting
the supportedSASLMechanisms attribute, and checking whether the
string "DIGEST-MD5" is present as a value of this attribute.
In the first stage of authentication, when the client is performing
an "initial authentication" as defined in section 2.1 of [4], the
client sends a bind request in which the version number is 3, the
authentication choice is sasl, the sasl mechanism name is "DIGEST-
MD5", and the credentials are absent. The client then waits for a
response from the server to this request.
The server will respond with a bind response in which the resultCode
is saslBindInProgress, and the serverSaslCreds field is present. The
contents of this field is a string defined by "digest-challenge" in
section 2.1.1 of [4]. The server SHOULD include a realm indication
and MUST indicate support for UTF-8.
The client will send a bind request with a distinct message id, in
which the version number is 3, the authentication choice is sasl, the
sasl mechanism name is "DIGEST-MD5", and the credentials contain the
string defined by "digest-response" in section 2.1.2 of [4]. The
serv-type is "ldap".
The server will respond with a bind response in which the resultCode
is either success, or an error indication. If the authentication is
successful and the server does not support subsequent authentication,
then the credentials field is absent. If the authentication is
successful and the server supports subsequent authentication, then
the credentials field contains the string defined by "response-auth"
in section 2.1.3 of [4]. Support for subsequent authentication is
OPTIONAL in clients and servers.
Wahl, et al. Standards Track [Page 8]
RFC 2829 Authentication Methods for LDAP May 2000
6.2. "simple" authentication choice under TLS encryption
A user who has a directory entry containing a userPassword attribute
MAY authenticate to the directory by performing a simple password
bind sequence following the negotiation of a TLS ciphersuite
providing connection confidentiality [6].
The client will use the Start TLS operation [5] to negotiate the use
of TLS security [6] on the connection to the LDAP server. The client
need not have bound to the directory beforehand.
For this authentication procedure to be successful, the client and
server MUST negotiate a ciphersuite which contains a bulk encryption
algorithm of appropriate strength. Recommendations on cipher suites
are given in section 10.
Following the successful completion of TLS negotiation, the client
MUST send an LDAP bind request with the version number of 3, the name
field containing the name of the user's entry, and the "simple"
authentication choice, containing a password.
The server will, for each value of the userPassword attribute in the
named user's entry, compare these for case-sensitive equality with
the client's presented password. If there is a match, then the
server will respond with resultCode success, otherwise the server
will respond with resultCode invalidCredentials.
6.3. Other authentication choices with TLS
It is also possible, following the negotiation of TLS, to perform a
SASL authentication which does not involve the exchange of plaintext
reusable passwords. In this case the client and server need not
negotiate a ciphersuite which provides confidentiality if the only
service required is data integrity.
7. Certificate-based authentication
LDAP implementations SHOULD support authentication via a client
certificate in TLS, as defined in section 7.1.
7.1. Certificate-based authentication with TLS
A user who has a public/private key pair in which the public key has
been signed by a Certification Authority may use this key pair to
authenticate to the directory server if the user's certificate is
requested by the server. The user's certificate subject field SHOULD
be the name of the user's directory entry, and the Certification
Authority must be sufficiently trusted by the directory server to
Wahl, et al. Standards Track [Page 9]
RFC 2829 Authentication Methods for LDAP May 2000
have issued the certificate in order that the server can process the
certificate. The means by which servers validate certificate paths
is outside the scope of this document.
A server MAY support mappings for certificates in which the subject
field name is different from the name of the user's directory entry.
A server which supports mappings of names MUST be capable of being
configured to support certificates for which no mapping is required.
The client will use the Start TLS operation [5] to negotiate the use
of TLS security [6] on the connection to the LDAP server. The client
need not have bound to the directory beforehand.
In the TLS negotiation, the server MUST request a certificate. The
client will provide its certificate to the server, and MUST perform a
private key-based encryption, proving it has the private key
associated with the certificate.
As deployments will require protection of sensitive data in transit,
the client and server MUST negotiate a ciphersuite which contains a
bulk encryption algorithm of appropriate strength. Recommendations
of cipher suites are given in section 10.
