Kerberos WORKING GROUP                                   S. Hartman, Ed.
Internet-Draft                                         Painless Security
Updates: 4120 (if approved)                                   K. Raeburn
Intended status: Standards Track                                     MIT
Expires: September 29, 2011                                       L. Zhu
                                                   Microsoft Corporation
                                                          March 28, 2011

 Kerberos Principal Name Canonicalization and KDC-Generated Cross-Realm


   The memo documents a method for a Kerberos Key Distribution Center
   (KDC) to respond to client requests for Kerberos tickets when the
   client does not have detailed configuration information on the realms
   of users or services.  The KDC will handle requests for principals in
   other realms by returning either a referral error or a cross-realm
   TGT to another realm on the referral path.  The clients will use this
   referral information to reach the realm of the target principal and
   then receive the ticket.  This memo also provides a mechanism for
   verifying that a request has not been tampered with in transit.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 29, 2011.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Conventions Used in This Document  . . . . . . . . . . . . . .  5
   3.  Requesting a Referral  . . . . . . . . . . . . . . . . . . . .  6
   4.  Realm Organization Model . . . . . . . . . . . . . . . . . . .  6
     4.1.  Trust Assumptions  . . . . . . . . . . . . . . . . . . . .  7
   5.  Enterprise Principal Name Type . . . . . . . . . . . . . . . .  8
   6.  Name Canonicalization  . . . . . . . . . . . . . . . . . . . .  8
   7.  Client Referrals . . . . . . . . . . . . . . . . . . . . . . . 10
   8.  Server Referrals . . . . . . . . . . . . . . . . . . . . . . . 11
   9.  Cross Realm Routing  . . . . . . . . . . . . . . . . . . . . . 12
   10. Caching Information  . . . . . . . . . . . . . . . . . . . . . 12
   11. Negotiation of FAST and Detecting Modified Requests  . . . . . 13
   12. Open Issues  . . . . . . . . . . . . . . . . . . . . . . . . . 14
   13. Number Assignments . . . . . . . . . . . . . . . . . . . . . . 14
   14. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   15. Security Considerations  . . . . . . . . . . . . . . . . . . . 15
     15.1. Shared-password case . . . . . . . . . . . . . . . . . . . 17
     15.2. Preauthentication data . . . . . . . . . . . . . . . . . . 18
   16. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 18
   17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     17.1. Normative References . . . . . . . . . . . . . . . . . . . 18
     17.2. Informative References . . . . . . . . . . . . . . . . . . 19
   Appendix A.  Compatibility with Earlier Implementations of
                Name Canonicalization . . . . . . . . . . . . . . . . 19
   Appendix B.  Document history [REMOVE BEFORE PUBLICATION]  . . . . 20
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21

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1.  Introduction

   Current implementations of the Kerberos AS and TGS protocols, as
   defined in [RFC4120], use principal names constructed from a known
   user or service name and realm.  A service name is typically
   constructed from a name of the service and the DNS host name of the
   computer that is providing the service.  Many existing deployments of
   Kerberos use a single Kerberos realm where all users and services
   would be using the same realm.  However in an environment where there
   are multiple Kerberos realms, the client needs to be able to
   determine what realm a particular user or service is in before making
   an AS or TGS request.  Traditionally this requires client
   configuration to make this possible.

   When having to deal with multiple realms, users are forced to know
   what realm they are in before they can obtain a ticket granting
   ticket (TGT) with an AS request.  However, in many cases the user
   would like to use a more familiar name that is not directly related
   to the realm of their Kerberos principal name.  A good example of
   this is an RFC 822 style email name.  This document describes a
   mechanism that would allow a user to specify a user principal name
   that is an alias for the user's Kerberos principal name.  In practice
   this would be the name that the user specifies to obtain a TGT from a
   Kerberos KDC.  The user principal name no longer has a direct
   relationship with the Kerberos principal or realm.  Thus the
   administrator is able to move the user's principal to other realms
   without the user having to know that it happened.

   Once a user has a TGT, they would like to be able to access services
   in any Kerberos realm for which there is an authentication path from
   the realm of their principal.  To do this requires that the client be
   able to determine what realm the target service principal is in
   before making the TGS request.  Current implementations of Kerberos
   typically have a table that maps DNS host names to corresponding
   Kerberos realms.  The user-supplied host name or its domain component
   is looked up in this table (often using the result of some form of
   host name lookup performed with insecure DNS queries, in violation of
   [RFC4120]).  The corresponding realm is then used to complete the
   target service principal name.  Even if insecure DNS queries were not
   used, managing this table is problematic.

