draft-brezak-spnego-http-02.txt [plain text]
Kerberos working group J.Brezak
Internet Draft Microsoft
Document: draft-brezak-spnego-http-02.txt
Category: Informational
November 2001
HTTP Authentication: SPNEGO Access Authentication
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026 [1]. Internet-Drafts are
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1. Abstract
This document describes how Microsoft's Internet Explorer (MSIE) and
Internet Information Services (IIS) incorporated in Windows 2000 use
Kerberos for security enhancements of web transactions. The HTTP
auth-scheme of "negotiate" is defined here; when the negotiation
results in the selection of Kerberos, the security services of
authentication and optionally impersonation are performed.
This document explains how HTTP authentication utilizes the SPNEGO
[7] GSSAPI mechanism. Details of SPNEGO implementation are not
provided in this document.
2. Conventions used in this document
In examples, "C:" and "S:" indicate lines sent by the client and
server respectively.
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 [3].
3. Access Authentication
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3.1 Reliance on the HTTP/1.1 Specification
This specification is a companion to the HTTP/1.1 specification [4]
and builds on the authentication mechanisms defined in [5]. It uses
the augmented BNF section 2.1 of that document, and relies on both
the non-terminals defined in that document and other aspects of the
HTTP/1.1 specification.
4. HTTP Negotiate Authentication Scheme
Use of Kerberos is wrapped in an HTTP auth-scheme of "Negotiate".
The auth-params exchanged use data formats defined for use with the
GSS-API [6]. In particular, they follow the formats set for the
SPNEGO [7] and Kerberos [8] mechanisms for GSSAPI. The "Negotiate"
auth-scheme calls for the use of SPNEGO GSSAPI tokens which the
specific mechanism type specifies.
The current implementation of this protocol is limited to the use of
SPNEGO with the Kerberos and Microsoft NTLM protocols.
4.1 The WWW-Authenticate Response Header
If the server receives a request for an access-protected object, and
an acceptable Authorization header has not been sent, the server
responds with a "401 Unauthorized" status code, and a "WWW-
Authenticate:" header as per the framework described in [4]. The
initial WWW-Authenticate header will not carry any gssapi-data.
The negotiate scheme will operate as follows:
challenge = "Negotiate" auth-data
auth-data = 1#( [gssapi-data] )
The meanings of the values of the directives used above are as
follows:
gssapi-data
If the gss_accept_security_context return a token for the
client, this directive contains the base64 encoding of an
InitialContextToken as defined in [6]. This is not present in
the initial response from the server.
A status code 200 status response can also carry a "WWW-
Authenticate" response header containing the final leg of an
authentication. In this case, the gssapi-data will be present.
Before using the contents of the response, the gssapi-data should be
processed by gss_init_security_context to determine the state of the
security context. If this function indicates success, the response
can be used by the application. Otherwise an appropriate action
based on the authentication status should be.
For example the authentication could have failed on the final leg if
mutual authentication was requested and the server was not able to
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prove its identity. In this case, the returned results are suspect.
It is not always possible to mutually authenticate the server before
the HTTP operation. POST methods are in this category.
When the Kerberos Version 5 GSSAPI mechanism [RFC-1964] is being
used, the HTTP server will be using a principal name of the form of
"http/<hostname>".
4.2 The Authorization Request Header
Upon receipt of the response containing a "WWW-Authenticate" header
from the server, the client is expected to retry the HTTP request,
passing a HTTP "Authorization" header line. This is defined
according to the framework described in [4] utilized as follows:
credentials = "Negotiate" auth-data2
auth-data2 = 1#( gssapi-data )
gssapi-data
This directive contains is the base64 encoding of an
InitialContextToken as defined in [6].
Any returned code other than a success 2xx code represents an
authentication error. If a 401 containing a "WWW-Authenticate"
header with "Negotiate" and gssapi-data is returned from the server,
it is a continuation of the authentication request.
A client may initiate a connection to the server with an
"Authorization" header containing the initial token for the server.
This form will bypass the initial 401 error from the server when the
client knows that the server will accept the Negotiate HTTP
authentication type.
5. Negotiate Operation Example
The client requests an access-protected document from server via a
GET method request. The URI of the document is
"http://www.nowhere.org/dir/index.html".
C: GET dir/index.html
The first time the client requests the document, no Authorization
header is sent, so the server responds with:
S: HTTP/1.1 401 Unauthorized
S: WWW-Authenticate: Negotiate
The client will obtain the user credentials using the SPNEGO GSSAPI
mechanism type to identify generate a GSSAPI message to be sent to
the server with a new request, including the following Authorization
header:
C: GET dir/index.html
C: Authorization: Negotiate a87421000492aa874209af8bc028
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The server will decode the gssapi-data and pass this to the SPNEGO
GSSAPI mechanism in the gss_accept_security_context function. If the
context is not complete, the server will respond with a 401 status
code with a WWW-Authenticate header containing the gssapi-data.
S: HTTP/1.1 401 Unauthorized
S: WWW-Authenticate: Negotiate 749efa7b23409c20b92356
The client will decode the gssapi-data and pass this into
gss_init_security_context and return the new gssapi-data to the
server.
C: GET dir/index.html
C: Authorization: Negotiate 89a8742aa8729a8b028
This cycle can continue until the security context is complete.
When the return value from the gss_accept_security_context function
indicates that the security context is complete, it may supply final
authentication data to be returned to the client. If the server has
more gssapi data to send to the client to complete the context it is
to be carried in WWW-Authenticate header with the final response
containing the HTTP body.
S: HTTP/1.1 200 Success
S: WWW-Authenticate: Negotiate ade0234568a4209af8bc0280289eca
The client will decode the gssapi-data and supply it to
gss_init_security_context using the context for this server. If the
status is successful from the final gss_init_security_context, the
response can be used by the application.
7. Security Considerations
The SPNEGO HTTP authentication facility is only used to provide
authentication of a user to server. It provides no facilities for
protecting the HTTP headers or data including the Authorization and
WWW-Authenticate headers that are used to implement this mechanism.
This mechanism is not used for HTTP authentication to HTTP proxies.
If an HTTP proxy is used between the client and server, it must take
care to not share authenticated connections between different
authenticated clients to the same server. If this is not honored,
then the server can easily lose track of security context
associations. A proxy that correctly honors client to server
authentication integrity will supply the "Proxy-support: Session-
Based-Authentication" HTTP header to the client in HTTP responses
from the proxy. The client MUST NOT utilize the SPNEGO HTTP
authentication mechanism through a proxy unless the proxy supplies
this header with the "401 Unauthorized" response from the server.
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When using the SPNEGO HTTP authentication facility with client
supplied data such as PUT and POST, the authentication should be
complete between the client and server before sending the user data.
The return status from the gss_init_security_context will indicate
with the security context is complete. At this point the data can be
sent to the server.
8. References
1 Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
3 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997
4 Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
HTTP/1.1", RFC 2616, June 1999.
5 Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., Leach,
P., Luotonen, A., Stewart, L., "HTTP Authentication: Basic and
Digest Access Authentication", RFC 2617, June 1999.
6 Linn, J., "Generic Security Service Application Program Interface,
Version 2", RFC 2078, January 1997.
7 Baize, E., Pinkas, D., "The Simple and Protected GSS-API
Negotiation Mechanism", RFC 2478, December 1998.
8 Linn, J., "The Kerberos Version 5 GSS-API Mechanism", RFC 1964,
June 1996.
10. Author's Addresses
John Brezak
Microsoft
One Microsoft Way
Redmond, Washington
Email: jbrezak@microsoft.com
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