NETWORK WORKING GROUP L. Zhu
Internet-Draft Microsoft Corporation
Updates: 4120 (if approved) J. Altman
Intended status: Standards Track Secure Endpoints
Expires: January 31, 2010 July 30, 2009
Initial and Pass Through Authentication Using Kerberos V5 and the GSS-
API (IAKERB)
draft-ietf-krb-wg-iakerb-02
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Abstract
This document defines extensions to the Kerberos protocol and the
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GSS-API Kerberos mechanism that enable a GSS-API Kerberos client to
exchange messages with the KDC using the GSS-API acceptor as the
proxy, by encapsulating the Kerberos messages inside GSS-API tokens.
With these extensions a client can obtain Kerberos tickets for
services where the KDC is not accessible to the client, but is
accessible to the application server.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . . 3
3. GSS-API Encapsulation . . . . . . . . . . . . . . . . . . . . . 3
4. Addresses in Tickets . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . . 8
8.2. Informative references . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
When authenticating using Kerberos V5, clients obtain tickets from a
KDC and present them to services. This model of operation cannot
work if the client does not have access to the KDC. For example, in
remote access scenarios, the client must initially authenticate to an
access point in order to gain full access to the network. Here the
client may be unable to directly contact the KDC either because it
does not have an IP address, or the access point packet filter does
not allow the client to send packets to the Internet before it
authenticates to the access point.
Recent advancements in extending Kerberos permit Kerberos
authentication to complete with the assistance of a proxy. The
Kerberos [RFC4120] pre-authentication framework [KRB-PAFW] prevents
the exposure of weak client keys over the open network. The Kerberos
support of anonymity [KRB-ANON] provides for privacy and further
complicates traffic analysis. The kdc-referrals option defined in
[KRB-PAFW] may reduce the number of messages exchanged while
obtaining a ticket to exactly two even in cross-realm
authentications.
Building upon these Kerberos extensions, this document extends
[RFC4120] and [RFC4121] such that the client can communicate with the
KDC using a Generic Security Service Application Program Interface
(GSS-API) [RFC2743] acceptor as the proxy. The GSS-API acceptor
relays the KDC request and reply messages between the client and the
KDC. The GSS-API acceptor, when relaying the Kerberos messages, is
called an IAKERB proxy. Consequently, IAKERB as defined in this
document requires the use of GSS-API.
2. Conventions Used in This Document
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 Encapsulation
The mechanism Objection Identifier (OID) for GSS-API IAKERB, in
accordance with the mechanism proposed by [RFC4178] for negotiating
protocol variations, is id-kerberos-iakerb:
id-kerberos-iakerb ::=
{ iso(1) org(3) dod(6) internet(1) security(5) kerberosV5(2)
iakerb(5) }
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All context establishment token of IAKERB MUST have the generic token
framing described in section 3.1 of [RFC2743] with the mechanism OID
being id-kerberos-iakerb.
The client starts by constructing the ticket request, and if the
ticket request is being made to the KDC, the client, instead of
contacting the KDC directly, encapsulates the request message into
the output token of the GSS_Init_security_context() call and returns
GSS_S_CONTINUE_NEEDED [RFC2743] indicating that at least one more
token is required in order to establish the context. The output
token is then passed for use as the input token to the
GSS_Accept_sec_context() call in accordance with GSS-API. The GSS-
API acceptor extracts the Kerberos request in the input token,
locates the target KDC, and sends the request on behalf of the
client. After receiving the KDC reply, the GSS-API acceptor then
encapsulates the reply message into the output token of
GSS_Accept_sec_context(). The GSS-API acceptor returns
GSS_S_CONTINUE_NEEDED [RFC2743] indicating that at least one more
token is required in order to establish the context. The output
token is passed to the initiator in accordance with GSS-API.
Client <---------> IAKERB proxy <---------> KDC
The innerToken described in section 3.1 of [RFC2743] and subsequent
GSS-API mechanism tokens have the following formats: it starts with a
two-octet token-identifier (TOK_ID), followed by an IAKERB message or
a Kerberos message.
Only one IAKERB specific message, namely the IAKERB_PROXY message, is
defined in this document. The TOK_ID values for Kerberos messages
are the same as defined in [RFC4121].
Token TOK_ID Value in Hex
--------------------------------------
IAKERB_PROXY 05 01
The content of the IAKERB_PROXY message is defined as an IAKERB-
HEADER structure immediately followed by a Kerberos message. The
Kerberos message can be an AS-REQ, an AS-REP, a TGS-REQ, a TGS-REP,
or a KRB-ERROR as defined in [RFC4120].
