Network Working Group L. Hornquist Astrand
Internet-Draft Apple, Inc.
Intended status: Standards Track S. Hartman
Expires: March 27, 2009 Painless Security, LLC
September 23, 2008
GSS-API: Delegate if approved by policy
draft-lha-gssapi-delegate-policy-01
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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 ability to act as a user
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 the ability to act as a user
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. If the client sets
the deleg_req_flag to gss_init_sec_context then the application
requests delegation. This requires client applications to know what
services should be trusted for delegation. In some cases, however, a
central authority is in a better position to know what services
should receive delegation than the client application. Some
mechanisms such as Kerberos [RFC4121] have a facility to allow a
realm administrator to communicate that a particular service is a
valid target for delegation. In Kerberos, the KDC can set the OK-AS-
DELEGATE flag in issued tickets. However even in such a case,
delegating to services for applications that do not need delegation
is problematic. So, it is desirable for a GSS-API client to be able
to request delegation if and only-if central policy recommends
delegation to the given target.
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: if
the deleg_policy_req flag is set, then delegation should be performed
if recommended by central policy. In addition, the C bindings are
extended to define the following constant to represent this new flag.
#define GSS_C_DELEG_POLICY_FLAG 32768
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4. GSS-API behavior
As before, if the GSS_C_DELEG_FLAG is set, the GSS-API mechanism
tries to delegate. Output ret_flags contains the flag
GSS_C_DELEG_FLAG if delegation is successful.
If the GSS_C_DELEG_POLICY_FLAG is set, the code delegates only if the
mechanism policy allows delegation. If delegation is done, the
output flag ret_flags contain both GSS_C_DELEG_FLAG and
GSS_C_DELEG_POLICY_FLAG on the initator and GSS_C_DELEG_FLAG on the
acceptor.
If both GSS_C_DELEG_FLAG and GSS_C_DELEG_POLICY_FLAG are set, then
delegation is attempted. However GSS_C_DELEG_POLICY_FLAG is only set
in ret_flags on the initiator if GSS_C_DELEG_POLICY_FLAG would have
been sufficient to request delegation.
GSS_C_DELEG_POLICY_FLAG is a local flag and is never sent over the
wire and thus will never end up in returning flags of the acceptor.
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5. Kerberos GSS-API behavior
If the GSS_C_DELEG_POLICY_FLAG is set, 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] is unclear in what the behavior of ok-as-delegate flag
should be on cross realm. This document clarify that behavior. In
addition to the service tickets ok-as-delegate flag the GSS-API
Kerberos 5 mech MUST also look at the all cross realm tickets
traversed between the users initial TGT and the service ticket. If
any of the intermediate cross realm TGT doesn't have the ok-as-
delegate flag set, the client MUST not delegate.
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6. Rationale
The flag GSS_C_DELEG_POLICY_FLAG shouldn't need to exist; the flag
that it's updating, GSS_C_DELEG_FLAG can in [RFC2743] be read as
behaving as GSS_C_DELEG_POLICY_FLAG is described in this document.
However, GSS_C_DELEG_POLICY_FLAG needs to exist because existing code
and user expectations depend on GSS-API mechanism implementations
that do not honor ok-as-delegate and always delegate.
In a more ideal world, the GSS_C_DELEG_FLAG would not have been
implemented as unconditional delegation. Such unconditional
delegation is not very security conscious and allows users to spread
their credentials all over the place, even to hosts that shouldn't be
trusted. The user is left with a choice that is very hard to make
without insight into how the system is deployed at this particular
installation: "Is it safe to delegate to this host?"
If GSS_C_DELEG_FLAG had been originally implemented to obey the ok-
as-delegate flag, then it would have been reasonable to define a
GSS_C_DELEG_FORCE_FLAG to override the site policy.
Today there are Kerberos implementations that don't support the ok-
as-delegate flag in the Kerberos database. If the implementation of
the GSS_C_DELEG_FLAG were changed to honor the ok-as-delegate flag,
users who deploy new client software, who often do so without
coordinating with the Kerberos administrators at their site, 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.
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7. Security Considerations
Introduce a flag what allows client to get help from the KDC when to
delegate to servers, will limit what servers that client delegate
too.
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8. IANA Considerations
This section needs to be revised to be consistent with the kitten
IANA draft.
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9. Acknowledgements
Thanks to Martin Rex, Ken Raeburn 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 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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