Kerberos over TLS: Using Kerberos Version 5 over the Transport Layer Security (TLS) Protocol
How a Kerberos client and its KDC can upgrade their TCP connection to TLS for privacy and extra authentication.
Josefsson, S., "Using Kerberos Version 5 over the Transport Layer Security (TLS) Protocol", RFC 6251, DOI 10.17487/RFC6251, May 2011, <https://www.rfc-editor.org/info/rfc6251>.
@misc{rfc6251,
series = {Request for Comments},
number = 6251,
howpublished = {RFC 6251},
publisher = {RFC Editor},
doi = {10.17487/RFC6251},
url = {https://www.rfc-editor.org/info/rfc6251},
author = {S. Josefsson},
title = {{Using Kerberos Version 5 over the Transport Layer Security (TLS) Protocol}},
pagetotal = 8,
year = 2011,
month = may,
} RFC text
Contents
Internet Engineering Task Force (IETF) S. Josefsson
Request for Comments: 6251 SJD AB
Category: Informational May 2011
ISSN: 2070-1721
Using Kerberos Version 5
over the Transport Layer Security (TLS) Protocol
Abstract
This document specifies how the Kerberos V5 protocol can be
transported over the Transport Layer Security (TLS) protocol in order
to provide additional security features.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
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). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see 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/rfc6251.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction and Background .....................................2
2. Kerberos V5 STARTTLS Extension ..................................3
3. Examples ........................................................4
4. STARTTLS-Aware KDC Discovery ....................................5
5. Server Certificates .............................................6
6. IANA Considerations .............................................7
7. Acknowledgements ................................................7
8. Security Considerations .........................................7
9. References ......................................................8
9.1. Normative References .......................................8
9.2. Informative References .....................................8
1. Introduction and Background
This document describes how a Kerberos V5 [RFC4120] implementation
may upgrade communication between clients and Key Distribution
Centers (KDCs) to use the Transport Layer Security (TLS) [RFC5246]
protocol.
The TLS protocol offers integrity- and privacy-protected exchanges
that can be authenticated using X.509 certificates, OpenPGP keys
[RFC6091], and usernames and passwords via Secure Remote Password
(SRP) [RFC5054].
There are several reasons to use Kerberos V5 over TLS.
o It prevents downgrade attacks affecting, e.g., encryption types
and pre-auth data negotiation. The encryption type field in
KDC-REQ and the METHOD-DATA field with the requested pre-auth
types from the server in KDC_ERR_PREAUTH_REQUIRED errors in
KDC-REP are sent without integrity or privacy protection in
Kerberos V5. This allows an active attacker to replace the
encryption type with a compromised encryption type, e.g., 56-bit
DES, or request that clients should use a broken pre-auth type.
Since clients in general cannot know the encryption types other
servers support, or the pre-auth types servers prefer or require,
it is difficult for the client to detect if there was a man in the
middle or if the remote server simply did not support a stronger
encryption type or preferred another pre-auth type.
o Kerberos exchanges are privacy protected. Parts of many Kerberos
packets are transferred without privacy protection (i.e.,
encryption). That part contains information, such as the client
principal name, the server principal name, the encryption types
supported by the client, the lifetime of tickets, etc. Revealing
such information is, in some threat models, considered a problem.
o It provides additional authentication against the KDC. In some
situations, users are equipped with smart cards with an RSA
authentication key. In others, users have an OpenPGP client on
their desktop, with a public OpenPGP key known to the server.
o It provides explicit server authentication of the KDC to the
client. In traditional Kerberos V5, authentication of the KDC is
proved as a side effect that the KDC knows your encryption key
(i.e., your password).
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 RFC 2119 [RFC2119].
2. Kerberos V5 STARTTLS Extension
The STARTTLS extension uses the Kerberos V5 TCP extension mechanism
[RFC5021]. The extension uses bit 0 in the extension bitmask.
The protocol is as follows. The client requests the extension by
setting the STARTTLS bit in the TCP extension mechanism bitmask.