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IAKERB-HEADER ::= SEQUENCE {
target-realm [1] UTF8String,
-- The name of the target realm.
cookie [2] OCTET STRING OPTIONAL,
-- Opaque data, if sent by the server,
-- MUST be copied by the client verbatim into
-- the next IAKRB_PROXY message.
...
}
The IAKERB-HEADER structure and all the Kerberos messages MUST be
encoded using Abstract Syntax Notation One (ASN.1) Distinguished
Encoding Rules (DER) [X680] [X690].
The IAKERB client fills out the IAKERB-HEADER structure as follows:
the target-realm contains the realm name the ticket request is
addressed to. In the initial message from the client, the cookie
field is absent. The client MUST specify a target-realm. If the
client does not know the realm of the client's true principal name
[REFERALS], it MUST specify a realm it knows. This can be the realm
of the client's host.
Upon receipt of the IAKERB_PROXY message, the GSS-API acceptor
inspects the target-realm field in the IAKERB_HEADER, and locates a
KDC of that realm, and sends the ticket request to that KDC.
The GSS-API server encapsulates the KDC reply message in the returned
IAKERB message. It fills out the target realm using the realm sent
by the client and the KDC reply message is included immediately
following the IAKERB-HEADER header.
When the GSS-API acceptor is unable to obtain an IP address for a KDC
in the client's realm, it sends a KRB_ERROR message with the code
KRB_AP_ERR_IAKERB_KDC_NOT_FOUND to the client and the context fails
to establish. There is no accompanying error data defined in this
document for this error code.
KRB_AP_ERR_IAKERB_KDC_NOT_FOUND 85
-- The IAKERB proxy could not find a KDC.
When the GSS-API acceptor has an IP address for a KDC in the client
realm, but does not receive a response from any KDC in the realm
(including in response to retries), it sends a KRB_ERROR message with
the code KRB_AP_ERR_IAKERB_KDC_NO_RESPONSE to the client and the
context fails to establish. There is no accompanying error data
defined in this document for this error code.
KRB_AP_ERR_IAKERB_KDC_NO_RESPONSE 86
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-- The KDC did not respond to the IAKERB proxy.
The IAKERB proxy can send opaque data in the cookie field of the
IAKERB-HEADER structure in the server reply to the client, in order
to, for example, minimize the amount of state information kept by the
GSS-API acceptor. The content and the encoding of the cookie field
is a local matter of the IAKERB proxy. The client MUST copy the
cookie verbatim from the previous server response whenever the cookie
is present into the subsequent tokens that contains an IAKERB_PROXY
message.
The client and the server can repeat the sequence of sending and
receiving the IAKERB messages as described above, in order to allow
the client interact with the KDC through the IAKERB proxy, and to
obtain Kerberos tickets as needed.
When obtaining the initial TGT, the client may start with an NT-
ENTERPRISE name type and the client host does not have a Kerberos
realm. To resolve the NT-ENTERPRISE name type, the client typically
starts with the client host realm and then finds out the true realm
of the client based on [REFERALS]. In this case the GSS-API client
can retrieve the realm of the GSS-API server as follows: the client
returns GSS_S_CONTINUE_NEEDED with the output token containing an
IAKERB message with an empty target-realm in the IAKERB-HEADER and no
Kerberos message following the IAKERB-HEADER structure. Upon receipt
of the realm request, the GSS-API server fills out the target realm
field using the realm of the server, and returns
GSS_S_CONTINUE_NEEDED with the output token containing the IAKERB
message with the server's realm and no Kerberos message following the
IAKERB-HEADER header. The GSS-API client can then use the returned
realm in subsequent IAKERB messages to resolve the NT-ENTERPRISE name
type. Since the GSS-API server can act as a Kerberos acceptor, it
always has a Kerberos realm in this case.
When the client obtained a service ticket, the client sends a
KRB_AP_REQ message to the server, and performs the client-server
application exchange as defined in [RFC4120] and [RFC4121].
For implementations conforming to this specification, both the
authenticator subkey and the GSS_EXTS_FINISHED extension as defined
in [PKU2U] MUST be present in the AP-REQ authenticator. This
checksum provides integrity protection for the messages exchanged
including the unauthenticated clear texts in the IAKERB-HEADER
structure.
If the pre-authentication data is encrypted in the long-term
password-based key of the principal, the risk of security exposures
is significant. Implementations SHOULD provide the AS_REQ armoring
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as defined in [KRB-PAFW] unless an alternative protection is
deployed. In addition, the anonymous Kerberos FAST option is
RECOMMENDED for the client to complicate traffic analysis.
