Network Working Group E. Stokes
Request for Comments: 2820 D. Byrne
Category: Informational IBM
May 2000 Access Control Requirements for LDAP
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright (C) The Internet Society (2000). All Rights Reserved.
This document describes the fundamental requirements of an access
control list (ACL) model for the Lightweight Directory Application
Protocol (LDAP) directory service. It is intended to be a gathering
place for access control requirements needed to provide authorized
access to and interoperability between directories.
The keywords "MUST", "SHOULD", and "MAY" used in this document are to
be interpreted as described in [bradner97].
The ability to securely access (replicate and distribute) directory
information throughout the network is necessary for successful
deployment. LDAP's acceptance as an access protocol for directory
information is driving the need to provide an access control model
definition for LDAP directory content among servers within an
enterprise and the Internet. Currently LDAP does not define an
access control model, but is needed to ensure consistent secure
access across heterogeneous LDAP implementations. The requirements
for access control are critical to the successful deployment and
acceptance of LDAP in the market place.
The RFC 2119 terminology is used in this document.
The major objective is to provide a simple, but secure, highly
efficient access control model for LDAP while also providing the
appropriate flexibility to meet the needs of both the Internet and
enterprise environments and policies.
This generally leads to several general requirements that are
This section is divided into several areas of requirements: general,
semantics/policy, usability, and nested groups (an unresolved issue).
The requirements are not in any priority order. Examples and
explanatory text is provided where deemed necessary. Usability is
perhaps the one set of requirements that is generally overlooked, but
must be addressed to provide a secure system. Usability is a security
issue, not just a nice design goal and requirement. If it is
impossible to set and manage a policy for a secure situation that a
human can understand, then what was set up will probably be non-
secure. We all need to think of usability as a functional security
G1. Model SHOULD be general enough to support extensibility to add
desirable features in the future.
G2. When in doubt, safer is better, especially when establishing
G3. ACL administration SHOULD be part of the LDAP protocol. Access
control information MUST be an LDAP attribute.
G4. Object reuse protection SHOULD be provided and MUST NOT inhibit
implementation of object reuse. The directory SHOULD support policy
controlling the re-creation of deleted DNs, particularly in cases
where they are re-created for the purpose of assigning them to a
subject other than the owner of the deleted DN.
3.2 Semantics / Policy
S1. Omitted as redundant; see U8.
S2. More specific policies must override less specific ones (e.g.
individual user entry in ACL SHOULD take precedence over group entry)
for the evaluation of an ACL.
S3. Multiple policies of equal specificity SHOULD be combined in
some easily-understood way (e.g. union or intersection). This is
best understood by example. Suppose user A belongs to 3 groups and
those 3 groups are listed on the ACL. Also suppose that the
permissions for each of those groups are not identical. Each group is
of equal specificity (e.g. each group is listed on the ACL) and the
policy for granting user A access (given the example) SHOULD be
combined in some easily understood way, such as by intersection or
union. For example, an intersection policy here may yield a more
limited access for user A than a union policy.
S4. Newly created directory entries SHOULD be subject to a secure
S5. Access policy SHOULD NOT be expressed in terms of attributes
which the directory administrator or his organization cannot
administer (e.g. groups whose membership is administered by another
S6. Access policy SHOULD NOT be expressed in terms of attributes
which are easily forged (e.g. IP addresses). There may be valid
reasons for enabling access based on attributes that are easily
forged and the behavior/implications of doing that should be
S7. Humans (including administrators) SHOULD NOT be required to
manage access policy on the basis of attributes which are not
"human-readable" (e.g. IP addresses).
S8. It MUST be possible to deny a subject the right to invoke a
directory operation. The system SHOULD NOT require a specific
implementation of denial (e.g. explicit denial, implicit denial).
S9. The system MUST be able (semantically) to support either
default-grant or default-deny semantics (not simultaneously).
S10. The system MUST be able to support either union semantics or
intersection semantics for aggregate subjects (not simultaneously).
S11. Absence of policy SHOULD be interpretable as grant or deny.
Deny takes precedence over grant among entries of equal specificity.
S12. ACL policy resolution MUST NOT depend on the order of entries
in the ACL.
S13. Rights management MUST have no side effects. Granting a
subject one right to an object MUST NOT implicitly grant the same or
any other subject a different right to the same object. Granting a
privilege attribute to one subject MUST NOT implicitly grant the same
privilege attribute to any other subject. Granting a privilege
attribute to one subject MUST NOT implicitly grant a different
privilege attribute to the same or any other subject. Definition: An
ACL's "scope" is defined as the set of directory objects governed by
the policy it defines; this set of objects is a sub-tree of the
directory. Changing the policy asserted by an ACL (by changing one
or more of its entries) MUST NOT implicitly change the policy
governed by an ACL in a different scope.
S14. It SHOULD be possible to apply a single policy to multiple
directory entries, even if those entries are in different subtrees.
Applying a single policy to multiple directory entries SHOULD NOT
require creation and storage of multiple copies of the policy data.