The server MUST verify that the client's certificate is valid. The
server will normally check that the certificate is issued by a known
CA, and that none of the certificates on the client's certificate
chain are invalid or revoked. There are several procedures by which
the server can perform these checks.
Following the successful completion of TLS negotiation, the client
will send an LDAP bind request with the SASL "EXTERNAL" mechanism.
8. Other mechanisms
The LDAP "simple" authentication choice is not suitable for
authentication on the Internet where there is no network or transport
layer confidentiality.
As LDAP includes native anonymous and plaintext authentication
methods, the "ANONYMOUS" and "PLAIN" SASL mechanisms are not used
with LDAP. If an authorization identity of a form different from a
DN is requested by the client, a mechanism that protects the password
in transit SHOULD be used.
The following SASL-based mechanisms are not considered in this
document: KERBEROS_V4, GSSAPI and SKEY.
Wahl, et al. Standards Track [Page 10]
RFC 2829 Authentication Methods for LDAP May 2000
The "EXTERNAL" SASL mechanism can be used to request the LDAP server
make use of security credentials exchanged by a lower layer. If a TLS
session has not been established between the client and server prior
to making the SASL EXTERNAL Bind request and there is no other
external source of authentication credentials (e.g. IP-level
security [8]), or if, during the process of establishing the TLS
session, the server did not request the client's authentication
credentials, the SASL EXTERNAL bind MUST fail with a result code of
inappropriateAuthentication. Any client authentication and
authorization state of the LDAP association is lost, so the LDAP
association is in an anonymous state after the failure.
9. Authorization Identity
The authorization identity is carried as part of the SASL credentials
field in the LDAP Bind request and response.
When the "EXTERNAL" mechanism is being negotiated, if the credentials
field is present, it contains an authorization identity of the
authzId form described below.
Other mechanisms define the location of the authorization identity in
the credentials field.
The authorization identity is a string in the UTF-8 character set,
corresponding to the following ABNF [7]:
; Specific predefined authorization (authz) id schemes are
; defined below -- new schemes may be defined in the future.
authzId = dnAuthzId / uAuthzId
; distinguished-name-based authz id.
dnAuthzId = "dn:" dn
dn = utf8string ; with syntax defined in RFC 2253
; unspecified userid, UTF-8 encoded.
uAuthzId = "u:" userid
userid = utf8string ; syntax unspecified
A utf8string is defined to be the UTF-8 encoding of one or more ISO
10646 characters.
All servers which support the storage of authentication credentials,
such as passwords or certificates, in the directory MUST support the
dnAuthzId choice.
Wahl, et al. Standards Track [Page 11]
RFC 2829 Authentication Methods for LDAP May 2000
The uAuthzId choice allows for compatibility with client applications
which wish to authenticate to a local directory but do not know their
own Distinguished Name or have a directory entry. The format of the
string is defined as only a sequence of UTF-8 encoded ISO 10646
characters, and further interpretation is subject to prior agreement
between the client and server.
For example, the userid could identify a user of a specific directory
service, or be a login name or the local-part of an RFC 822 email
address. In general a uAuthzId MUST NOT be assumed to be globally
unique.
Additional authorization identity schemes MAY be defined in future
versions of this document.
10. TLS Ciphersuites
The following ciphersuites defined in [6] MUST NOT be used for
confidentiality protection of passwords or data:
TLS_NULL_WITH_NULL_NULL
TLS_RSA_WITH_NULL_MD5
TLS_RSA_WITH_NULL_SHA
The following ciphersuites defined in [6] can be cracked easily (less
than a week of CPU time on a standard CPU in 1997). The client and
server SHOULD carefully consider the value of the password or data
being protected before using these ciphersuites:
TLS_RSA_EXPORT_WITH_RC4_40_MD5
TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5
TLS_RSA_EXPORT_WITH_DES40_CBC_SHA
TLS_DH_DSS_EXPORT_WITH_DES40_CBC_SHA
TLS_DH_RSA_EXPORT_WITH_DES40_CBC_SHA
TLS_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA
TLS_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA
TLS_DH_anon_EXPORT_WITH_RC4_40_MD5
TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA
The following ciphersuites are vulnerable to man-in-the-middle
attacks, and SHOULD NOT be used to protect passwords or sensitive
data, unless the network configuration is such that the danger of a
man-in-the-middle attack is tolerable:
Wahl, et al. Standards Track [Page 12]
RFC 2829 Authentication Methods for LDAP May 2000
TLS_DH_anon_EXPORT_WITH_RC4_40_MD5
TLS_DH_anon_WITH_RC4_128_MD5
TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA
TLS_DH_anon_WITH_DES_CBC_SHA
TLS_DH_anon_WITH_3DES_EDE_CBC_SHA
A client or server that supports TLS MUST support at least
TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA.
