    Internet Engineering Task Force (IETF)              L. Hornquist Astrand
    Request for Comments: 5896                                   Apple, Inc.
    Updates: 4120                                                 S. Hartman
    Category: Standards Track                         Painless Security, LLC
    ISSN: 2070-1721                                                June 2010

       Generic Security Service Application Program Interface (GSS-API):
                         Delegate if Approved by Policy

    Abstract

       Several Generic Security Service Application Program Interface
       (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, including
       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

       This is an Internet Standards Track document.

       This document is a product of the Internet Engineering Task Force
       (IETF).  It represents the consensus of the IETF community.  It has
       received public review and has been approved for publication by the
       Internet Engineering Steering Group (IESG).  Further information on
       Internet Standards is available in Section 2 of RFC 5741.

       Information about the current status of this document, any errata,
       and how to provide feedback on it may be obtained at
       http://www.rfc-editor.org/info/rfc5896.

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    RFC 5896         GSS-API: Delegate if Approved by Policy       June 2010

    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
       Provisions Relating to IETF Documents
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       publication of this document.  Please review these documents
       carefully, as they describe your rights and restrictions with respect
       to this document.  Code Components extracted from this document must
       include Simplified BSD License text as described in Section 4.e of
       the Trust Legal Provisions and are provided without warranty as
       described in the Simplified BSD License.

    Table of Contents

       1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 2
       2.  Requirements Notation . . . . . . . . . . . . . . . . . . . . . 3
       3.  GSS-API flag, C binding . . . . . . . . . . . . . . . . . . . . 3
       4.  GSS-API Behavior  . . . . . . . . . . . . . . . . . . . . . . . 3
       5.  Kerberos GSS-API Behavior . . . . . . . . . . . . . . . . . . . 4
       6.  Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
       7.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
       8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 5
       9.  Normative References  . . . . . . . . . . . . . . . . . . . . . 5

    1.  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, including
       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

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       facility to allow an administrator to communicate that a particular
       service is 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.

    2.  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].

    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

    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

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       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 is successful, deleg_state will return TRUE in the
       initiator and acceptor.  When delegation was successful, the
       deleg_state will return TRUE in the initiator and acceptor.
       Additionally, if the mechanism-specific policy recommended
       delegation, the deleg_policy_state will additionally return TRUE for
       the initiator (only).

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

    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 by [RFC2743].
       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 the 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.

    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 has no reason to trust.

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       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.

    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 spoofable 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 document.

       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.

    8.  Acknowledgements

       Thanks to Disco Vince Giffin, Thomas Maslen, Ken Raeburn, Martin Rex,
       Alexey Melnikov, Jacques Vidrine, Tom Yu, Hilarie Orman, and Shawn
       Emery for reviewing the document and providing suggestions for
       improvements.

    9.  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.

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    RFC 5896         GSS-API: Delegate if Approved by Policy       June 2010

       [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.

    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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