Network Working Group L. Hornquist Astrand
Internet-Draft Apple, Inc.
Intended status: Standards Track S. Hartman
Expires: June 12, 2009 Painless Security, LLC
December 9, 2008
GSS-API: Delegate if approved by policy
draft-lha-gssapi-delegate-policy-03
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Abstract
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 as well as
CIFS 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).
Table of Contents
1. Requirements Notation . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. GSS-API flag, C binding . . . . . . . . . . . . . . . . . . . 5
4. GSS-API behavior . . . . . . . . . . . . . . . . . . . . . . . 6
5. Kerberos GSS-API behavior . . . . . . . . . . . . . . . . . . 7
6. Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
10. Normative References . . . . . . . . . . . . . . . . . . . . . 12
Appendix A. Change history . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
Intellectual Property and Copyright Statements . . . . . . . . . . 15
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1. 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].
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2. 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 as well as
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 and administrator to communicate that a particular
service 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 would 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.
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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
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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 was
successful, deleg_state will return TRUE in both the initiator and
acceptor (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 was allows and successful,
deleg_state will return TRUE in both initiator and acceptor. In
addition, deleg_policy_state will be set in the initiator.
If the initiator set both the deleg_req_flag and
deleg_policy_req_flag, delegation will attempted unconditionally.
When delegation was successful, deleg_state will be returned TRUE in
the initiator and acceptor. However, the deleg_policy_state will
additionally be returned TRUE for the initiator (only) if the
mechanism-specific policy recommended delegation.
Note that deleg_policy_req_flag and deleg_policy_state apply the the
initiator only. There state is never sent over the wire.
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5. Kerberos GSS-API behavior
If the initiator sets the deleg_policy_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.
[RFC4120] does not adequately describe the behavior of OK-AS-DELEGATE
flag in a cross realm environment. This document clarifies that
behavior. In addition to the service tickets' OK-AS-DELEGATE flag,
the GSS-API Kerberos mechanism 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.
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6. Rationale
Strictly speaking, the existing deleg_req_flag could be interpreted
as in [RFC2743] as behaving as deleg_policy_req_flag 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 have 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.
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7. Security Considerations
Introduce 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 spoof-able on the network.
Delegating the user's TGT is still to power and dangerous. Ideally
one would delegate specific service tickets, but this is out of scope
of this draft.
Failure to specify deleg_policy_req_flag on the part of the client
can at worst result in NOT delegating credentials -- fails closed, a
desirable security property.
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8. IANA Considerations
This document doesnt have any IANA considerations, all registrations
are part of draft-ietf-kitten-gssapi-extensions-iana. RFC-EDIOR:
please remove this section.
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9. Acknowledgements
Thanks to Disco Vince Giffin, Thomas Maslen, Ken Raeburn, Martin Rex,
Jacques Vidrine and Tom Yu for reviewing the document and provided
suggestions for improvements.
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10. 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.
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Appendix A. Change history
RFC-EDITOR: please remove this section.
o Version 03: Feedback from Thomas Maslen. Remove IANA
considerations, Sam will work in the text into IANA draft as part
of the initial registry submission.
o Version 02: Comments from Disco and Jacques. Use deleg_req_flag
instead of GSS_C_DELEG_FLAG for all places that discuesses the
flag.
o Version 01: Document that GSS_C_DELEG_POLICY_FLAG is a local flag
from Martin Rex. Provide rationale as requested by Tom Yu. Ran
spell checker over document.
o Version 00: Inital draft by Love and cleaned up by Sam.
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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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