


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 
   http://www.ietf.org/ietf/1id-abstracts.txt  
    
   The list of Internet-Draft Shadow Directories can be accessed at 
   http://www.ietf.org/shadow.html. 
    
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 
   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]. 
    
    
  
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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 
   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. 
    
  
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   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 
   "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: 
    
  
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        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 
   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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