11. SASL service name for LDAP
For use with SASL [2], a protocol must specify a service name to be
used with various SASL mechanisms, such as GSSAPI. For LDAP, the
service name is "ldap", which has been registered with the IANA as a
GSSAPI service name.
12. Security Considerations
Security issues are discussed throughout this memo; the
(unsurprising) conclusion is that mandatory security is important,
and that session encryption is required when snooping is a problem.
Servers are encouraged to prevent modifications by anonymous users.
Servers may also wish to minimize denial of service attacks by timing
out idle connections, and returning the unwillingToPerform result
code rather than performing computationally expensive operations
requested by unauthorized clients.
A connection on which the client has not performed the Start TLS
operation or negotiated a suitable SASL mechanism for connection
integrity and encryption services is subject to man-in-the-middle
attacks to view and modify information in transit.
Additional security considerations relating to the EXTERNAL mechanism
to negotiate TLS can be found in [2], [5] and [6].
13. Acknowledgements
This document is a product of the LDAPEXT Working Group of the IETF.
The contributions of its members is greatly appreciated.
Wahl, et al. Standards Track [Page 13]
RFC 2829 Authentication Methods for LDAP May 2000
14. Bibliography
[1] Wahl, M., Howes, T. and S. Kille, "Lightweight Directory Access
Protocol (v3)", RFC 2251, December 1997.
[2] Myers, J., "Simple Authentication and Security Layer (SASL)", RFC
2222, October 1997.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[4] Leach, P. and C. Newman, "Using Digest Authentication as a SASL
Mechanism", RFC 2831, May 2000.
[5] Hodges, J., Morgan, R. and M. Wahl, "Lightweight Directory Access
Protocol (v3): Extension for Transport Layer Security", RFC 2830,
May 2000.
[6] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
2246, January 1999.
[7] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[8] Kent, S. and R. Atkinson, "Security Architecture for the Internet
Protocol", RFC 2401, November 1998.
Wahl, et al. Standards Track [Page 14]
RFC 2829 Authentication Methods for LDAP May 2000
15. Authors' Addresses
Mark Wahl
Sun Microsystems, Inc.
8911 Capital of Texas Hwy #4140
Austin TX 78759
USA
EMail: M.Wahl@innosoft.com
Harald Tveit Alvestrand
EDB Maxware
Pirsenteret
N-7462 Trondheim, Norway
Phone: +47 73 54 57 97
EMail: Harald@Alvestrand.no
Jeff Hodges
Oblix, Inc.
18922 Forge Drive
Cupertino, CA 95014
USA
Phone: +1-408-861-6656
EMail: JHodges@oblix.com
RL "Bob" Morgan
Computing and Communications
University of Washington
Seattle, WA 98105
USA
Phone: +1-206-221-3307
EMail: rlmorgan@washington.edu
Wahl, et al. Standards Track [Page 15]
RFC 2829 Authentication Methods for LDAP May 2000
16. Full Copyright Statement
Copyright (C) The Internet Society (2000). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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Network Working Group J. Hodges
Request for Comments: 2830 Oblix Inc.
Category: Standards Track R. Morgan
Univ of Washington
M. Wahl
Sun Microsystems, Inc.
May 2000
Lightweight Directory Access Protocol (v3):
Extension for Transport Layer Security
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 Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
This document defines the "Start Transport Layer Security (TLS)
Operation" for LDAP [LDAPv3, TLS]. This operation provides for TLS
establishment in an LDAP association and is defined in terms of an
LDAP extended request.