   This traditional mechanism requires that each client have very
   detailed configuration information about the hosts that are providing
   services and their corresponding realms.  Having client side
   configuration information can be very costly from an administration
   point of view-- especially if there are many realms and computers in
   the environment.

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   There are also cases where specific DNS aliases (local names) have
   been setup in an organization to refer to a server in another
   organization (remote server).  The server has different DNS names in
   each organization and each organization has a Kerberos realm that is
   configured to service DNS names within that organization.  Ideally
   users are able to authenticate to the server in the other
   organization using the local server name.  This would mean that the
   local realm be able to produce a ticket to the remote server under
   its name.  The administrator in the local realm could give that
   remote server an identity in the local realm and then have that
   remote server maintain a separate secret for each alias it is known
   as.  Alternatively the administrator could arrange to have the local
   realm issue a referral to the remote realm and notify the requesting
   client of the server's remote name that should be used in order to
   request a ticket.

   This memo proposes a solution for these problems and simplifies
   administration by minimizing the configuration information needed on
   each computer using Kerberos.  Specifically it describes a mechanism
   to allow the KDC to handle canonicalization of names, provide for
   principal aliases for users and services and allow the KDC to
   determine the trusted realm authentication path by being able to
   generate referrals to other realms in order to locate principals.

   Two kinds of KDC referrals are introduced in this memo:

   1. Client referrals, in which the client doesn't know which realm
      contains a user account.
   2. Server referrals, in which the client doesn't know which realm
      contains a server account.

   These two types of referrals introduce new opportunities for an
   attacker.  In order to avoid these attacks, a mechanism is provided
   to protect the integrity of the request between the client and KDC.
   This mechanism compliments the Flexible Authentication through Secure
   Tunnels (FAST) facility provided in
   [I-D.ietf-krb-wg-preauth-framework].  A mechanism is provided to
   negotiate the availability of FAST.  Among other benefits this can be
   used to protect errors generated by the referral process.

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].

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3.  Requesting a Referral

   In order to request referrals as defined in later sections, the
   Kerberos client MUST explicitly request the canonicalize KDC option
   (bit 15) [RFC4120] for the AS-REQ or TGS-REQ.  This flag indicates to
   the KDC that the client is prepared to receive a reply that contains
   a principal name other than the one requested.

          KDCOptions ::= KerberosFlags
                   -- canonicalize (15)
                   -- other KDCOptions values omitted

   The client should expect, when sending names with the "canonicalize"
   KDC option, that names in the KDC's reply MAY be different than the
   name in the request.  A referral TGT is a cross realm TGT that is
   returned with the server name of the ticket being different from the
   server name in the request [RFC4120].

4.  Realm Organization Model

   This memo assumes that the world of principals is arranged on
   multiple levels: the realm, the enterprise, and the world.  A KDC may
   issue tickets for any principal in its realm or cross-realm tickets
   for realms with which it has a direct cross-realm relationship.  The
   KDC also has access to a trusted name service that can resolve any
   name from within its enterprise into a realm closer along the
   authentication path to the service.  This trusted name service
   removes the need to use an un-trusted DNS lookup for name resolution.

   For example, consider the following configuration, where lines
   indicate cross-realm relationships:

                      /        \
                     /          \

   In this configuration, all users in the EXAMPLE.COM enterprise could
   have principal names such as alice@EXAMPLE.COM, with the same realm
   portion.  In addition, servers at EXAMPLE.COM should be able to have
   DNS host names from any DNS domain independent of what Kerberos realm
   their principals reside in.

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4.1.  Trust Assumptions

   Two realms participate in any cross-realm relationship: an issuing
   realm issues a cross-realm ticket and a consuming realm uses this
   ticket.  There is a degree of trust of the issuing realm by the
   consuming realm implicit in this relationship.  Whenever a service in
   the consuming realm permits an authentication path containing the
   issuing realm, that service trusts the issuing realm to accurately
   represent the identity of the authenticated principal and any
   information about the transited path.  If the consuming realm's KDC
   sets the transited policy checked flag, the KDC is making the same
   trust assumption a service would.

   This trust is transitive across a multi-hop authentication path.  The
   service's realm trusts each hop along the authentication path closer
   to the client to accurately represent the authenticated identity and
   to accurately represent transited information.  Any KDC along this
   path could impersonate the client.

   KDC signed or issued authorization data often implies additional
   trust.  The implications of such trust from a security and
   operational standpoint is an ongoing topic of discussion during the
   development of this specification.  As such, such discussion is out
   of scope for this memo.