4. Addresses in Tickets
In IAKERB, the machine sending requests to the KDC is the GSS-API
acceptor and not the client. As a result, the client should not
include its addresses in any KDC requests for two reasons. First,
the KDC may reject the forwarded request as being from the wrong
client. Second, in the case of initial authentication for a dial-up
client, the client machine may not yet possess a network address.
Hence, as allowed by [RFC4120], the addresses field of the AS-REQ and
TGS-REQ requests SHOULD be blank and the caddr field of the ticket
SHOULD similarly be left blank.
5. Security Considerations
A typical IAKERB client sends the AS_REQ with pre-authentication data
encrypted in the long-term keys of the user before the server is
authenticated. This enables offline attacks by un-trusted servers.
To mitigate this threat, the client SHOULD use Kerberos
FAST[KRB-PAFW] and require KDC authentication to protect the user's
credentials.
The client name is in clear text in the authentication exchange
messages and ticket granting service exchanges according to [RFC4120]
whereas the client name is encrypted in client- server application
exchange messages. By using the IAKERB proxy to relay the ticket
requests and responses, the client's identity could be revealed in
the client-server traffic where the same identity could have been
concealed if IAKERB were not used. Hence, to complicate traffic
analysis and provide privacy for the IAKERB client, the IAKERB client
SHOULD request the anonymous Kerberos FAST option [KRB-PAFW].
Similar to other network access protocols, IAKERB allows an
unauthenticated client (possibly outside the security perimeter of an
organization) to send messages that are proxied to interior servers.
To reduce attack surface, firewall filters can be applied to allow
from which hosts the client requests can be proxied and the proxy can
further restrict the set of realms to which the requests can be
proxied.
In a scenario where DNS SRV RR's are being used to locate the KDC,
IAKERB is being used, and an external attacker can modify DNS
responses to the IAKERB proxy, there are several countermeasures to
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prevent arbitrary messages from being sent to internal servers:
1. KDC port numbers can be statically configured on the IAKERB
proxy. In this case, the messages will always be sent to KDC's.
For an organization that runs KDC's on a static port (usually
port 88) and does not run any other servers on the same port,
this countermeasure would be easy to administer and should be
effective.
2. The proxy can do application level sanity checking and filtering.
This countermeasure should eliminate many of the above attacks.
3. DNS security can be deployed. This countermeasure is probably
overkill for this particular problem, but if an organization has
already deployed DNS security for other reasons, then it might
make sense to leverage it here. Note that Kerberos could be used
to protect the DNS exchanges. The initial DNS SRV KDC lookup by
the proxy will be unprotected, but an attack here is at most a
denial of service (the initial lookup will be for the proxy's KDC
to facilitate Kerberos protection of subsequent DNS exchanges
between itself and the DNS server).
6. Acknowledgements
Jonathan Trostle, Michael Swift, Bernard Aboba and Glen Zorn wrote
earlier revision of this document.
The hallway conversations between Larry Zhu and Nicolas Williams
formed the basis of this document.
7. IANA Considerations
There is no IANA action required for this document.
8. References
8.1. Normative References
[GSS-EXTS]
Emery, S., "Kerberos Version 5 GSS-API Channel Binding
Hash Agility",
draft-ietf-krb-wg-gss-cb-hash-agility-03.txt (work in
progress), 2007.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
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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.
[RFC3961] Raeburn, K., "Encryption and Checksum Specifications for
Kerberos 5", RFC 3961, February 2005.
[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.
[RFC4178] Zhu, L., Leach, P., Jaganathan, K., and W. Ingersoll, "The
Simple and Protected Generic Security Service Application
Program Interface (GSS-API) Negotiation Mechanism",
RFC 4178, October 2005.
8.2. Informative references
[KRB-ANON]
Zhu, L. and P. Leach, "Kerberos Anonymity Support",
draft-ietf-krb-wg-anon-04.txt (work in progress), 2007.
[KRB-PAFW]
Zhu, L. and S. Hartman, "A Generalized Framework for
Kerberos Pre-Authentication",
draft-ietf-krb-wg-preauth-framework-06.txt (work in
progress), 2007.
[PKU2U] Zhu, L. and J. Altman, "Public Key Cryptography Based
User-to-User Authentication - (PKU2U)", draft-zhu-pku2u
(work in progress), 2007.
[REFERALS]
Raeburn, K. and L. Zhu, "Kerberos Principal Name
Canonicalization and KDC-Generated Cross-Realm Referrals",
draft-ietf-krb-wg-kerberos-referral (work in progress),
2009.
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Authors' Addresses
Larry Zhu
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
Email: larry.zhu@microsoft.com
Jeffery Altman
Secure Endpoints
255 W 94th St
New York, NY 10025
US
Email: jaltman@secure-endpoints.com
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