(How to deal with extension negotiation failures at this point is
described in [RFC5021].) After the server has sent the 4-octet value
0x00000000 to indicate support of this extension, the stream will be
controlled by the TLS protocol and its framing. The TLS protocol is
initiated by the client.
Typically, the client initiates the TLS handshake protocol by sending
a client hello, the server responds, and the handshake continues
until it either succeeds or fails.
If, for any reason, the handshake fails, the STARTTLS protocol will
also fail, and the TLS error is used as the error indication. In
this case, no further messages can be exchanged over the same TCP
session.
If the handshake succeeds, the Kerberos V5 authentication protocol is
performed within the protected TLS channel, like a normal TCP
Kerberos V5 exchange. In particular, this means that every Kerberos
V5 packet will be prefixed by a 4-octet length field that indicates
the length of the Kerberos V5 packet.
When no further Kerberos V5 messages need to be transferred in the
TLS session, the TLS session MUST be shut down properly using the
close_notify alert. When the TLS session is shut down, the TCP
connection cannot be re-used to send any further data and MUST be
closed.
3. Examples
A complete packet flow for a successful AS-REQ/REP exchange protected
by this mechanism will be as follows. The "STARTTLS-bit" is a
4-octet value with only the bit allocated for this extension set, and
| is the binary OR operation.
Client Server
[ Kerberos V5 TCP extension mechanism negotiation starts ]
0x80000000 | STARTTLS-bit -------->
0x00000000
<--------
[ TLS negotiation starts ]
ClientHello -------->
ServerHello
Certificate*
ServerKeyExchange*
CertificateRequest*
<-------- ServerHelloDone
Certificate*
ClientKeyExchange
CertificateVerify*
[ChangeCipherSpec]
Finished -------->
[ChangeCipherSpec]
<-------- Finished
[ Kerberos V5 negotiation starts ]
4-octet length field
Kerberos V5 AS-REQ -------->
4-octet length field
Kerberos V5 AS-REP
<--------
* Indicates optional or situation-dependent messages that are not
always sent
4. STARTTLS-Aware KDC Discovery
Section 7.2.3 of Kerberos V5 [RFC4120] describes how Domain Name
System (DNS) SRV records [RFC2782] can be used to find the address of
a KDC. We define a new Service of "kerberos-tls" to indicate that
the particular KDC is intended to support this STARTTLS extension.
The Proto (tcp), Realm, TTL, Class, SRV, Priority, Weight, Port, and
Target have the same meaning as in RFC 4120.
For example:
_kerberos-tls._tcp.EXAMPLE.COM. IN SRV 0 0 88 kdc1.example.com.
_kerberos-tls._tcp.EXAMPLE.COM. IN SRV 1 0 88 kdc2.example.com.
5. Server Certificates
The TLS protocol may be used in a mode that provides server
authentication using, for example, X.509 and OpenPGP.
A goal for the protocol described in this memo is that it should be
as easy to implement and deploy on clients as support for UDP/TCP.
Since many client environments do not have access to long-term
storage, or to long-term storage that is sufficiently secure to
enable validation of server certificates, the Kerberos V5 STARTTLS
protocol does not require clients to verify server certificates. If
server certification had been required, then environments with
constrained clients such as those mentioned would be forced to
disable TLS; this would arguably be worse than TLS without server
certificate validation, as the use of TLS, even without server
certificate validation, protects against some attacks that Kerberos
V5 over UDP/TCP does not. For example, even without server
certificate validation, TLS does protect against passive network
sniffing aimed at tracking Kerberos service usage by a given client.
However, note that the use of TLS without server certificate
verification opens up a range of active attacks such as man in the
middle.
When clients have the ability, they MUST validate the server
certificate. For this reason, if a KDC presents an X.509 server
certificate over TLS, it MUST contain an otherName Subject
Alternative Name (SAN) identified using a type-id of
id-krb5starttls-san. The intention is to bind the server certificate
to the Kerberos realm for the purpose of using Kerberos V5 STARTTLS.