   Administrators have several tools to limit trust caused by cross-
   realm relationships.  A service or KDC can control what
   authentication paths are acceptable.  For example if a given realm is
   not permitted on the authentication path for a particular client then
   that realm cannot affect trust placed in that client principal.
   Consuming realms can exercise significant control by deciding what
   principals to place on an access-control list.  If no client using a
   given issuing realm in authentication paths is permitted to access a
   resource, then that issuing realm is not trusted in access decisions
   regarding that resource.

   Creating a cross-realm relationship implies relatively little
   inherent trust in the issuing realm.  Significant trust only applies
   as principals dependent on that issuing realm are given access to
   resources.  However, two deployment constraints may imply
   significantly greater trust is implied by the initial cross-realm
   relationship.  First, a number of realms provide access to any
   principal to some resources.  Access decisions involving these
   resources involve a degree of trust in all issuing realms in the
   transited graph.  Secondly, many realms do not significantly
   constrain what principals users of that realm may grant access.  In
   these realms, creating a cross-realm relationship delegates the
   decision to trust that realm to users of the consuming realm.  In

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   this situation, creating the cross-realm relationship is the primary
   trust decision point under the administrator's control.

5.  Enterprise Principal Name Type

   The NT-ENTERPRISE type principal name contains one component, a
   string of realm-defined content, which is intended to be used as an
   alias for another principal name in some realm in the enterprise.  It
   is used for conveying the alias name, not for the real principal
   names within the realms, and thus is only useful when name
   canonicalization is requested.

   The intent is to allow unification of email and security principal
   names.  For example, all users at EXAMPLE.COM may have a client
   principal name of the form "joe@EXAMPLE.COM" even though the
   principals are contained in multiple realms.  This global name is
   again an alias for the true client principal name, which indicates
   what realm contains the principal.  Thus, accounts "alice" in the
   realm DEV.EXAMPLE.COM and "bob" in ADMIN.EXAMPLE.COM may log on as
   "alice@EXAMPLE.COM" and "bob@EXAMPLE.COM".

   This utilizes a new principal name type, as the KDC-REQ message only
   contains a single client realm field, and the realm portion of this
   name corresponds to the Kerberos realm with which the request is
   made.  Thus, the entire name "alice@EXAMPLE.COM" is transmitted as a
   single component in the client name field of the AS-REQ message, with
   a name type of NT-ENTERPRISE [RFC4120] (and the local realm name).
   The KDC will recognize this name type and then transform the
   requested name into the true principal name if the client account
   resides in the local realm.  The true principal name can have a name
   type different from the requested name type.  Typically the true
   principal name will be a NT-PRINCIPAL [RFC4120].

6.  Name Canonicalization

   A service or account may have multiple principal names.  For example,
   if a host is known by multiple names, host-based services on it may
   be known by multiple names in order to prevent the client from
   needing a secure directory service to determine the correct hostname
   to use.  In order that the host should not need to be updated
   whenever a new alias is created, the KDC may provide the mapping
   information to the client in the credential acquisition process.

   If the "canonicalize" KDC option is set, then the KDC MAY change the
   client and server principal names and types in the AS response and
   ticket returned from the name type of the client name in the request.

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   In a TGS exchange, the server principal name and type may be changed.

   For example the AS request may specify a client name of "bob@
   EXAMPLE.COM" as an NT-ENTERPRISE name with the "canonicalize" KDC
   option set and the KDC will return with a client name of "104567" as
   a NT-UID.

   (It is assumed that the client discovers whether the KDC supports the
   NT-ENTERPRISE name type via out of band mechanisms.)

   See Section 11 for a mechanism to detect modification of the request
   between the client and KDC.  However for best protection, Flexible
   Authentication through Secure Tunneling (FAST)
   [I-D.ietf-krb-wg-preauth-framework] or another mechanism that
   protects the entire KDC exchange SHOULD be used.  Clients MAY reject
   responses from a KDC where the client or server name is changed if
   the KDC does not support such a mechanism.  Clients SHOULD reject an
   AS response that changes the server name unless the response is
   protected by such a mechanism or the new server name is one
   explicitly expected by the client.  For example, many clients permit
   the realm name to be changed in an AS response even if the response
   is not protected.  See Section 15 for a discussion of the tradeoffs
   in allowing unprotected responses.

