J.Brezak Internet Draft Microsoft Document: draft-brezak-kerberos-http-00.txt Category: Informational November 17,2003 Expires: May 17,2003 HTTP Authentication: Kerberos Authentication As implemented in Microsoft Windows 2000 Status of this Memo This document is an Internet-Draft and is subject to all provisions of Section 10 of RFC2026 [1] except that the right to create derivative works is not granted. 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 Hypertext Transport Protocol (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 Simple and Protected GSS-API Negotiation 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 [1]. Brezak Category - Informational 1 HTTP Kerberos Access Authentication November 2003 3. Introduction Microsoft has provided support for Kerberos authentication in MSIE and IIS in addition to other mechanisms. This provides the benefits of the Kerberos v5 protocol for Web applications. Support for Kerberos authentication is based on other previously defined mechanisms such as SPNEGO and the Generic Security Services Application Program Interface(GSSAPI). 3. Access Authentication 3.1 Reliance on the HTTP/1.1 Specification This specification is a companion to the HTTP/1.1 specification [1] and builds on the authentication mechanisms defined in [2]. 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 [3]. In particular, they follow the formats set for the SPNEGO [4] and Kerberos [5] 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(NT Lan Manager) 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 [1]. 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 [3]. This is not present in the initial response from the server. Brezak Category - Informational 2 HTTP Kerberos Access Authentication November 2003 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 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 [5] is being used, the HTTP server will be using a principal name of the form of "HTTP/". 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 [1] 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 [3]. 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: Brezak Category - Informational 3 HTTP Kerberos Access Authentication November 2003 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 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. Brezak Category - Informational 4 HTTP Kerberos Access Authentication November 2003 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. 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 10. Author's Addresses John Brezak Microsoft One Microsoft Way Redmond, Washington Email: jbrezak@microsoft.com Brezak Category - Informational 5 HTTP Kerberos Access Authentication November 2003 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 English. 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 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE." Brezak Category - Informational 6