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Kerberos working group J.Brezak
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Internet Draft Microsoft
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Document: draft-brezak-spnego-http-00.txt
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Category: Informational
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HTTP Authentication: SPNEGO Access Authentication
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This document is an Internet-Draft and is in full conformance with
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all provisions of Section 10 of RFC2026 [1]. Internet-Drafts are
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working documents of the Internet Engineering Task Force (IETF), its
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areas, and its working groups. Note that other groups may also
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distribute working documents as Internet-Drafts. Internet-Drafts are
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draft documents valid for a maximum of six months and may be
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updated, replaced, or obsoleted by other documents at any time. It
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is inappropriate to use Internet- Drafts as reference material or to
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cite them other than as "work in progress."
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The list of current Internet-Drafts can be accessed at
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http://www.ietf.org/ietf/1id-abstracts.txt
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The list of Internet-Draft Shadow Directories can be accessed at
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http://www.ietf.org/shadow.html.
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This document describes how Microsoft�s Internet Explorer 5.0 and
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Internet Information Services 5.0 use Kerberos for security
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enhancements of web transactions. The HTTP auth-scheme of
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'negotiate' is defined here; when the negotiation results in the
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selection of Kerberos, the security services of authentication and
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optionally impersonation are performed.
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2. Conventions used in this document
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In examples, "C:" and "S:" indicate lines sent by the client and
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
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"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
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this document are to be interpreted as described in RFC-2119 [3].
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3. Access Authentication
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3.1 Reliance on the HTTP/1.1 Specification
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This specification is a companion to the HTTP/1.1 specification [4]
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and builds on the authentication mechanisms defined in [5]. It uses
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SPNEGO Access Authentication September 2001
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the augmented BNF section 2.1 of that document, and relies on both
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the non-terminals defined in that document and other aspects of the
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HTTP/1.1 specification.
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4. HTTP Negotiate Authentication Scheme
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Use of Kerberos is wrapped in an HTTP auth-scheme of "Negotiate".
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The auth-params exchanged use data formats defined for use with the
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GSS-API [6]. In particular, they follow the formats set for the
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SPNEGO [7] and Kerberos [8] "mechanisms" for GSSAPI. The
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"Negotiate" auth-scheme calls for the use of SPNEGO GSSAPI tokens
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which the specific mechanism type specifies.
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4.1 The WWW-Authenticate Response Header
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If the server receives a request for an access-protected object, and
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an acceptable Authorization header is not sent, the server responds
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with a "401 Unauthorized" status code, and a WWW-Authenticate header
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as per the framework described in [4]. The negotiate scheme will
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challenge = "Negotiate" auth-data
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auth-data = 1#( [gssapi-data] )
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The meanings of the values of the directives used above are as
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If the gss_accept_security_context return a token for the
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client, this directive contains is the base64 encoding of an
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InitialContextToken as defined in [6].
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A status code 200 response can also carry a WWW-Authenticate
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response header containing the final leg of a authentication. Before
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using the contents of the response, the gssapi-data should be
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processed by gss_init_security_context to determine the state of the
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security context. If this function indicates success, the response
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can be used by the application. Otherwise an appropriate action
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based on the authentication status should be.
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For example the authentication could have failed on the final leg if
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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.
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It is not always possible to mutually authenticate the server before
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the HTTP operation. POST methods are in this category.
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When the Kerberos Version 5 GSSAPI mechanism [RFC-1964] is being
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used, the HTTP server will be using a principal name of the form of
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4.2 The Authorization Request Header
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SPNEGO Access Authentication September 2001
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The client is expected to retry the request, passing an
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Authorization header line, which is defined according to the
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framework described in [4] utilized as follow:
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credentials = "Negotiate" auth-data2
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auth-data2 = 1#( gssapi-data )
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This directive contains is the base64 encoding of an
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InitialContextToken as defined in [6].
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If a directive or its value is improper, or required directives are
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missing, the propose response is 400 Bad Request. If a 401
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Unauthorized status code is returned, the contents of the WWW-
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Authenticate response header is used to continue the authentication
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as long as the opaque value is the same.
