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
2.  Introduction
3.  GSS-API flag, c binding
4.  GSS-API behavior
5.  Kerberos GSS-API behavior
6.  Rationale
7.  Security Considerations
8.  IANA Considerations
9.  Acknowledgements
10.  Normative References
Appendix A.  Change history
§  Authors' Addresses
§  Intellectual Property and Copyright Statements

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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

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] (Linn, J., “Generic Security Service Application Program Interface Version 2, Update 1,” January 2000.) 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] (Zhu, L., Jaganathan, K., and S. Hartman, “The Kerberos Version 5 Generic Security Service Application Program Interface (GSS-API) Mechanism: Version 2,” July 2005.) 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] (Wray, J., “Generic Security Service API Version 2 : C-bindings,” January 2000.) is defined. Finally, the behavior for the Kerberos mechanism [RFC4121] (Zhu, L., Jaganathan, K., and S. Hartman, “The Kerberos Version 5 Generic Security Service Application Program Interface (GSS-API) Mechanism: Version 2,” July 2005.) is specified.

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

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.

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] (Neuman, C., Yu, T., Hartman, S., and K. Raeburn, “The Kerberos Network Authentication Service (V5),” July 2005.) in the service ticket. Other policy checks MAY be applied.

[RFC4120] (Neuman, C., Yu, T., Hartman, S., and K. Raeburn, “The Kerberos Network Authentication Service (V5),” July 2005.) 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.

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] (Linn, J., “Generic Security Service Application Program Interface Version 2, Update 1,” January 2000.) 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.

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.

8.  IANA Considerations

This section needs to be revised to be consistent with the kitten IANA draft.

9.  Acknowledgements

Thanks to Martin Rex, Ken Raeburn and Tom Yu for reviewing the document and provided suggestions for improvements.

10. Normative References

Appendix A.  Change history

RFC-EDITOR: please remove this section.

• 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.
• Version 00: Inital draft by Love and cleaned up by Sam.

Authors' Addresses

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