The value field of the otherName should contain the realm as the
"Realm" ASN.1 type.
id-krb5starttls-san OBJECT IDENTIFIER ::=
{ iso(1) identified-organization(3) dod(6) internet(1)
private(4) enterprise(1) gnu(11591)
shishi(6) krb5starttls-san(1) }
To validate a server certificate, the client MAY use local
configuration (e.g., a list that maps the Kerberos realm to a copy of
the server's certificate) and compare that with the authentication
information provided from the server via TLS. For illustration, the
server certificate could be an X.509 certificate or an OpenPGP key.
In this mode, the client needs no processing related to id-
krb5starttls-san.
When the server presents an X.509 server certificate, clients MAY use
"Certification Path Validation" as described in [RFC5280] to validate
the KDC server certificate. In addition, unless the client can
otherwise verify that the server certificate is bound to the KDC of
the target realm, the client MUST verify that the server certificate
contains the id-krb5starttls-san SAN and that the value is identical
to the intended Kerberos realm.
6. IANA Considerations
Per [RFC5021], the IANA has allocated a bit (value 0) in the
"Kerberos TCP Extensions" registry for Krb5 over TLS, the extension
described in this document.
7. Acknowledgements
Miguel A. Garcia, Sam Hartman, Jeffrey Hutzelman, Magnus Nystroem,
and Peter Saint-Andre (in alphabetical order) provided comments that
improved the protocol and document.
8. Security Considerations
The security considerations in Kerberos V5, TLS, and the Kerberos V5
TCP extension mechanism are inherited.
Note that TLS does not protect against man-in-the-middle attacks
unless clients verify the KDC's credentials (X.509 certificate,
OpenPGP key, etc.) correctly. Although certificate validation adds
an extra layer of protection, that is not considered strictly
necessary to improve the security profile of Kerberos V5 as outlined
in this document.
If server authentication is used, some information about the server
(such as its name) is visible to passive attackers.
To protect against the inherent downgrade attack in the extension
framework, implementations SHOULD offer a policy mode that requires
this extension to always be successfully negotiated, for a particular
realm, or generally. For interoperability with implementations that
do not support this extension, the policy mode SHOULD be disabled by
default.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
July 2005.
[RFC5021] Josefsson, S., "Extended Kerberos Version 5 Key
Distribution Center (KDC) Exchanges over TCP", RFC 5021,
August 2007.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[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.
9.2. Informative References
[RFC5054] Taylor, D., Wu, T., Mavrogiannopoulos, N., and T. Perrin,
"Using the Secure Remote Password (SRP) Protocol for TLS
Authentication", RFC 5054, November 2007.
[RFC6091] Mavrogiannopoulos, N. and D. Gillmor, "Using OpenPGP Keys
for Transport Layer Security (TLS) Authentication",
RFC 6091, February 2011.
Author's Address
Simon Josefsson
Simon Josefsson Datakonsult AB
Hagagatan 24
Stockholm 113 47
Sweden
EMail: simon@josefsson.org
URI: http://josefsson.org/
ISO Plain Language
On this page
§1Why Put Kerberos Inside TLS
The document describes how a Kerberos V5 (RFC 4120) implementation may upgrade communication between clients and Key Distribution Centers to use the Transport Layer Security protocol (RFC 5246). TLS offers integrity- and privacy-protected exchanges that can be authenticated using X.509 certificates, OpenPGP keys (RFC 6091), and usernames and passwords via Secure Remote Password (RFC 5054).
There are several reasons to use Kerberos V5 over TLS. The first is that it prevents downgrade attacks affecting, for example, encryption types and pre-authentication data negotiation. The encryption type field in the KDC-REQ and the METHOD-DATA field with the requested pre-authentication types, sent by the server in KDC_ERR_PREAUTH_REQUIRED errors in the KDC-REP, are sent without integrity or privacy protection in Kerberos V5. That allows an active attacker to replace the encryption type with a compromised one, such as 56-bit DES, or to request that clients use a broken pre-authentication type. Since clients in general cannot know which encryption types other servers support, or which pre-authentication types servers prefer or require, it is difficult for the client to detect whether there was a man in the middle or whether the remote server simply did not support a stronger encryption type or preferred another pre-authentication type.
