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

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.

Copyright Notice

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   document authors.  All rights reserved.

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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
   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
   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.

   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.

   [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