Kerberos working group                                         J.Brezak
Internet Draft                                                Microsoft
Document: draft-brezak-spnego-http-04.txt
Category: Informational
                                                           October 2002

           HTTP Authentication: SPNEGO Access Authentication
                 As implemented in Microsoft Windows 2000

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
   working documents of the Internet Engineering Task Force (IETF), its
   areas, and its working groups. Note that other groups may also
   distribute working documents as Internet-Drafts. Internet-Drafts are
   draft documents valid for a maximum of six months and may be
   updated, replaced, or obsoleted by other documents at any time. It
   is inappropriate to use Internet- Drafts as reference material or to
   cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

1. Abstract

   This document describes how the Microsoft Internet Explorer (MSIE)
   and Internet Information Services (IIS) incorporated in Microsoft
   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

   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",
   this document are to be interpreted as described in RFC-2119 [3].

Brezak                 Category - Informational                      1
                  HTTP SPNEGO Access Authentication      October 2002

3. Access Authentication

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

        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.

Brezak                 Category - Informational                      2
                  HTTP SPNEGO Access Authentication      October 2002

   For example the authentication could have failed on the final leg if
   mutual authentication was requested and the server was not able to
   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

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 )

        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

        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

Brezak                 Category - Informational                      3
                  HTTP SPNEGO Access Authentication      October 2002

        C: GET dir/index.html
        C: Authorization: Negotiate a87421000492aa874209af8bc028

   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

        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.

Brezak                 Category - Informational                      4
                  HTTP SPNEGO Access Authentication      October 2002

   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
   One Microsoft Way
   Redmond, Washington

Brezak                 Category - Informational                      5
                  HTTP SPNEGO Access Authentication      October 2002

Full Copyright Statement

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph
   are included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an

Brezak                 Category - Informational                      6