   In order to enable one party in a user-to-user exchange to confirm
   the identity of another when only the alias is known, the KDC MAY
   include the following authorization data element, wrapped in AD-KDC-
   ISSUED, in the initial credentials and copy it from a ticket-granting
   ticket into additional credentials:

   AD-LOGIN-ALIAS ::= SEQUENCE { -- ad-type number TBD --
     login-aliases  [0] SEQUENCE(1..MAX) OF PrincipalName,

   The login-aliases field lists one or more of the aliases the
   principal may have.

   The recipient of this authenticator must check the AD-LOGIN-ALIAS
   names, if present, in addition to the normal client name field,
   against the identity of the party with which it wishes to
   authenticate; either should be allowed to match.  (Note that this is
   not backwards compatible with [RFC4120]; if the server side of the
   user-to-user exchange does not support this extension, and does not
   know the true principal name, authentication may fail if the alias is
   sought in the client name field.)

   The use of AD-KDC-ISSUED authorization data elements in cross-realm
   cases has not been well explored at this writing; hence we will only

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   specify the inclusion of this data in the one-realm case.  The alias
   information SHOULD be dropped in the general cross-realm case.
   However, a realm MAY implement a policy of accepting and re-signing
   (wrapping in a new AD-KDC-ISSUED element) alias information provided
   by certain trusted realms, in the cross-realm ticket-granting

   The canonical principal name for an alias may not be in the form of a
   ticket-granting service name, as (in a case of server name
   canonicalization) that would be construed as a case of cross-realm
   referral, described below.

7.  Client Referrals

   The simplest form of ticket referral is for a user requesting a
   ticket using an AS-REQ.  In this case, the client machine will send
   the AS-REQ to a convenient realm trusted to map principals, for
   example the realm of the client machine.  In the case of the name
   alice@EXAMPLE.COM, the client MAY optimistically choose to send the
   request to EXAMPLE.COM.  The realm in the AS-REQ is always the name
   of the realm that the request is for as specified in [RFC4120].

   The KDC will try to lookup the name in its local account database.
   If the account is present in the realm of the request, it SHOULD
   return a KDC reply structure with the appropriate ticket.

   If the account is not present in the realm specified in the request
   and the "canonicalize" KDC option is set, the KDC may look up the
   client principal name using some kind of name service or directory
   service.  If this lookup is unsuccessful, it MUST return the error
   KDC_ERR_C_PRINCIPAL_UNKNOWN [RFC4120].  If the lookup is successful,
   it MUST return an error KDC_ERR_WRONG_REALM [RFC4120] and in the
   error message the crealm field will contain either the true realm of
   the client or another realm that MAY have better information about
   the client's true realm.  The client MUST NOT use the cname returned
   in this error message.

   If the client receives a KDC_ERR_WRONG_REALM error, it will issue a
   new AS request with the same client principal name used to generate
   the first referral to the realm specified by the realm field of the
   Kerberos error message corresponding to the first request.  (The
   client realm name will be updated in the new request to refer to this
   new realm.)  The client SHOULD repeat these steps until it finds the
   true realm of the client.  To avoid infinite referral loops, an
   implementation should limit the number of referrals.  A suggested
   limit is 5 referrals before giving up.

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   Since the same client name is sent to the referring and referred-to
   realms, both realms must recognize the same client names.  In
   particular, the referring realm cannot (usefully) define principal
   name aliases that the referred-to realm will not know.

   The true principal name of the client, returned in AS-REQ, can be
   validated in a subsequent TGS message exchange where its value is
   communicated back to the KDC via the authenticator in the PA-TGS-REQ
   padata [RFC4120].  However, this requires trusting the referred-to
   realm's KDCs.  Clients should limit the referral mappings they will
   accept to realms trusted via some local policy.  Some possible
   factors that might be taken into consideration for such a policy
   might include:

   o  Any realm indicated by the local KDC, if the returned KRB-ERROR
      message is protected by some additional means, for example FAST
   o  A list of realms configured by an administrator
   o  Any realm accepted by the user when explicitly prompted

   There is currently no provision for changing the client name in a
   client referral response, because there is no method for verifying
   that a man-in-the-middle attack did not change the requested name in
   the request on the way to the KDC.

8.  Server Referrals

   The primary difference in server referrals is that the KDC returns a
   referral TGT rather than an error message as is done in the client
   referrals.  There needs to be a place to include in the reply
   information about what realm contains the server; this is done by
   returning information about the server name in the pre-authentication
   data field of the KDC reply [RFC4120], as specified later in this

   If the "canonicalize" flag in the KDC options is set and the KDC
   doesn't find the principal locally, either as a regular principal or
   as an alias for another local principal, the KDC MAY return a cross-
   realm ticket granting ticket to the next hop on the trust path
   towards a realm that may be able to resolve the principal name.  The
   true principal name of the server SHALL be returned in the padata of
   the reply if it is different from what is specified the request.