1. Conventions Used in this Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [ReqsKeywords].
2. The Start TLS Request
This section describes the Start TLS extended request and extended
response themselves: how to form the request, the form of the
response, and enumerates the various result codes the client MUST be
prepared to handle.
The section following this one then describes how to sequence an
overall Start TLS Operation.
Hodges, et al. Standards Track [Page 1]
RFC 2830 LDAPv3: Extension for Transport Layer Security May 2000
2.1. Requesting TLS Establishment
A client may perform a Start TLS operation by transmitting an LDAP
PDU containing an ExtendedRequest [LDAPv3] specifying the OID for the
Start TLS operation:
1.3.6.1.4.1.1466.20037
An LDAP ExtendedRequest is defined as follows:
ExtendedRequest ::= [APPLICATION 23] SEQUENCE {
requestName [0] LDAPOID,
requestValue [1] OCTET STRING OPTIONAL }
A Start TLS extended request is formed by setting the requestName
field to the OID string given above. The requestValue field is
absent. The client MUST NOT send any PDUs on this connection
following this request until it receives a Start TLS extended
response.
When a Start TLS extended request is made, the server MUST return an
LDAP PDU containing a Start TLS extended response. An LDAP
ExtendedResponse is defined as follows:
ExtendedResponse ::= [APPLICATION 24] SEQUENCE {
COMPONENTS OF LDAPResult,
responseName [10] LDAPOID OPTIONAL,
response [11] OCTET STRING OPTIONAL }
A Start TLS extended response MUST contain a responseName field which
MUST be set to the same string as that in the responseName field
present in the Start TLS extended request. The response field is
absent. The server MUST set the resultCode field to either success or
one of the other values outlined in section 2.3.
2.2. "Success" Response
If the ExtendedResponse contains a resultCode of success, this
indicates that the server is willing and able to negotiate TLS. Refer
to section 3, below, for details.
2.3. Response other than "success"
If the ExtendedResponse contains a resultCode other than success,
this indicates that the server is unwilling or unable to negotiate
TLS.
Hodges, et al. Standards Track [Page 2]
RFC 2830 LDAPv3: Extension for Transport Layer Security May 2000
If the Start TLS extended request was not successful, the resultCode
will be one of:
operationsError (operations sequencing incorrect; e.g. TLS already
established)
protocolError (TLS not supported or incorrect PDU structure)
referral (this server doesn't do TLS, try this one)
unavailable (e.g. some major problem with TLS, or server is
shutting down)
The server MUST return operationsError if the client violates any of
the Start TLS extended operation sequencing requirements described in
section 3, below.
If the server does not support TLS (whether by design or by current
configuration), it MUST set the resultCode to protocolError (see
section 4.1.1 of [LDAPv3]), or to referral. The server MUST include
an actual referral value in the LDAP Result if it returns a
resultCode of referral. The client's current session is unaffected if
the server does not support TLS. The client MAY proceed with any LDAP
operation, or it MAY close the connection.
The server MUST return unavailable if it supports TLS but cannot
establish a TLS connection for some reason, e.g. the certificate
server not responding, it cannot contact its TLS implementation, or
if the server is in process of shutting down. The client MAY retry
the StartTLS operation, or it MAY proceed with any other LDAP
operation, or it MAY close the connection.
3. Sequencing of the Start TLS Operation
This section describes the overall procedures clients and servers
MUST follow for TLS establishment. These procedures take into
consideration various aspects of the overall security of the LDAP
association including discovery of resultant security level and
assertion of the client's authorization identity.
Note that the precise effects, on a client's authorization identity,
of establishing TLS on an LDAP association are described in detail in
section 5.
Hodges, et al. Standards Track [Page 3]
RFC 2830 LDAPv3: Extension for Transport Layer Security May 2000
3.1. Requesting to Start TLS on an LDAP Association
The client MAY send the Start TLS extended request at any time after
establishing an LDAP association, except that in the following cases
the client MUST NOT send a Start TLS extended request:
- if TLS is currently established on the connection, or
- during a multi-stage SASL negotiation, or
- if there are any LDAP operations outstanding on the connection.
