Internet Engineering Task Force (IETF) S. Hartman, Ed.

Internet Engineering Task Force (IETF) S. Hartman, Ed.
Request for Comments: 6806 Painless Security
Updates: 4120 K. Raeburn
Category: Standards Track MIT
ISSN: 2070-1721 L. Zhu
Microsoft Corporation
November 2012

Kerberos Principal Name Canonicalization and Cross-Realm Referrals

Abstract

This 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
Ticket-Granting Ticket (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. This memo updates RFC 4120.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6806.

Copyright Notice

Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.

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Provisions Relating to IETF Documents
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publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect

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to this document. Code Components extracted from this document must
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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . 4
3. Requesting a Referral . . . . . . . . . . . . . . . . . . . . 4
4. Realm Organization Model . . . . . . . . . . . . . . . . . . . 5
4.1. Trust Assumptions . . . . . . . . . . . . . . . . . . . . 5
5. Enterprise Principal Name Type . . . . . . . . . . . . . . . . 6
6. Name Canonicalization . . . . . . . . . . . . . . . . . . . . 7
7. Client Referrals . . . . . . . . . . . . . . . . . . . . . . . 9
8. Server Referrals . . . . . . . . . . . . . . . . . . . . . . . 10
9. Cross-Realm Routing . . . . . . . . . . . . . . . . . . . . . 11
10. Caching Information . . . . . . . . . . . . . . . . . . . . . 11
11. Negotiation of FAST and Detecting Modified Requests . . . . . 12
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
13. Security Considerations . . . . . . . . . . . . . . . . . . . 13
13.1. Shared-Password Case . . . . . . . . . . . . . . . . . . . 16
13.2. Pre-Authentication Data . . . . . . . . . . . . . . . . . 16
14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17
15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
15.1. Normative References . . . . . . . . . . . . . . . . . . . 17
15.2. Informative References . . . . . . . . . . . . . . . . . . 17
Appendix A. Compatibility with Earlier Implementations of
Name Canonicalization . . . . . . . . . . . . . . . . 18

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1. Introduction

Current implementations of the Kerberos Authentication Service (AS)
and Ticket-Granting Service (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 email name in the style described in [RFC5322]. 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 TGT has been obtained, the user 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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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 complements the Flexible Authentication Secure Tunnels
(FAST) facility provided in [RFC6113]. 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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].

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

When sending names with the "canonicalize" KDC option, the client
should expect 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].

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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 untrusted DNS lookup for name resolution.

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

EXAMPLE.COM
/ \
/ \
ADMIN.EXAMPLE.COM DEV.EXAMPLE.COM

In this configuration, all users in the EXAMPLE.COM enterprise could
have principal names, such as [email protected], 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.

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 implied by 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 that 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

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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 characteristics may increase the
trust 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 constrain the set of principals to which 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 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 "[email protected]", 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
"[email protected]" and "[email protected]".

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This utilizes a new principal name type, as the KDC-REQ message only
contains a single client realm (crealm) field, and the realm portion
of this name corresponds to the Kerberos realm with which the request
is made. Thus, the entire name "[email protected]" 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 an 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 host name
to use. In order to avoid the need to update the host 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 those in the request. Names MUST NOT be changed
in the response to a TGS request, although it is common for KDCs to
maintain a set of aliases for service principals. Regardless of
which alias a client requests, the same service key is used.
However, in the TGS request, the client receives a ticket for the
alias requested. Services MUST NOT make distinctions based on which
alias is in the issued ticket, because the service name in a ticket
is not cryptographically protected and can be changed by parties
other than the KDC.

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
an NT-UID [RFC4120].

(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 the best protection,
Flexible Authentication Secure Tunneling (FAST) [RFC6113] 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.

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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 13 for a
discussion of the tradeoffs in allowing unprotected responses.

In order to permit authorization decisions to be made based on
aliases as well as the canonicalized form of a principal name, 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 80 --
login-aliases [0] SEQUENCE (SIZE (1..MAX)) OF PrincipalName,
...
}

The login-aliases field lists one or more of the aliases the
principal is known by.