   The client will use this referral information to request a chain of
   cross-realm ticket granting tickets until it reaches the realm of the
   server, and can then expect to receive a valid service ticket.

   However an implementation should limit the number of referrals that

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   it processes to avoid infinite referral loops.  A suggested limit is
   5 referrals before giving up.

   The client may cache the mapping of the requested name to the name of
   the next realm to use and the principal name to ask for.  (See
   Section 10.)

   Here is an example of a client requesting a service ticket for a
   service in realm DEV.EXAMPLE.COM where the client is in

      +NC = Canonicalize KDCOption set
      C: TGS-REQ sname=http/ +NC to ADMIN.EXAMPLE.COM
      C: TGS-REQ sname=http/ +NC to EXAMPLE.COM
      C: TGS-REQ sname=http/ +NC to DEV.EXAMPLE.COM
      S: TGS-REP sname=http/

   Note that any referral or alias processing of the server name in
   user-to-user authentication should use the same data as client name
   canonicalization or referral.  Otherwise, the name used by one user
   to log in may not be useable by another for user-to-user
   authentication to the first.

9.  Cross Realm Routing

   RFC 4120 permits a KDC to return a closer referral ticket when a
   cross-realm TGT is requested.  This specification extends this
   behavior when the canonicalize flag is set.  When this flag is set, a
   KDC MAY return a TGT for a realm closer to the service for any
   service.  XXX What should go in the realm field of the request to the
   next realm?

10.  Caching Information

   It is possible that the client may wish to get additional credentials
   for the same service principal, perhaps with different authorization-
   data restrictions or other changed attributes.  The return of a
   server referral from a KDC can be taken as an indication that the
   requested principal does not currently exist in the local realm.
   Clearly, it would reduce network traffic if the clients could cache
   that information and use it when acquiring the second set of
   credentials for a service, rather than always having to re-check with
   the local KDC to see if the name has been created locally.

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   When the TGT expires, the previously returned referral from the local
   KDC should be considered invalid, and the local KDC must be asked
   again for information for the desired service principal name.  (Note
   that the client may get back multiple referral TGTs from the local
   KDC to the same remote realm, with different lifetimes.  The lifetime
   information SHOULD be properly associated with the requested service
   principal names.  Simply having another TGT for the same remote realm
   does not extend the validity of previously acquired information about
   one service principal name.)

   Accordingly, KDC authors and maintainers should consider what factors
   (e.g., DNS alias lifetimes) they may or may not wish to incorporate
   into credential expiration times in cases of referrals.

11.  Negotiation of FAST and Detecting Modified Requests

   Implementations of this specification MUST support the FAST
   negotiation mechanism described in this section.  This mechanism
   provides detection of KDC requests modified by an attacker when those
   requests result in a reply instead of an error.  In addition, this
   mechanism provides a secure way to detect if a KDC supports FAST.

   Clients conforming to this specification MUST send a new pre-
   authentication data of type PA-REQ-ENC-PA-REP (TBD1) in all AS
   requests and MAY send this padata type in TGS requests.  The value of
   this padata item SHOULD be empty and MUST be ignored by a receiving
   KDC.  Sending this padata item indicates support for this negotiation
   mechanism.  KDCs conforming to this specification must always set the
   ticket flag enc-pa-rep(15) in all the issued tickets.  This ticket
   flag indicates KDC support for the mechanism.

   The KDC response is extended to support an additional field
   containing encrypted pre-authentication data.

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          EncKDCRepPart   ::= SEQUENCE {
                 key                [0] EncryptionKey,
                 last-req           [1] LastReq,
                 nonce              [2] UInt32,
                 key-expiration     [3] KerberosTime OPTIONAL,
                 flags              [4] TicketFlags,
                 authtime           [5] KerberosTime,
                 starttime          [6] KerberosTime OPTIONAL,
                 endtime            [7] KerberosTime,
                 renew-till         [8] KerberosTime OPTIONAL,
                 srealm             [9] Realm,
                 sname             [10] PrincipalName,
                 caddr             [11] HostAddresses OPTIONAL,
                 encrypted-pa-data [12] SEQUENCE OF PA-DATA OPTIONAL

   The The encrypted-pa-data element MUST be absent unless either the
   canonicalize KDC option is set or the PA-REQ-ENC-PA-REP padata item
   is sent.