The result of violating any of these requirements is a resultCode of
operationsError, as described above in section 2.3.
The client MAY have already performed a Bind operation when it sends
a Start TLS request, or the client might have not yet bound.
If the client did not establish a TLS connection before sending any
other requests, and the server requires the client to establish a TLS
connection before performing a particular request, the server MUST
reject that request with a confidentialityRequired or
strongAuthRequired result. The client MAY send a Start TLS extended
request, or it MAY choose to close the connection.
3.2. Starting TLS
The server will return an extended response with the resultCode of
success if it is willing and able to negotiate TLS. It will return
other resultCodes, documented above, if it is unable.
In the successful case, the client, which has ceased to transfer LDAP
requests on the connection, MUST either begin a TLS negotiation or
close the connection. The client will send PDUs in the TLS Record
Protocol directly over the underlying transport connection to the
server to initiate TLS negotiation [TLS].
3.3. TLS Version Negotiation
Negotiating the version of TLS or SSL to be used is a part of the TLS
Handshake Protocol, as documented in [TLS]. Please refer to that
document for details.
3.4. Discovery of Resultant Security Level
After a TLS connection is established on an LDAP association, both
parties MUST individually decide whether or not to continue based on
the privacy level achieved. Ascertaining the TLS connection's privacy
level is implementation dependent, and accomplished by communicating
with one's respective local TLS implementation.
Hodges, et al. Standards Track [Page 4]
RFC 2830 LDAPv3: Extension for Transport Layer Security May 2000
If the client or server decides that the level of authentication or
privacy is not high enough for it to continue, it SHOULD gracefully
close the TLS connection immediately after the TLS negotiation has
completed (see sections 4.1 and 5.2, below).
The client MAY attempt to Start TLS again, or MAY send an unbind
request, or send any other LDAP request.
3.5. Assertion of Client's Authorization Identity
The client MAY, upon receipt of a Start TLS extended response
indicating success, assert that a specific authorization identity be
utilized in determining the client's authorization status. The client
accomplishes this via an LDAP Bind request specifying a SASL
mechanism of "EXTERNAL" [SASL]. See section 5.1.2, below.
3.6. Server Identity Check
The client MUST check its understanding of the server's hostname
against the server's identity as presented in the server's
Certificate message, in order to prevent man-in-the-middle attacks.
Matching is performed according to these rules:
- The client MUST use the server hostname it used to open the LDAP
connection as the value to compare against the server name as
expressed in the server's certificate. The client MUST NOT use the
server's canonical DNS name or any other derived form of name.
- If a subjectAltName extension of type dNSName is present in the
certificate, it SHOULD be used as the source of the server's
identity.
- Matching is case-insensitive.
- The "*" wildcard character is allowed. If present, it applies only
to the left-most name component.
E.g. *.bar.com would match a.bar.com, b.bar.com, etc. but not
bar.com. If more than one identity of a given type is present in the
certificate (e.g. more than one dNSName name), a match in any one of
the set is considered acceptable.
If the hostname does not match the dNSName-based identity in the
certificate per the above check, user-oriented clients SHOULD either
notify the user (clients MAY give the user the opportunity to
Hodges, et al. Standards Track [Page 5]
RFC 2830 LDAPv3: Extension for Transport Layer Security May 2000
continue with the connection in any case) or terminate the connection
and indicate that the server's identity is suspect. Automated clients
SHOULD close the connection, returning and/or logging an error
indicating that the server's identity is suspect.
Beyond the server identity checks described in this section, clients
SHOULD be prepared to do further checking to ensure that the server
is authorized to provide the service it is observed to provide. The
client MAY need to make use of local policy information.
3.7. Refresh of Server Capabilities Information
The client MUST refresh any cached server capabilities information
(e.g. from the server's root DSE; see section 3.4 of [LDAPv3]) upon
TLS session establishment. This is necessary to protect against
active-intermediary attacks which may have altered any server
capabilities information retrieved prior to TLS establishment. The
server MAY advertise different capabilities after TLS establishment.