The second reason is that Kerberos exchanges are not wholly privacy protected. Parts of many Kerberos packets are transferred without encryption, and that part contains information such as the client principal name, the server principal name, the encryption types supported by the client and the lifetime of tickets. Revealing such information is, in some threat models, considered a problem.
The third reason is that it provides additional authentication against the KDC. In some situations users are equipped with smart cards holding an RSA authentication key, and in others they have an OpenPGP client on their desktop with a public key known to the server. The fourth is that it provides explicit server authentication of the KDC to the client: in traditional Kerberos V5, authentication of the KDC is proved as a side effect of the KDC knowing your encryption key, that is, your password.
Kerberos encrypts the tickets it hands out, but the first conversation, where the client and the server agree on how strong to be, is out in the open. Someone in the middle can rewrite that agreement and talk the client into using an encryption type that is no longer safe. Wrapping the conversation in TLS closes that gap, and it also hides who is asking for what.
- No downgrade attacksThe choice of encryption type and pre-authentication type can no longer be rewritten in transit.
- Privacy for the negotiationPrincipal names, supported encryption types and ticket lifetimes are no longer visible on the wire.
- Extra authentication of the clientThe KDC can also check a smart card key or an OpenPGP key.
- Explicit authentication of the KDCThe server proves who it is with a certificate, rather than only by knowing your password.
§2The STARTTLS Extension
The STARTTLS extension uses the Kerberos V5 TCP extension mechanism (RFC 5021), and uses bit 0 in the extension bitmask. The protocol is as follows. The client requests the extension by setting the STARTTLS bit in the TCP extension mechanism bitmask; how to deal with extension negotiation failures at this point is described in RFC 5021. After the server has sent the 4-octet value 0x00000000 to indicate support of this extension, the stream is controlled by the TLS protocol and its framing. The TLS protocol is initiated by the client.
Typically the client initiates the TLS handshake protocol by sending a client hello, the server responds, and the handshake continues until it either succeeds or fails. If for any reason the handshake fails, the STARTTLS protocol also fails and the TLS error is used as the error indication. In that case no further messages can be exchanged over the same TCP session.
If the handshake succeeds, the Kerberos V5 authentication protocol is performed within the protected TLS channel, like a normal TCP Kerberos V5 exchange. In particular, every Kerberos V5 packet will be prefixed by a 4-octet length field that indicates the length of the Kerberos V5 packet. When no further Kerberos V5 messages need to be transferred in the TLS session, the TLS session MUST be shut down properly using the close_notify alert. When the TLS session is shut down, the TCP connection cannot be reused to send any further data and MUST be closed.
The upgrade happens on the same TCP connection the Kerberos exchange would have used. The client asks for it, the server confirms, and from then on the connection belongs to TLS. If the handshake fails the exchange is over, because the connection cannot be reused. Once it works, the Kerberos messages look exactly as they do over plain TCP, with a length prefix, but they are now inside the encrypted channel.
- 0x80000000 | STARTTLS-bitThe Kerberos TCP extension negotiation starts.
- 0x00000000The server confirms that it supports the extension.
- ClientHelloThe TLS handshake begins.
- ServerHello, Certificate, ServerKeyExchange, CertificateRequest, ServerHelloDone
- Certificate, ClientKeyExchange, CertificateVerify, ChangeCipherSpec, Finished
- ChangeCipherSpec, Finished
- 4-octet length field and Kerberos V5 AS-REQThe Kerberos exchange now runs inside the protected channel.