   If the PA-REQ-ENC-PA-REP padata item is sent in the request, then the
   KDC MUST include a PA-REQ-ENC-PA-REP padata item in the encrypted-pa-
   data item of any generated KDC reply.  The PA-REQ-ENC-PA-REP pa-data
   value contains the checksum computed over the type AS-REQ or TGS-REQ
   in the request.  The checksum key is the reply key and the checksum
   type is the required checksum type for the encryption type of the
   reply key, and the key usage number is KEY_USAGE_AS_REQ (56).  If the
   KDC supports FAST, then the KDC MUST include a padata of type PA-FX-
   FAST in any encrypted-pa-data sequence it generates.  The value for
   this padata item should be empty.

   A client MUST reject a response for which it sent PA-REQ-ENC-PA-REP
   if the ENC-PA-REP ticket flag is set and the PA-REQ-ENC-PA-REP padata
   item is absent or the checksum is not successfully verified.

12.  Open Issues

   When should client name aliases be included in credentials?  Should
   all known client name aliases be included, or only the one used at
   initial ticket acquisition?

13.  Number Assignments

   Most number registries in the Kerberos protocol have not been turned
   over to IANA for management at the time of this writing, hence this
   is not an "IANA Considerations" section.

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   Various values do need assigning for this draft:

14.  IANA Considerations

   In the Kerberos pre-authentication and typed data registry at http://
   kerberos-parameters.xhtml#pre-authentication, the PA-REQ-ENC-PA-REP
   pa-data item should be registered.  Because of existing
   implementations the value 149 is strongly preferred.

15.  Security Considerations

   For the AS exchange case, it is important that the logon mechanism
   not trust a name that has not been used to authenticate the user.
   For example, the name that the user enters as part of a logon
   exchange may not be the name that the user authenticates as, given
   that the KDC_ERR_WRONG_REALM error may have been returned.  The
   relevant Kerberos naming information for logon (if any), is the
   client name and client realm in the service ticket targeted at the
   workstation that was obtained using the user's initial TGT.  That is,
   rather than trusting the client name in the AS response, a
   workstation SHOULD perform an AP-REQ authentication against itself as
   a service and use the client name in the ticket issued for its
   service by the KDC.

   How the client name and client realm is mapped into a local account
   for logon is a local matter, but the client logon mechanism MUST use
   additional information such as the client realm and/or authorization
   attributes from the service ticket presented to the workstation by
   the user, when mapping the logon credentials to a local account on
   the workstation.

   Not all fields in an RFC 4120 KDC reply are protected.  None of the
   fields in an RFC 4120 AS request are protected and some information
   in a TGS request may not be protected.  The referrals mechanism
   creates several opportunities for attack because of these unprotected
   fields.  FAST [I-D.ietf-krb-wg-preauth-framework] can be used to
   completely mitigate these issues by protecting both the KDC request
   and response.  However, FAST requires that a client obtain an armor
   ticket before authenticating.  Not all realms permit all clients to
   obtain armor tickets.  Also, while it is expected to be uncommon, a
   client might wish to use name canonicalization while obtaining an
   armor ticket.  The mechanism in Section 11 detects modification of
   the request between the KDC and client, mitigating some attacks.

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   There is a wide deployed base of implementations that use name
   canonicalization or server referrals that uses neither the
   negotiation mechanism nor FAST.  So, implementations may be faced
   with only the limited protection afforded by RFC 4120, by the
   negotiation mechanism discussed in this document, or by FAST.  All
   three situations are important to consider from a security

   An attacker cannot mount a downgrade attack against a client.  The
   negotiation mechanism described in this document is securely
   indicated by the presence of a ticket flag.  So, a client will detect
   if the facility was available but not used.  It is possible for an
   attacker to strip the indication that a client supports the
   negotiation facility.  The client will learn from the response that
   this happened, but the KDC will not learn that the client is
   attacked.  So, for a single round-trip Kerberos exchange, the KDC may
   believe the exchange was successful when the client detects an
   attack.  Packet loss or client failure can produce a similar result;
   this is not a significant vulnerability.  The negotiation facility
   described in this document securely indicates the presence of FAST,
   so if a client wishes to use FAST when it is available, an attacker
   cannot force the client to downgrade away from FAST.  An attacker MAY
   be able to prevent a client from obtaining an armor ticket, for
   example by responding to a request for anonymous PKINIT with an error

   If FAST is used, then the communications between the client and KDC
   are protected.  However name canonicalization places a new
   responsibility for mapping principals onto the KDC.  This can
   increase the number of KDCs involved in an authentication which adds
   additional trusted third parties to the exchange.

