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.
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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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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
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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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[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]
