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Internet Engineering Task Force (IETF)              L. Hornquist AstrandRequest for Comments: 5896                                   Apple, Inc.Updates:4120                                                 S. HartmanCategory: Standards Track                         Painless Security, LLCISSN: 2070-1721                                                June 2010Generic Security Service Application Program Interface (GSS-API):Delegate if Approved by PolicyAbstract   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 (RFC2743).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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc5896.Hornquist Astrand & Hartman  Standards Track                    [Page 1]

RFC 5896         GSS-API: Delegate if Approved by Policy       June 2010Copyright Notice   Copyright (c) 2010 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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 Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .22.  Requirements Notation . . . . . . . . . . . . . . . . . . . . .33.  GSS-API flag, C binding . . . . . . . . . . . . . . . . . . . .34.  GSS-API Behavior  . . . . . . . . . . . . . . . . . . . . . . .35.  Kerberos GSS-API Behavior . . . . . . . . . . . . . . . . . . .46.  Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . .47.  Security Considerations . . . . . . . . . . . . . . . . . . . .58.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . .59.  Normative References  . . . . . . . . . . . . . . . . . . . . .51.  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 aHornquist 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 327684.  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 outputHornquist 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.eduHornquist Astrand & Hartman  Standards Track                    [Page 6]