- 4-octet length field and Kerberos V5 AS-REP
§4Finding a KDC That Supports It
Section 7.2.3 of Kerberos V5 (RFC 4120) describes how Domain Name System SRV records (RFC 2782) can be used to find the address of a KDC. This document defines a new service of kerberos-tls to indicate that the particular KDC is intended to support the STARTTLS extension. The Proto, Realm, TTL, Class, SRV, Priority, Weight, Port and Target have the same meaning as in RFC 4120.
The example given in the document is a pair of SRV records for the service name kerberos-tls over TCP in the realm EXAMPLE.COM, pointing at two different KDC hosts on port 88, with different priorities and weights.
A client that wants the extra protection has to find a server that offers it, and the existing way of finding Kerberos servers is DNS. So the memo adds a second service name, kerberos-tls, published in the same way, which says: these servers support the upgrade.
_kerberos-tls._tcp.EXAMPLE.COM. IN SRV 0 0 88 kdc1.example.com.
_kerberos-tls._tcp.EXAMPLE.COM. IN SRV 1 0 88 kdc2.example.com.§5Server Certificates
The TLS protocol may be used in a mode that provides server authentication using, for example, X.509 and OpenPGP. A goal for the protocol is that it should be as easy to implement and deploy on clients as support for UDP or TCP. Since many client environments do not have access to long-term storage, or to long-term storage that is sufficiently secure to enable validation of server certificates, the Kerberos V5 STARTTLS protocol does not require clients to verify server certificates. If server certification had been required, environments with constrained clients would be forced to disable TLS, which would arguably be worse than TLS without server certificate validation, since the use of TLS even without server certificate validation protects against some attacks that Kerberos V5 over UDP or TCP does not. For example, even without server certificate validation, TLS protects against passive network sniffing aimed at tracking Kerberos service usage by a given client.
The use of TLS without server certificate verification does, however, open up a range of active attacks such as man in the middle. When clients have the ability, they MUST validate the server certificate. For this reason, if a KDC presents an X.509 server certificate over TLS, it MUST contain an otherName Subject Alternative Name identified using a type id of id-krb5starttls-san. The intention is to bind the server certificate to the Kerberos realm for the purpose of using Kerberos V5 STARTTLS, and the value field of the otherName should contain the realm as the Realm ASN.1 type.
To validate a server certificate, the client MAY use local configuration, for example a list that maps the Kerberos realm to a copy of the server's certificate, and compare that with the authentication information provided by the server via TLS. The server certificate could be an X.509 certificate or an OpenPGP key, and in this mode the client needs no processing related to id-krb5starttls-san. When the server presents an X.509 server certificate, clients MAY use Certification Path Validation as described in RFC 5280 to validate it. In addition, unless the client can otherwise verify that the server certificate is bound to the KDC of the target realm, the client MUST verify that the certificate contains the id-krb5starttls-san alternative name and that the value is identical to the intended Kerberos realm.
The document makes certificate checking optional, because many clients have nowhere safe to keep a copy of the server's key. A client that cannot check the certificate gets protection against passive snooping but not against an active attacker. A client that can check it must, and the certificate has to name the realm in a dedicated field.
§8Security Considerations
The security considerations in Kerberos V5, TLS and the Kerberos V5 TCP extension mechanism are inherited. TLS does not protect against man-in-the-middle attacks unless clients verify the KDC's credentials, such as an X.509 certificate or an OpenPGP key, correctly. Although certificate validation adds an extra layer of protection, that is not considered strictly necessary to improve the security profile of Kerberos V5 as outlined in this document.
If server authentication is used, some information about the server, such as its name, is visible to passive attackers. To protect against the inherent downgrade attack in the extension framework, implementations SHOULD offer a policy mode that requires this extension to always be successfully negotiated, for a particular realm or generally. For interoperability with implementations that do not support the extension, the policy mode SHOULD be disabled by default.
Two points to keep in mind. TLS only stops a man in the middle if the client checks who it is talking to. And because the extension itself is negotiated, an attacker could try to make the negotiation fail; a client that can be told to insist on the extension closes that door, though the memo leaves that setting off by default so that older servers still work.