4. Closing a TLS Connection
4.1. Graceful Closure
Either the client or server MAY terminate the TLS connection on an
LDAP association by sending a TLS closure alert. This will leave the
LDAP association intact.
Before closing a TLS connection, the client MUST either wait for any
outstanding LDAP operations to complete, or explicitly abandon them
[LDAPv3].
After the initiator of a close has sent a closure alert, it MUST
discard any TLS messages until it has received an alert from the
other party. It will cease to send TLS Record Protocol PDUs, and
following the receipt of the alert, MAY send and receive LDAP PDUs.
The other party, if it receives a closure alert, MUST immediately
transmit a TLS closure alert. It will subsequently cease to send TLS
Record Protocol PDUs, and MAY send and receive LDAP PDUs.
4.2. Abrupt Closure
Either the client or server MAY abruptly close the entire LDAP
association and any TLS connection established on it by dropping the
underlying TCP connection. A server MAY beforehand send the client a
Notice of Disconnection [LDAPv3] in this case.
Hodges, et al. Standards Track [Page 6]
RFC 2830 LDAPv3: Extension for Transport Layer Security May 2000
5. Effects of TLS on a Client's Authorization Identity
This section describes the effects on a client's authorization
identity brought about by establishing TLS on an LDAP association.
The default effects are described first, and next the facilities for
client assertion of authorization identity are discussed including
error conditions. Lastly, the effects of closing the TLS connection
are described.
Authorization identities and related concepts are defined in
[AuthMeth].
5.1. TLS Connection Establishment Effects
5.1.1. Default Effects
Upon establishment of the TLS connection onto the LDAP association,
any previously established authentication and authorization
identities MUST remain in force, including anonymous state. This
holds even in the case where the server requests client
authentication via TLS -- e.g. requests the client to supply its
certificate during TLS negotiation (see [TLS]).
5.1.2. Client Assertion of Authorization Identity
A client MAY either implicitly request that its LDAP authorization
identity be derived from its authenticated TLS credentials or it MAY
explicitly provide an authorization identity and assert that it be
used in combination with its authenticated TLS credentials. The
former is known as an implicit assertion, and the latter as an
explicit assertion.
5.1.2.1. Implicit Assertion
An implicit authorization identity assertion is accomplished after
TLS establishment by invoking a Bind request of the SASL form using
the "EXTERNAL" mechanism name [SASL, LDAPv3] that SHALL NOT include
the optional credentials octet string (found within the
SaslCredentials sequence in the Bind Request). The server will derive
the client's authorization identity from the authentication identity
supplied in the client's TLS credentials (typically a public key
certificate) according to local policy. The underlying mechanics of
how this is accomplished are implementation specific.
Hodges, et al. Standards Track [Page 7]
RFC 2830 LDAPv3: Extension for Transport Layer Security May 2000
5.1.2.2. Explicit Assertion
An explicit authorization identity assertion is accomplished after
TLS establishment by invoking a Bind request of the SASL form using
the "EXTERNAL" mechanism name [SASL, LDAPv3] that SHALL include the
credentials octet string. This string MUST be constructed as
documented in section 9 of [AuthMeth].
5.1.2.3. Error Conditions
For either form of assertion, the server MUST verify that the
client's authentication identity as supplied in its TLS credentials
is permitted to be mapped to the asserted authorization identity. The
server MUST reject the Bind operation with an invalidCredentials
resultCode in the Bind response if the client is not so authorized.
Additionally, with either form of assertion, if a TLS session has not
been established between the client and server prior to making the
SASL EXTERNAL Bind request and there is no other external source of
authentication credentials (e.g. IP-level security [IPSEC]), or if,
during the process of establishing the TLS session, the server did
not request the client's authentication credentials, the SASL
EXTERNAL bind MUST fail with a result code of
inappropriateAuthentication.
After the above Bind operation failures, any client authentication
and authorization state of the LDAP association is lost, so the LDAP
association is in an anonymous state after the failure. TLS
connection state is unaffected, though a server MAY end the TLS
connection, via a TLS close_notify message, based on the Bind failure
(as it MAY at any time).
