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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: March 27, 2009                           Painless Security, LLC
                                                      September 23, 2008


                GSS-API: Delegate if approved by policy
                  draft-lha-gssapi-delegate-policy-01

Status of this Memo

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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 ability to act as a user
   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 the ability to act as a user
   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.  If the client sets
   the deleg_req_flag to gss_init_sec_context then the application
   requests delegation.  This requires client applications to know what
   services should be trusted for delegation.  In some cases, however, a
   central authority is in a better position to know what services
   should receive delegation than the client application.  Some
   mechanisms such as Kerberos [RFC4121] have a facility to allow a
   realm administrator to communicate that a particular service is a
   valid target for delegation.  In Kerberos, the KDC can set the OK-AS-
   DELEGATE flag in issued tickets.  However even in such a case,
   delegating to services for applications that do not need delegation
   is problematic.  So, it is desirable for a GSS-API client to be able
   to request delegation if and only-if central policy recommends
   delegation to the given target.

   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: if
   the deleg_policy_req flag is set, then delegation should be performed
   if recommended by central policy.  In addition, the C bindings are
   extended to define the following constant to represent this new flag.



   #define GSS_C_DELEG_POLICY_FLAG 32768









































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4.  GSS-API behavior

   As before, if the GSS_C_DELEG_FLAG is set, the GSS-API mechanism
   tries to delegate.  Output ret_flags contains the flag
   GSS_C_DELEG_FLAG if delegation is successful.

   If the GSS_C_DELEG_POLICY_FLAG is set, the code delegates only if the
   mechanism policy allows delegation.  If delegation is done, the
   output flag ret_flags contain both GSS_C_DELEG_FLAG and
   GSS_C_DELEG_POLICY_FLAG on the initator and GSS_C_DELEG_FLAG on the
   acceptor.

   If both GSS_C_DELEG_FLAG and GSS_C_DELEG_POLICY_FLAG are set, then
   delegation is attempted.  However GSS_C_DELEG_POLICY_FLAG is only set
   in ret_flags on the initiator if GSS_C_DELEG_POLICY_FLAG would have
   been sufficient to request delegation.

   GSS_C_DELEG_POLICY_FLAG is a local flag and is never sent over the
   wire and thus will never end up in returning flags of the acceptor.
































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5.  Kerberos GSS-API behavior

   If the GSS_C_DELEG_POLICY_FLAG is set, 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] is unclear in what the behavior of ok-as-delegate flag
   should be on cross realm.  This document clarify that behavior.  In
   addition to the service tickets ok-as-delegate flag the GSS-API
   Kerberos 5 mech MUST also look at the all cross realm tickets
   traversed between the users initial TGT and the service ticket.  If
   any of the intermediate cross realm TGT doesn't have the ok-as-
   delegate flag set, the client MUST not delegate.






































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6.  Rationale

   The flag GSS_C_DELEG_POLICY_FLAG shouldn't need to exist; the flag
   that it's updating, GSS_C_DELEG_FLAG can in [RFC2743] be read as
   behaving as GSS_C_DELEG_POLICY_FLAG is described in this document.

   However, GSS_C_DELEG_POLICY_FLAG needs to exist because existing code
   and user expectations depend on GSS-API mechanism implementations
   that do not honor ok-as-delegate and always delegate.

   In a more ideal world, the GSS_C_DELEG_FLAG would not have been
   implemented as unconditional delegation.  Such unconditional
   delegation is not very security conscious and allows users to spread
   their credentials all over the place, even to hosts that shouldn't be
   trusted.  The user is left with a choice that is very hard to make
   without insight into how the system is deployed at this particular
   installation: "Is it safe to delegate to this host?"

   If GSS_C_DELEG_FLAG had been originally implemented to obey the ok-
   as-delegate flag, then it would have been reasonable to define a
   GSS_C_DELEG_FORCE_FLAG to override the site policy.

   Today there are Kerberos implementations that don't support the ok-
   as-delegate flag in the Kerberos database.  If the implementation of
   the GSS_C_DELEG_FLAG were changed to honor the ok-as-delegate flag,
   users who deploy new client software, who often do so without
   coordinating with the Kerberos administrators at their site, 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.




















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7.  Security Considerations

   Introduce a flag what allows client to get help from the KDC when to
   delegate to servers, will limit what servers that client delegate
   too.














































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8.  IANA Considerations

   This section needs to be revised to be consistent with the kitten
   IANA draft.















































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9.  Acknowledgements

   Thanks to Martin Rex, Ken Raeburn 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 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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