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5. Negotiate Operation Example
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The user is logged onto realm A.COM as user@A.COM. The web server is
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in realm B using the principal http/server@B.COM. Realm B.COM trusts
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The client requests an access-protected document from server via a
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GET method request. The URI of the document is
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"http://www.nowhere.org/dir/index.html".
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The first time the client requests the document, no Authorization
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header is sent, so the server responds with:
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HTTP/1.1 401 Unauthorized
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WWW-Authenticate: Negotiate
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The client will obtain the user credentials using the SPNEGO GSSAPI
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mechanism type to identify generate a GSSAPI message to be sent to
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the server with a new request, including the following Authorization
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Authorization: Negotiate
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2a87421000492ade0234568ac0289eca874209af8bc028
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The server will decode the gssapi-data and pass this to the SPNEGO
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GSSAPI mechanism in the gss_accept_security_context function. The
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return value from the gss_accept_security_context function can
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indicate the security context is complete and supply final
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authentication data to be returned to the client. If the server has
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more gssapi data to send to the client to complete the context it is
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to be carried in WWW-Authenticate header with the final response.
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The response will be sent to the client, including the following
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WWW-Authenticate: Negotiate ade0234568ac874209af8bc0280289eca
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SPNEGO Access Authentication September 2001
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The client will decode the gssapi-data and supply it to
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gss_init_security_context using the context for this server. If the
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status is successful from the final gss_init_security_context, the
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response can be used by the application.
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7. Security Considerations
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The SPNEGO HTTP authentication facility is only used to provide
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authentication of a user to server. It provides no facilities for
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protecting the HTTP headers or data including the Authorization and
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WWW-Authenticate headers that are used to implement this mechanism.
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This mechanism is not used for HTTP authentication to HTTP proxies.
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If an HTTP proxy is used between the client and server, it must take
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care to not share authenticated connections between different
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authenticated clients to the same server. If this is not honored,
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then the server can easily lose track of security context
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associations. A proxy that correctly honors client to server
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authentication integrity will supply the "Proxy-support: Session-
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Based-Authentication" HTTP header to the client in HTTP responses
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from the proxy. The client MUST NOT utilize the SPNEGO HTTP
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authentication mechanism through a proxy unless the proxy supplies
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this header with the 401 Unauthorized response from the server.
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1 Bradner, S., "The Internet Standards Process -- Revision 3", BCP
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9, RFC 2026, October 1996.
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3 Bradner, S., "Key words for use in RFCs to Indicate Requirement
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Levels", BCP 14, RFC 2119, March 1997
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4 Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
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Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
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HTTP/1.1", RFC 2616, June 1999.
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5 Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., Leach,
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P., Luotonen, A., Stewart, L., "HTTP Authentication: Basic and
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Digest Access Authentication", RFC 2617, June 1999.
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6 Linn, J., "Generic Security Service Application Program Interface,
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Version 2", RFC 2078, January 1997.
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7 Baize, E., Pinkas, D., "The Simple and Protected GSS-API
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Negotiation Mechanism", RFC 2478, December 1998.
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8 Linn, J., "The Kerberos Version 5 GSS-API Mechanism", RFC 1964,
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SPNEGO Access Authentication September 2001
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10. Author's Addresses
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Email: jbrezak@microsoft.com
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SPNEGO Access Authentication September 2001
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Full Copyright Statement
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Copyright (C) The Internet Society (2001). All Rights Reserved.
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This document and translations of it may be copied and furnished to
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others, and derivative works that comment on or otherwise explain it
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or assist in its implementation may be prepared, copied, published
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and distributed, in whole or in part, without restriction of any
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kind, provided that the above copyright notice and this paragraph
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are included on all such copies and derivative works. However, this
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document itself may not be modified in any way, such as by removing
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the copyright notice or references to the Internet Society or other
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Internet organizations, except as needed for the purpose of
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developing Internet standards in which case the procedures for
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copyrights defined in the Internet Standards process must be
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followed, or as required to translate it into languages other than
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The limited permissions granted above are perpetual and will not be
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revoked by the Internet Society or its successors or assigns.
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This document and the information contained herein is provided on an
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"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
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TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."
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Brezak Category � Informational 6
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doc/standardisation/draft-brezak-spnego-http-00.txt
