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Network Working Group                                      E. StokesRequest for Comments: 2820                                  D. ByrneCategory: Informational                                          IBM                                                          B. Blakley                                                              Dascom                                                           P. Behera                                                            Netscape                                                            May 2000Access Control Requirements for LDAPStatus 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 Notice   Copyright (C) The Internet Society (2000).  All Rights Reserved.Abstract   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].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.   TheRFC 2119 terminology is used in this document.Stokes, et al.               Informational                      [Page 1]

RFC 2820          Access Control Requirements for LDAP          May 20002.  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 policies.   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 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 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.Stokes, et al.               Informational                      [Page 2]

RFC 2820          Access Control Requirements for LDAP          May 2000   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   default policy.   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   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 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 aStokes, et al.               Informational                      [Page 3]

RFC 2820          Access Control Requirements for LDAP          May 2000   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   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 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.Stokes, et al.               Informational                      [Page 4]

RFC 2820          Access Control Requirements for LDAP          May 2000   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 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   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 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.Stokes, et al.               Informational                      [Page 5]

RFC 2820          Access Control Requirements for LDAP          May 2000   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.5.  Glossary   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.Stokes, et al.               Informational                      [Page 6]

RFC 2820          Access Control Requirements for LDAP          May 2000   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   [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.Stokes, et al.               Informational                      [Page 7]

RFC 2820          Access Control Requirements for LDAP          May 20007. Authors' Addresses   Bob Blakley   Dascom   5515 Balcones Drive   Austin, TX 78731   USA   Phone: +1 512 458 4037  ext 5012   Fax:   +1 512 458 2377   EMail: blakley@dascom.com   Ellen Stokes   IBM   11400 Burnet Rd   Austin, TX 78758   USA   Phone: +1 512 838 3725   Fax:   +1 512 838 0156   EMail: stokes@austin.ibm.com   Debbie Byrne   IBM   11400 Burnet Rd   Austin, TX 78758   USA   Phone: +1 512 838 1930   Fax:   +1 512 838 8597   EMail: djbyrne@us.ibm.com   Prasanta Behera   Netscape   501 Ellis Street   Mountain View, CA 94043   USA   Phone: +1 650 937 4948   Fax:   +1 650 528-4164   EMail: prasanta@netscape.comStokes, et al.               Informational                      [Page 8]

RFC 2820          Access Control Requirements for LDAP          May 20008.  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.Stokes, et al.               Informational                      [Page 9]