   If only the negotiation mechanism is used, then the request from the
   client to the KDC is protected, but not all of the response is
   protected.  In particular, the client name is not protected; the
   ticket is also not protected.  An attacker can potentially modify
   these fields.  Modification of the client name will result in a
   denial of service.  When the client attempts to authenticate to a
   service (including the TGS), it constructs an AP-REQ message.  This
   message includes a client name which MUST match the client name in
   the ticket according to RFC 4120.  Thus if the client name is
   changed, the resulting ticket will fail when used.  This is
   undesirable because the authentication is separated from the later
   failure, which may confuse problem determination.  If the ticket is
   replaced with another ticket, then later authentication to a service
   will fail because the client will not know the session key for the
   other ticket.  If the ticket is simply modified, then authentication
   to a service will fail as with RFC 4120.  More significant attacks

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   are possible if a KDC violates the requirements of RFC 4120 and
   issues two tickets with the same session key or if a service violates
   the requirements of RFC 4120 and does not check the client name
   against that in the ticket.

   There is an additional attack possible when FAST is not used against
   KDC_ERR_WRONG_REALM.  Since this is an error response not an AS
   response, it is not protected by the negotiation mechanism.  Thus, an
   attacker may be able to convince a client to authenticate to a realm
   other than the one intended.  If an attacker is off-path this may
   give the attacker an advantage in attacking the client's credentials.
   Also, see the discussion of shared passwords below.

   More serious attacks are possible if no protection beyond RFC 4120 is
   used.  In this case, neither the client name nor the service name is
   protected between the client and KDC.  In the general case, if an
   attacker changes the client name, then authentication will fail
   because the client will not have the right credentials (password,
   certificate , or other) to authenticate as the user selected by the
   attacker.  However, see the discussion of shared passwords below.
   Changing the server name can be a very significant attack.  For
   example if a user is authenticating in order to send some
   confidential information, then the attacker could gain this
   information by directing the user to a server under the attacker's
   control.  The server name in the response is protected by RFC 4120,
   but not the one in the request.  Fortunately, users are typically
   authenticating to the "krbtgt" service in an AS exchange.  Clients
   that permit changes to the server name when no protection beyond RFC
   4120 is in use SHOULD carefully restrict what service names are
   acceptable.  One critical case to consider is the password changing
   service.  When a user authenticates to change their password they use
   an AS authentication directly to the password changing service.
   Clients MUST restrict service name changes sufficiently that the
   client ends up talking to the correct password changing service.

15.1.  Shared-password case

   A special case to examine is when the user is known (or correctly
   suspected) to use the same password for multiple accounts.  A man-in-
   the-middle attacker can either alter the request on its way to the
   KDC, changing the client principal name, or reply to the client with
   a response previously send by the KDC in response to a request from
   the attacker.  The response received by the client can then be
   decrypted by the user, though if the default "salt" generated from
   the principal name is used to produce the user's key, a PA-ETYPE-INFO
   or PA-ETYPE-INFO2 preauth record may need to be added or altered by
   the attacker to cause the client software to generate the key needed
   for the message it will receive.  None of this requires the attacker

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   to know the user's password, and without further checking, could
   cause the user to unknowingly use the wrong credentials.

   In normal [RFC4120] operation, a generated AP-REQ message includes in
   the Authenticator field a copy of the client's idea of its own
   principal name.  If this differs from the name in the KDC-generated
   Ticket, the application server will reject the message.

   With client name canonicalization as described in this document, the
   client may get its principal name from the response from the KDC.
   Using the wrong credentials may provide an advantage to an attacker.
   For example if a client uses one principal for administrative
   operations and one for less privileged operation, an attacker may
   coerce a client into using the wrong privilege to either cause some
   later operation to succeed or fail.

15.2.  Preauthentication data

   In cases of credential renewal, forwarding, or validation, if
   credentials are sent to the KDC that are not an initial ticket-
   granting ticket for the client's home realm, the encryption key used
   to protect the TGS exchange is one known to a third party (namely,
   the service for which the credential was issued).  Consequently, in
   such an exchange, the protection described earlier may be compromised
   by the service.  This is not generally believed to be a problem.  If
   it is, some form of explicit TGS armor could be added to FAST.

16.  Acknowledgments

   John Brezak, Mike Swift, and Jonathan Trostle wrote the initial
   version of this document.

   Karthik Jaganathan contributed to earlier versions.

   Sam Hartman's work on this document was funded by the MIT Kerberos

17.  References

17.1.  Normative References

              Hartman, S. and L. Zhu, "A Generalized Framework for
              Kerberos Pre-Authentication",
              draft-ietf-krb-wg-preauth-framework-17 (work in progress),
              June 2010.