In addition to permitting authorization based on aliases, this
permits user-to-user exchanges where the party receiving the
authenticator knows the other party only by an alias. The recipient
of such an authenticator SHOULD 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
specify the inclusion of this data in the one-realm case. The AD-
LOGIN-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 service.

The canonical principal name for an alias MUST 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.

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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
[email protected], 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 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]; 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 AS request 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.

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-REP, 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:

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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

One common approach for limiting the realms from which referrals are
accepted is to limit referrals to realms that can construct an
authentication path back to the service principal of the local
machine. This tends to work well when realms are generally within an
organization and all realms that can form an authentication path back
to the local machine have some reasonable level of mapping trust.
Deployments involving more complex trust, for example, high
probability of malicious realms, are likely to need more complex
policy and MAY need to prompt the user before accepting some
referrals.

There is currently no provision for changing the client name in a
client referral response.

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.

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 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
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).

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

+NC = Canonicalize KDCOption set
C: TGS-REQ sname=http/foo.dev.example.com +NC to ADMIN.EXAMPLE.COM
S: TGS-REP sname=krbtgt/[email protected]
C: TGS-REQ sname=http/foo.dev.example.com +NC to EXAMPLE.COM
S: TGS-REP sname=krbtgt/[email protected]
C: TGS-REQ sname=http/foo.dev.example.com +NC to DEV.EXAMPLE.COM
S: TGS-REP sname=http/[email protected]

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 as discussed in the previous section. When a client follows
such a referral, it includes the realm of the referred-to realm in
the generated request.

When the canonicalize flag is not set, the rules defined in RFC 4120
apply.

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 recheck with
the local KDC to see if the name has been created locally.

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

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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 new pre-
authentication data of type PA-REQ-ENC-PA-REP (149) in all AS
requests and MAY send this padata type in TGS requests. The value of
this padata item SHOULD be empty and its value 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 [RFC4120] is extended to support an additional field
containing encrypted pre-authentication data.

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 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.

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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 padata item
MUST be empty on sending, and the contents of the padata item MUST be
ignored on receiving.

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. IANA Considerations

PA-REQ-ENC-PA-REP has been registered in the Kerveros "Pre-
authentication and Typed Data" registry
<http://www.iana.org/assignments/kerberos-parameters>.

13. 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 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 are 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 a KDC reply defined by RFC 4120 are protected.
None of the fields defined in RFC 4120 for AS request are protected,
and some information in a TGS request may not be protected. The
referrals mechanism creates several opportunities for attack because

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of these unprotected fields. FAST [RFC6113] 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 described in Section 11 detects
modification of the request between the KDC and client, mitigating
some attacks.

There is a widely deployed base of implementations that use name
canonicalization or server referrals that use 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 standpoint.

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 Public Key
Cryptography for Initial Authentication in Kerberos (PKINIT) with an
error response.

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

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service (including the TGS), it constructs an AP-REQ message. This
message includes a client name that 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 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.

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

In normal operation as described in [RFC4120], 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.

13.2. Pre-Authentication 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.

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14. 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
Consortium.

15. References

15.1. Normative References

[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.

[RFC6113] Hartman, S. and L. Zhu, "A Generalized Framework for
Kerberos Pre-Authentication", RFC 6113, April 2011.

15.2. 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.

[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 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.

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Appendix A. Compatibility with Earlier Implementations of Name
Canonicalization

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

1) Windows include an additional encrypted padata element. The
preauth data type definition in the encrypted preauth data is as
follows:

PA-SVR-REFERRAL-INFO 20

PA-SVR-REFERRAL-DATA ::= SEQUENCE {
referred-name [1] PrincipalName OPTIONAL,
referred-realm [0] Realm
}}

The referred-principal is never sent. The referred-realm is
included in TGS replies and includes the realm name of the
realm to which the client is referred. This information is
redundant with the realm in the second component of the
returned TGT.

2) 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
(1.3.6.1.4.1.311.20.2.3), and the value field of the type
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 trust 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
arisen.

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Authors' Addresses

Sam Hartman (editor)
Painless Security

EMail: [email protected]

Kenneth Raeburn
Massachusetts Institute of Technology

EMail: [email protected]

Larry Zhu
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US

EMail: [email protected]

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