5.2. TLS Connection Closure Effects
Closure of the TLS connection MUST cause the LDAP association to move
to an anonymous authentication and authorization state regardless of
the state established over TLS and regardless of the authentication
and authorization state prior to TLS connection establishment.
6. Security Considerations
The goals of using the TLS protocol with LDAP are to ensure
connection confidentiality and integrity, and to optionally provide
for authentication. TLS expressly provides these capabilities, as
described in [TLS].
Hodges, et al. Standards Track [Page 8]
RFC 2830 LDAPv3: Extension for Transport Layer Security May 2000
All security gained via use of the Start TLS operation is gained by
the use of TLS itself. The Start TLS operation, on its own, does not
provide any additional security.
The use of TLS does not provide or ensure for confidentiality and/or
non-repudiation of the data housed by an LDAP-based directory server.
Nor does it secure the data from inspection by the server
administrators. Once established, TLS only provides for and ensures
confidentiality and integrity of the operations and data in transit
over the LDAP association, and only if the implementations on the
client and server support and negotiate it.
The level of security provided though the use of TLS depends directly
on both the quality of the TLS implementation used and the style of
usage of that implementation. Additionally, an active-intermediary
attacker can remove the Start TLS extended operation from the
supportedExtension attribute of the root DSE. Therefore, both parties
SHOULD independently ascertain and consent to the security level
achieved once TLS is established and before beginning use of the TLS
connection. For example, the security level of the TLS connection
might have been negotiated down to plaintext.
Clients SHOULD either warn the user when the security level achieved
does not provide confidentiality and/or integrity protection, or be
configurable to refuse to proceed without an acceptable level of
security.
Client and server implementors SHOULD take measures to ensure proper
protection of credentials and other confidential data where such
measures are not otherwise provided by the TLS implementation.
Server implementors SHOULD allow for server administrators to elect
whether and when connection confidentiality and/or integrity is
required, as well as elect whether and when client authentication via
TLS is required.
7. Acknowledgements
The authors thank Tim Howes, Paul Hoffman, John Kristian, Shirish
Rai, Jonathan Trostle, Harald Alvestrand, and Marcus Leech for their
contributions to this document.
Hodges, et al. Standards Track [Page 9]
RFC 2830 LDAPv3: Extension for Transport Layer Security May 2000
8. References
[AuthMeth] Wahl, M., Alvestrand, H., Hodges, J. and R. Morgan,
"Authentication Methods for LDAP", RFC 2829, May 2000.
[IPSEC] Kent, S. and R. Atkinson, "Security Architecture for
the Internet Protocol", RFC 2401, November 1998.
[LDAPv3] Wahl, M., Kille S. and T. Howes, "Lightweight
Directory Access Protocol (v3)", RFC 2251, December
1997.
[ReqsKeywords] Bradner, S., "Key Words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[SASL] Myers, J., "Simple Authentication and Security Layer
(SASL)", RFC 2222, October 1997.
[TLS] Dierks, T. and C. Allen. "The TLS Protocol Version
1.0", RFC 2246, January 1999.
9. Authors' Addresses
Jeff Hodges
Oblix, Inc.
18922 Forge Drive
Cupertino, CA 95014
USA
Phone: +1-408-861-6656
EMail: JHodges@oblix.com
RL "Bob" Morgan
Computing and Communications
University of Washington
Seattle, WA
USA
Phone: +1-206-221-3307
EMail: rlmorgan@washington.edu
Mark Wahl
Sun Microsystems, Inc.
8911 Capital of Texas Hwy #4140
Austin TX 78759
USA
EMail: M.Wahl@innosoft.com
Hodges, et al. Standards Track [Page 10]
RFC 2830 LDAPv3: Extension for Transport Layer Security May 2000
10. Intellectual Property Rights Notices
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
Hodges, et al. Standards Track [Page 11]
RFC 2830 LDAPv3: Extension for Transport Layer Security May 2000
11. Full Copyright Statement
Copyright (C) The Internet Society (2000). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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