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draft-ietf-ldapext-authmeth-04.txt
1999-08-19 04:13:30 +08:00
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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INTERNET-DRAFT Authentication Methods for LDAP June 1999
; 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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INTERNET-DRAFT Authentication Methods for LDAP June 1999
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.
Wahl, Alvestrand, Hodges, Morgan Page 14
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