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Versions: 00 01 02 03 04 05 RFC 5896

Network Working Group                               L. Hornquist Astrand
Internet-Draft                                               Apple, Inc.
Intended status: Standards Track                              S. Hartman
Expires: June 11, 2009                            Painless Security, LLC
                                                        December 8, 2008


                GSS-API: Delegate if approved by policy
                  draft-lha-gssapi-delegate-policy-02

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Abstract

   Several GSS-API applications work in a multi-tiered architecture,
   where the server takes advantage of delegated user credentials to act
   on behalf of the user and contact additional servers.  In effect, the
   server acts as an agent on behalf of the user.  Examples include web
   applications that need to access e-mail or file servers as well as
   CIFS file servers.  However, delegating the user credentials to a
   party who is not sufficiently trusted is problematic from a security
   standpoint.  Kerberos provides a flag called OK-AS-DELEGATE that
   allows the administrator of a Kerberos realm to communicate that a
   particular service is trusted for delegation.  This specification
   adds support for this flag and similar facilities in other
   authentication mechanisms to GSS-API (RFC 2743).


Table of Contents

   1.  Requirements Notation  . . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  GSS-API flag, C binding  . . . . . . . . . . . . . . . . . . .  5
   4.  GSS-API behavior . . . . . . . . . . . . . . . . . . . . . . .  6
   5.  Kerberos GSS-API behavior  . . . . . . . . . . . . . . . . . .  7
   6.  Rationale  . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
   10. Normative References . . . . . . . . . . . . . . . . . . . . . 12
   Appendix A.  Change history  . . . . . . . . . . . . . . . . . . . 13
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
   Intellectual Property and Copyright Statements . . . . . . . . . . 15




















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1.  Requirements Notation

   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 [RFC2119].














































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2.  Introduction

   Several GSS-API applications work in a multi-tiered architecture,
   where the server takes advantage of delegated user credentials to act
   on behalf of the user and contact additional servers.  In effect, the
   server acts as an agent on behalf of the user.  Examples include web
   applications that need to access e-mail or file servers as well as
   CIFS file servers.  However, delegating user credentials to a party
   who is not sufficiently trusted is problematic from a security
   standpoint.

   Today, GSS-API [RFC2743] leaves the determination of whether
   delegation is desired to the client application.  An application
   requests delegation by setting the deleg_req_flag when calling
   init_sec_context.  This requires client applications to know what
   services should be trusted for delegation.

   However blindly delegating to services for applications that do not
   need delegation is problematic.  In some cases a central authority is
   in a better position than the client application to know what
   services should receive delegation.  Some GSS-API mechanisms have a
   facility to allow and administrator to communicate that a particular
   service an appropriate target for delegation.  For example, a
   Kerberos [RFC4121] KDC can set the OK-AS-DELEGATE flag in issued
   tickets as such an indication.  It would is desirable to expose this
   knowledge to the GSS-API client so the client can request delegation
   if and only-if central policy recommends delegation to the given
   service.

   This specification adds a new input flag to gss_init_sec_context() to
   request delegation when approved by central policy.  In addition, a
   constant value to be used in the GSS-API C bindings [RFC2744] is
   defined.  Finally, the behavior for the Kerberos mechanism [RFC4121]
   is specified.

















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3.  GSS-API flag, C binding

   The gss_init_sec_context API is extended to gain a new input flag
   deleg_policy_req_flag, and a new output flag, deleg_policy_state
   BOOLEAN.  If the deleg_policy_req_flag is set, then delegation should
   be performed if recommended by central policy.  When delegation was
   recommended by the central policy and when delegation was done, the
   output flag deleg_policy_state will be set.

   In addition, the C bindings are extended to define the following
   constant to represent both deleg_policy_req_flag and
   deleg_policy_state (just like GSS_C_DELEG_FLAG maps to two flags).



   #define GSS_C_DELEG_POLICY_FLAG 32768



































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4.  GSS-API behavior

   As before, if the deleg_req_flag is set, the GSS-API mechanism will
   attempt delegation of user credentials.  When delegation was
   successful, deleg_state will return TRUE in both the initiator and
   acceptor (gss_init_sec_context and gss_accept_sec_context
   respectively).

   Similarly, if the deleg_policy_req_flag is set, then the GSS-API
   mechanism will attempt delegation if the mechanism-specific policy
   recommends to do so.  When delegation was allows and successful,
   deleg_state will return TRUE in both initiator and acceptor.  In
   addition, deleg_policy_state will be set in the acceptor.

   If the initiator set both the deleg_req_flag and
   deleg_policy_req_flag, delegation will attempted unconditionally.
   When delegation was successful, deleg_state will be returned TRUE in
   the initiator and acceptor.  However, the deleg_policy_state will
   additionally be returned TRUE for the initiator (only) if the
   mechanism-specific policy recommended delegation.

   Note that deleg_policy_req_flag and deleg_policy_state apply the the
   initiator only.  There state is never sent over the wire.




























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5.  Kerberos GSS-API behavior

   If the initiator sets the deleg_policy_req_flag, the Kerberos GSS-API
   mechanism MUST only delegate if OK-AS-DELEGATE is set [RFC4120] in
   the service ticket.  Other policy checks MAY be applied.

