Internet-Draft                                     E. Stokes
          LDAP Extensions WG                                  D. Byrne
          Intended Category: Informational                  B. Blakley
          Expires: 16 February 1998                                IBM
                                                             P. Behera
                                                      16 February 1998
                      Access Control Requirements for LDAP
             This document is an Internet Draft. Internet Drafts are
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             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 RFC 2119
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             terminology is used in this document.
          1.  Introduction
             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.
          2.  Objectives
             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
             This generally leads to several general requirements that
             are discussed below.
          3.  Requirements
             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
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             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 requirement.
          3.1  General
             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 defaults.
             G3.  ACL administration SHOULD be part of the LDAP
             protocol.  Access control information MUST be an LDAP
             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.  Policy MUST be administrable on a per-object
             granularity or finer.
             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
             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
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             for user A than a union policy.
             S4.  Newly created directory entries SHOULD be subject to
             a secure default policy.
             S5.  Access policy SHOULD NOT be expressed in terms of
             attributes which the directory administrator or his
             organization cannot control (e.g. groups whose membership
             is administered by another organization).
             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 documented.
             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
             S8.  Explicit denial SHOULD NOT be supported (i.e.
             negative rights). If explicit denial is supported,
             explicit "don't care" SHOULD also be supported to allow
             administrators to independently state policies they are
             competent to manage.
             S9.  The system MUST be able (semantically) to support
             either default-grant or default-deny semantics (not
             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 a right to an object MUST NOT
             implicitly grant any other subject the same right to that
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             object.  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
             S14.  Nested groups SHOULD NOT be supported.  Nested
             groups refer to the ability for an administrator to place
             a group DN on the ACL where that group DN may include
             other group DNs. Nested groups ease administration and
             maintenance. However, the administrator may not readily
             know the consequences of his actions.  For example, if
             the administrator adds group A to the ACL and group A
             includes group B, then from the administrator's
             perspective group B is implicitly added and may give
             access to a user (in group B) who should not have access.
             This is consistent with S13.  This does not imply that
             nested groups cannot be implemented in the model (see
             definition of SHOULD NOT).
          3.3  Usability (Manageability)
             U1.  When in doubt, simpler is better, both at the
             interface and in the implementation.
             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
             U4.  Administrators SHOULD NOT be required to evaluate
             arbitrary Boolean predicates in order to create or
             understand policy.
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             U5.  Administrators SHOULD NOT be required to know the
             sensitivity of every attribute of every entry (dynamic
             schema makes this impossible anyway).
             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-directory-entry basis.
             U8.  Control of access to individual directory entry
             attributes (not just the whole directory entry) MUST be
             U9.  Administrator MUST be able to coarsen access policy
             granularity by grouping attributes with similar access
             U10.  Control of access on a per-user granularity MUST be
             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
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             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 not.
             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 strucutre MUST be
             able to be protected from view if so desired by the
             U15.  It MUST be possible to create publicly readable
             entries, which may be read even by unauthenticated
             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 the requirements.
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          5.  Glossary
             This glossary is intended to aid the novice not versed in
             depth about access control.  It contains a list [2] of
             terms and their definitions that are commonly used in
             discussing access control.
             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
             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
             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.
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             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 policy applies.
             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 policy applies.
          6.  References
             [1] Steve Kille, Tim Howes, M. Wahl, "Lightweight
             Directory Access Protocol (v3)", INTERNET-DRAFT <draft-
             ietf-asid-ldapv3-protocol-07.txt>, August 1997.
             [2] ECMA, "Security in Open Systems: A Security
             Framework" ECMA TR/46, July 1988
             Bob Blakley                        Ellen Stokes
             IBM                                IBM
             11400 Burnet Rd                    11400 Burnet Rd
             Austin, TX 78758                   Austin, TX 78758
             USA                                USA
             mail-to:      mail-to:
             phone: +1 512 838 8133             phone: +1 512 838 3725
             fax:   +1 512 838 0156             fax:   +1 512 838 0156
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          Internet-Draft        ACL Requirements       23 January 1998
             Debbie Byrne                       Prasanta Behera
             IBM                                Netscape
             11400 Burnet Rd                    501 Ellis Street
             Austin, TX 78758                   Mountain View, CA 94043
             USA                                USA
             mail-to:    mail-to:
             phone: +1 512 838 1960             phone: +1 650 937 4948
             fax:   +1 512 838 0156             fax:   +1 650 528-4164
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