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
              Kerberos Network Authentication Service (V5)", RFC 4120,
              July 2005.

17.2.  Informative References

   [RFC4556]  Zhu, L. and B. Tung, "Public Key Cryptography for Initial
              Authentication in Kerberos (PKINIT)", RFC 4556, June 2006.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, May 2008.

   [XPR]      Trostle, J., Kosinovsky, I., and M. Swift, "Implementation
              of Crossrealm Referral Handling in the MIT Kerberos
              Client",  Network and Distributed System Security
              Symposium, February 2001.

Appendix A.  Compatibility with Earlier Implementations of Name

   The Microsoft Windows 2000 and Windows 2003 releases included an
   earlier form of name-canonicalization [XPR].  Here are the

   2) The preauth data type definition in the encrypted preauth data is
      as follows:

          PA-SVR-REFERRAL-INFO       20

                 referred-name   [1] PrincipalName OPTIONAL,
                 referred-realm  [0] Realm

   3) When PKINIT ([RFC4556]) is used, the NT-ENTERPRISE client name is
      encoded as a Subject Alternative Name (SAN) extension [RFC5280] in
      the client's X.509 certificate.  The type of the otherName field
      for this SAN extension is AnotherName [RFC5280].  The type-id
      field of the type AnotherName is id-ms-sc-logon-upn
      ( and the value field of the type

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      AnotherName is a KerberosString [RFC4120].  The value of this
      KerberosString type is the single component in the name-string
      [RFC4120] sequence for the corresponding NT-ENTERPRISE name type.

   In Microsoft's current implementation through the use of global
   catalogs any domain in one forest is reachable from any other domain
   in the same forest or another trusted forest with 3 or less
   referrals.  A forest is a collection of realms with hierarchical
   trust relationships: there can be multiple trust trees in a forest;
   each child and parent realm pair and each root realm pair have
   bidirectional transitive direct rusts between them.

   While we might want to permit multiple aliases to exist and even be
   reported in AD-LOGIN-ALIAS, the Microsoft implementation permits only
   one NT-ENTERPRISE alias to exist, so this question had not previously

Appendix B.  Document history [REMOVE BEFORE PUBLICATION]

   12 Refactor to take advantage of FAST and new protected negotiation
      mechanism instead of providing our own.  Simplify significantly
      based on this.  Remove the true principal name support for now
      pending discussion in the WG.  Add the new protected negotiation
   11 Changed title.  Better protection on server referral preauth data.
      Support server name canonicalization.  Rename ReferralInfo to
      ClientReferralInfo.  Disallow alias mapping to a TGT principal.
      Explain why no name change in client referrals.  Add empty IANA
      Considerations.  Add some notes on preauth data protection during
      renewal etc.
   10 Separate enterprise principal names into a separate section.  Add
      a little wording to suggest server principal name canonicalization
      might be allowed; not fleshed out.  Advise against AD-KDC-ISSUED
      in cronn-realm cases.  Advise policy checks on returned client
      referral info, since there's no security.  List number
      assignments.  Add security analysis of shared-password case.  No
      longer plan to remove Microsoft appendix.  Add referral-valid-
      until field.
   09 Changed to EXAMPLE.COM instead of using Morgan Stanley's domain.
      Rewrote description of existing practice.  (Don't name the lookup
      table consulted.  Mention that DNS "canonicalization" is contrary
      to [RFC4120].)  Noted Microsoft behavior should be moved out into
      a separate document.  Changed some second-person references in the
      introduction to identify the proper parties.  Changed PA-CLIENT-
      CANONICALIZED to use a separate type for the actual referral data,
      add an extension marker to that type, and change the checksum key
      from the "returned session key" to the "AS reply key".  Changed

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      AD-LOGIN-ALIAS to contain a sequence of names, to be contained in
      AD-KDC-ISSUED instead of AD-IF-RELEVANT, and to drop the no longer
      needed separate checksum.  Attempt to clarify the cache lifetime
      of referral information.
   08 Moved Microsoft implementation info to appendix.  Clarify lack of
      local server name canonicalization.  Added optional authz-data for
      login alias, to support user-to-user case.  Added requested-
      principal-name to ServerReferralData.  Added discussion of caching
      information, and referral TGT lifetime.
   07 Re-issued with new editor.  Fixed up some references.  Started

Authors' Addresses

   Sam hartman (editor)
   Painless Security


   Kenneth Raeburn
   Massachusetts Institute of Technology


   Larry Zhu
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052


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