The system SHOULD NOT require a specific implementation (e.g. nested
groups, named ACLs) of support for policy sharing.
3.3 Usability (Manageability)
U1. When in doubt, simpler is better, both at the interface and in
U2. Subjects MUST be drawn from the "natural" LDAP namespace; they
should be DNs.
U3. It SHOULD NOT be possible via ACL administration to lock all
users, including all administrators, out of the directory.
U4. Administrators SHOULD NOT be required to evaluate arbitrary
Boolean predicates in order to create or understand policy.
U5. Administrators SHOULD be able to administer access to
directories and their attributes based on their sensitivity, without
having to understand the semantics of individual schema elements and
their attributes (see U9).
U6. Management of access to resources in an entire subtree SHOULD
require only one ACL (at the subtree root). Note that this makes
access control based explicitly on attribute types very hard, unless
you constrain the types of entries in subtrees. For example, another
attribute is added to an entry. That attribute may fall outside the
grouping covered by the ACL and hence require additional
administration where the desired affect is indeed a different ACL.
Access control information specified in one administrative area MUST
NOT have jurisdiction in another area. You SHOULD NOT be able to
control access to the aliased entry in the alias. You SHOULD be able
to control access to the alias name.
U7. Override of subtree policy MUST be supported on a per-
U8. Control of access to individual directory entry attributes (not
just the whole directory entry) MUST be supported.
U9. Administrator MUST be able to coarsen access policy granularity
by grouping attributes with similar access sensitivities.
U10. Control of access on a per-user granularity MUST be supported.
U11. Administrator MUST be able to aggregate users (for example, by
assigning them to groups or roles) to simplify administration.
U12. It MUST be possible to review "effective access" of any user,
group, or role to any entry's attributes. This aids the administrator
in setting the correct policy.
U13. A single administrator SHOULD be able to define policy for the
entire directory tree. An administrator MUST be able to delegate
policy administration for specific subtrees to other users. This
allows for the partitioning of the entire directory tree for policy
administration, but still allows a single policy to be defined for
the entire tree independent of partitioning. (Partition in this
context means scope of administration). An administrator MUST be able
to create new partitions at any point in the directory tree, and MUST
be able to merge a superior and subordinate partition. An
administrator MUST be able to configure whether delegated access
control information from superior partitions is to be accepted or
U14. It MUST be possible to authorize users to traverse directory
structure even if they are not authorized to examine or modify some
traversed entries; it MUST also be possible to prohibit this. The
tree structure MUST be able to be protected from view if so desired
by the administrator.
U15. It MUST be possible to create publicly readable entries, which
may be read even by unauthenticated clients.
U16. The model for combining multiple access control list entries
referring to a single individual MUST be easy to understand.
U17. Administrator MUST be able to determine where inherited policy
information comes from, that is, where ACLs are located and which
ACLs were applied. Where inheritance of ACLs is applied, it must be
able to be shown how/where that new ACL is derived from.
U18. It SHOULD be possible for the administrator to configure the
access control system to permit users to grant additional access
control rights for entries which they create.
4. Security Considerations
Access control is a security consideration. This documents addresses
This glossary is intended to aid the novice not versed in depth about
access control. It contains a list of terms and their definitions
that are commonly used in discussing access control [emca].
Access control - The prevention of use of a resource by unidentified
and/or unauthorized entities in any other that an authorized manner.
Access control list - A set of control attributes. It is a list,
associated with a security object or a group of security objects.
The list contains the names of security subjects and the type of
access that may be granted.
Access control policy - A set of rules, part of a security policy, by
which human users, or their representatives, are authenticated and by
which access by these users to applications and other services and
security objects is granted or denied.
Access context - The context, in terms of such variables as location,
time of day, level of security of the underlying associations, etc.,
in which an access to a security object is made.
Authorization - The granting of access to a security object.
Authorization policy - A set of rules, part of an access control
policy, by which access by security subjects to security objects is
granted or denied. An authorization policy may be defined in terms
of access control lists, capabilities, or attributes assigned to
security subjects, security objects, or both.
Control attributes - Attributes, associated with a security object
that, when matched against the privilege attributes of a security
subject, are used to grant or deny access to the security object. An
access control list or list of rights or time of day range are
examples of control attributes.
Credentials - Data that serve to establish the claimed identity of a
security subject relative to a given security domain.
Privilege attributes - Attributes, associated with a security subject
that, when matched against control attributes of a security object,
are used to grant or deny access to that subject. Group and role
memberships are examples of privilege attributes.
Security attributes - A general term covering both privilege
attributes and control attributes. The use of security attributes is
defined by a security policy.
Security object - An entity in a passive role to which a security
Security policy - A general term covering both access control
policies and authorization policies.
Security subject - An entity in an active role to which a security
[ldap] Kille, S., Howes, T. and M. Wahl, "Lightweight Directory
Access Protocol (v3)", RFC 2251, August 1997.
[ecma] ECMA, "Security in Open Systems: A Security Framework"
ECMA TR/46, July 1988.
[bradner97] Bradner, S., "Key Words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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