Internet DRAFT - draft-ietf-krb-wg-kerberos-referrals
draft-ietf-krb-wg-kerberos-referrals
Kerberos WORKING GROUP S. Hartman, Ed.
Internet-Draft Painless Security
Updates: 4120 (if approved) K. Raeburn
Intended status: Standards Track MIT
Expires: March 27, 2013 L. Zhu
Microsoft Corporation
September 23, 2012
Kerberos Principal Name Canonicalization and KDC-Generated Cross-Realm
Referrals
draft-ietf-krb-wg-kerberos-referrals-15
Abstract
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. This
memo updates RFC 4120.
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
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and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 27, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions Used in This Document . . . . . . . . . . . . . . 5
3. Requesting a Referral . . . . . . . . . . . . . . . . . . . . 5
4. Realm Organization Model . . . . . . . . . . . . . . . . . . . 6
4.1. Trust Assumptions . . . . . . . . . . . . . . . . . . . . 6
5. Enterprise Principal Name Type . . . . . . . . . . . . . . . . 7
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. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
13. Security Considerations . . . . . . . . . . . . . . . . . . . 14
13.1. Shared-password case . . . . . . . . . . . . . . . . . . . 17
13.2. Preauthentication data . . . . . . . . . . . . . . . . . . 17
14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17
15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
15.1. Normative References . . . . . . . . . . . . . . . . . . . 18
15.2. Informative References . . . . . . . . . . . . . . . . . . 18
Appendix A. Compatibility with Earlier Implementations of
Name Canonicalization . . . . . . . . . . . . . . . . 18
Appendix B. Document history [REMOVE BEFORE PUBLICATION] . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
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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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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 through 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
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
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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:
EXAMPLE.COM
/ \
/ \
ADMIN.EXAMPLE.COM DEV.EXAMPLE.COM
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.
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 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
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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 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 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 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
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"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 those in the request. Names MUST NOT be changed
in the response to a TGS request, although it is common for KDCs to
maintain ta 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
whichever alias is 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
a 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 best protection, Flexible
Authentication through Secure Tunneling (FAST) [RFC6113] or another
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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 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(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
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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 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 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
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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
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/EXAMPLE.COM@ADMIN.EXAMPLE.COM
C: TGS-REQ sname=http/foo.dev.example.com +NC to EXAMPLE.COM
S: TGS-REP sname=krbtgt/DEV.EXAMPLE.COM@EXAMPLE.COM
C: TGS-REQ sname=http/foo.dev.example.com +NC to DEV.EXAMPLE.COM
S: TGS-REP sname=http/foo.dev.example.com@DEV.EXAMPLE.COM
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, RFC 4120's rules 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 re-check 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
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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 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 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
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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. IANA Considerations
In the Kerberos pre-authentication and typed data registry at http://
www.iana.org/assignments/kerberos-parameters/
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. [RFC editor
please remove the prior sentence when publishing.]
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 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
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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 [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 in Section 11 detects modification of the request
between the KDC and client, mitigating some attacks.
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
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 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.
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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
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.
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Clients MUST restrict service name changes sufficiently that the
client ends up talking to the correct password changing service.
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 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
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.
13.2. 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.
14. Acknowledgments
John Brezak, Mike Swift, and Jonathan Trostle wrote the initial
version of this document.
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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.
[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
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:
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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 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
arisen.
Appendix B. Document history [REMOVE BEFORE PUBLICATION]
13 Better reflect that we are not solving the gnuftp.raeburn.org use
case. Clean up other references to information in padata. Fix
the Microsoft appendix based on discussions with them
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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
mechanism.
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
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
history.
Authors' Addresses
Sam hartman (editor)
Painless Security
Email: hartmans-ietf@mit.edu
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Kenneth Raeburn
Massachusetts Institute of Technology
Email: raeburn@mit.edu
Larry Zhu
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
Email: lzhu@microsoft.com
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