draft-zhu-ws-kerb-03.txt   [plain text]

NETWORK WORKING GROUP                                             L. Zhu
Internet-Draft                                     Microsoft Corporation
Updates: 4120 (if approved)                                    J. Altman
Intended status: Standards Track                        Secure Endpoints
Expires: January 10, 2008                                   July 9, 2007

 Initial and Pass Through Authentication Using Kerberos V5 and the GSS-
                              API (IAKERB)

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Copyright Notice

   Copyright (C) The IETF Trust (2007).


   This document defines extensions to the Kerberos protocol and the
   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

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   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
   Intellectual Property and Copyright Statements . . . . . . . . . . 11

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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

   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",
   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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   The initial 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, and any subsequent IAKERB
   context establishment token MUST NOT have this token framing.

   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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               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.

   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.

               -- 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

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   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

   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 comforming to this specification, the
   authenticator subkey in the AP-REQ MUST alway be present, and the
   Exts field in the GSS-API authenticator [GSS-EXTS] MUST contain an
   extension of the type GSS_EXTS_IAKERB_FINISHED and the extension data
   contains the ASN.1 DER encoding of the structure IAKERB-FINISHED.

           GSS_EXTS_IAKERB_FINISHED             TBD
                --- Data type for the IAKERB checksum.

           IAKERB-FINISHED ::= {
                iakerb-messages [1] Checksum,
                    -- Contains the checksum of the GSS-API tokens
                    -- exchanged between the initiator and the acceptor,
                    -- and prior to the containing AP_REQ GSS-API token.
                    -- The checksum is performed over the GSS-API tokens
                    -- in the order that the tokens were sent.

   The iakerb-messages field in the IAKERB-FINISHED structure contains a
   checksum of all the GSS-API tokens exchanged between the initiator
   and the acceptor, and prior to the GSS-API token containing the
   AP_REQ.  This checksum is performed over these GSS-API tokens in the
   order that the tokens were sent.  In the parlance of [RFC3961], the
   checksum type is the required checksum type for the enctype of the
   subkey in the authenticator, the protocol key for the checksum
   operation is the authenticator subkey, and the key usage number is

           KEY_USAGE_IAKERB_FINISHED            42

   The GSS-API acceptor MUST then verify the checksum contained in the
   GSS_EXTS_IAKERB_FINISHED extension.  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

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   is significant.  Implementations SHOULD provide the AS_REQ armoring
   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

   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.

   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
   prevent arbitrary messages from being sent to internal servers:

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   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

   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

              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
              Requirement Levels", BCP 14, RFC 2119, March 1997.

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   [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

              Zhu, L. and P. Leach, "Kerberos Anonymity Support",
              draft-ietf-krb-wg-anon-04.txt (work in progress), 2007.

              Zhu, L. and S. Hartman, "A Generalized Framework for
              Kerberos Pre-Authentication",
              draft-ietf-krb-wg-preauth-framework-06.txt (work in
              progress), 2007.

Authors' Addresses

   Larry Zhu
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052

   Email: lzhu@microsoft.com

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   Jeffery Altman
   Secure Endpoints
   255 W 94th St
   New York, NY  10025

   Email: jaltman@secure-endpoints.com

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