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rfc:rfc5896

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.

Hornquist Astrand & Hartman Standards Track [Page 1] 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
 (http://trustee.ietf.org/license-info) in effect on the date of
 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

Hornquist Astrand & Hartman Standards Track [Page 2] RFC 5896 GSS-API: Delegate if Approved by Policy June 2010

 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

Hornquist Astrand & Hartman Standards Track [Page 3] RFC 5896 GSS-API: Delegate if Approved by Policy June 2010

 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.

Hornquist Astrand & Hartman Standards Track [Page 4] RFC 5896 GSS-API: Delegate if Approved by Policy June 2010

 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.

Hornquist Astrand & Hartman Standards Track [Page 5] 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

Hornquist Astrand & Hartman Standards Track [Page 6]

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