Generic Security Service Application Program Interface (GSS-API): Delegate if Approved by Policy
draft-lha-gssapi-delegate-policy-05

Network Working Group                               L. Hornquist Astrand
Internet-Draft                                               Apple, Inc.
Intended status: Standards Track                              S. Hartman
Expires: September 23, 2010                       Painless Security, LLC
                                                          March 22, 2010

                GSS-API: Delegate if approved by policy
                  draft-lha-gssapi-delegate-policy-05

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 (Common Internet File System) 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).

Status of this Memo

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Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   described in the BSD License.

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

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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 an 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 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 is
   successful, deleg_state will return TRUE in both the initiator and
   acceptor output state (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 is allowed and successful,
   deleg_state will return TRUE in both initiator and acceptor output
   state.  In addition, deleg_policy_state will be set in the initiator
   output state.

   If the initiator sets both the deleg_req_flag and
   deleg_policy_req_flag, delegation will be 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
   initiator only.  Their 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 (and not
   deleg_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.  If the initiator sets deleg_req_flag (and not
   deleg_policy_req_flag) the behavior will be as defined before.  If
   the initiator set both the deleg_req_flag and deleg_policy_req_flag,
   delegation will be attempted unconditionally.

   [RFC4120] does not adequately describe the behavior of OK-AS-DELEGATE
   flag in a cross realm environment.  This document clarifies that
   behavior.  If the initiator sets the deleg_policy_req_flag, the GSS-
   API Kerberos mechanism MUST examine the OK-AS-DELEGATE flag in the
   service ticket, and it 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 deleg_req_flag behavior in [RFC2743] could be
   interpreted the same as deleg_policy_req_flag is 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

   This document introduces 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 too powerful and dangerous.
   Ideally one would delegate specific service tickets, but this is out
   of scope of this draft.

   A client's failure to specify deleg_policy_req_flag can at worst
   result in NOT delegating credentials.  This means that the client
   does not expand its trust, which is generally safer than the
   alternative.

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8.  IANA Considerations

   This document doesnt have any IANA considerations, all registrations
   are part of draft-ietf-kitten-gssapi-extensions-iana.  RFC-EDIOR:
   please remove this section.

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9.  Acknowledgements

   Thanks to Disco Vince Giffin, Thomas Maslen, Ken Raeburn, Martin Rex,
   Alexey Melnikov, Jacques Vidrine, Tom Yu and Hilarie Orman, Shawn
   Emery 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 05: GEN-ART review.  SEC-DIR review.  Shawn Emery review.

   o  Version 04: Feedback from Thomas Maslen.  Clarify chapter 5.

   o  Version 03: Feedback from Thomas Maslen.  Remove IANA
      considerations, Sam will work in the text into IANA draft as part
      of the initial registry submission.

   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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