   [RFC4120] does not adequately describe the behavior of OK-AS-DELEGATE
   flag in a cross realm environment.  This document clarifies that
   behavior.  In addition to the service tickets' OK-AS-DELEGATE flag,
   the GSS-API Kerberos mechanism MUST examine all cross realm tickets
   in the traversal from the user's initial ticket-granting-ticket (TGT)
   to the service ticket.  If any of the intermediate cross realm TGTs
   do not have the OK-AS-DELEGATE flag set, the mechanism MUST not
   delegate credentials.





































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6.  Rationale

   Strictly speaking, the existing deleg_req_flag could be interpreted
   as in [RFC2743] as behaving as deleg_policy_req_flag described in
   this document.  However in practice the new flag is required because
   existing applications and user expectations depend upon GSS-API
   mechanism implementations without the described behavior, i.e. they
   do not respect OK-AS-DELEGATE.

   In hind sight, the deleg_req_flag should not have been implemented to
   mean unconditional delegation.  Such promiscuous delegation reduces
   overall security by unnecessarily exposing user credentials,
   including to hosts and services that the user have no reason to
   trust.

   Today there are Kerberos implementations that do not support the OK-
   AS-DELEGATE flag in the Kerberos database.  If the implementation of
   the deleg_req_flag were changed to honor the OK-AS-DELEGATE flag,
   users who deploy new client software, would never achieve credential
   delegation because the KDC would never issue a ticket with the OK-AS-
   DELEGATE flag set.  Changing the client software behavior in this way
   would cause a negative user experience for those users.  This is
   compounded by the fact that users often deploy new software without
   coordinating with site administrators.



























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7.  Security Considerations

   Introduce a flag that allows the client to get help from the KDC in
   determining to which servers one should delegate credentials, and the
   servers to which the client can delegate.

   The new flag deleg_policy_req_flag is not communicated over the wire,
   and thus does not present a new opportunity for spoofing or
   downgrading policy in and of itself.

   Mechanisms should use a trusted/authenticated means of determining
   delegation policy, and it must not be spoof-able on the network.

   Delegating the user's TGT is still to power and dangerous.  Ideally
   one would delegate specific service tickets, but this is out of scope
   of this draft.

   Failure to specify deleg_policy_req_flag on the part of the client
   can at worst result in NOT delegating credentials -- fails closed, a
   desirable security property.































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8.  IANA Considerations

   This document extends GSS-API Generic name space and Languange
   Bindings-specific GSS-API Namespace.  If the IANA registration draft
   is not in WG-LC by May 2009, please drop this section and publish.
   In that case this flags will be part of initial registration.

   +--------------------+----------------------------------------------+
   |        Field       |                     Value                    |
   +--------------------+----------------------------------------------+
   |  Registration type |                   Instance                   |
   |                    |                                              |
   |      Bindings      |             Generic and C binding            |
   |                    |                                              |
   |     Object type    |                 Context-flag                 |
   |                    |                                              |
   |    Symbol names    |            GSS_C_DELEG_POLICY_FLAG           |
   |                    |                                              |
   |     Binding of     | deleg_policy_req_flag and deleg_policy_state |
   |                    |                                              |
   |   Constant value   |                     32768                    |
   |                    |                                              |
   |     Description    |             Delegate policy flag             |
   |                    |                                              |
   |      Reference     |  RFC XXX, RFC-EDITOR: replace this with the  |
   |                    |               right RFC number               |
   |                    |                                              |
   |   Expert Reviewer  |             Kitten working group             |
   |                    |                                              |
   |   Expert reviewer  |                                              |
   |      comments      |                                              |
   |                    |                                              |
   |       Status       |                  Registered                  |
   +--------------------+----------------------------------------------+

                             IANA Registration















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9.  Acknowledgements

   Thanks to Disco Vince Giffin, Ken Raeburn, Martin Rex, Jacques
   Vidrine and Tom Yu for reviewing the document and provided
   suggestions for improvements.














































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10.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2743]  Linn, J., "Generic Security Service Application Program
              Interface Version 2, Update 1", RFC 2743, January 2000.

   [RFC2744]  Wray, J., "Generic Security Service API Version 2 :
              C-bindings", RFC 2744, January 2000.

   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
              Kerberos Network Authentication Service (V5)", RFC 4120,
              July 2005.

   [RFC4121]  Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos
              Version 5 Generic Security Service Application Program
              Interface (GSS-API) Mechanism: Version 2", RFC 4121,
              July 2005.
































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Appendix A.  Change history

   RFC-EDITOR: please remove this section.

   o  Version 02: Comments from Disco and Jacques.  Use deleg_req_flag
      instead of GSS_C_DELEG_FLAG for all places that discuesses the
      flag.

   o  Version 01: Document that GSS_C_DELEG_POLICY_FLAG is a local flag
      from Martin Rex. Provide rationale as requested by Tom Yu.  Ran
      spell checker over document.

   o  Version 00: Inital draft by Love and cleaned up by Sam.






































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Authors' Addresses

   Love Hornquist Astrand
   Apple, Inc.

   Email: lha@apple.com


   Sam Hartman
   Painless Security, LLC

   Email: hartmans-ietf@mit.edu







































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