IIS authentication methods: What and when

Internet Information Server 4.0 employs three authentication methods: anonymous, basic, and NT Challenge/Response. Let's look at these three methods, then we'll discuss the additional methods offered by IIS 5.0.

The anonymous authentication method doesn't require the visiting user to provide logon credentials but they are still authenticated to the IIS host. This authentication uses a pre-defined account under which all anonymous users are logged in. This account is named IUSR_<servername> (where servername is the NetBIOS name of the IIS host system).

The basic authentication requires visitors to provide a username and a password. These credentials are transmitted in easily decrypted Base64. So basic authentication is essentially clear text. Basic authentication is part of the HTTP 1.0 standard and is supported by all browsers and Web server systems.

The NT Challenge/Response authentication method was developed in response to the basic authentication method. NT Challenge/Response employs a one-way hash process to protect the password during transit between client and server. This method is a definite security improvement, but the hash function is known and transferred passwords can be discovered with focused effort. NT Challenge/Response is supported only on Microsoft Web browsers and Microsoft Web servers since it uses a proprietary hash function.

The limitations of NT Challenge/Response make it a poor choice for Internet Web servers

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that require actual authentication for each visitor. A preferred method is to employ the basic authentication method within an SSL connection. SSL is a widely supported communications encryption solution. This combination of mechanisms provides for individual authentication without unnecessary exposure of credentials.

IIS 5.0 offers two additional authentication methods: Integrated Windows authentication and Digest authentication. Integrated Windows is the same as NT Challenge/Response, but if the client is a Windows 2000 system with IIS 5.0+ and is a domain member then Kerberos is used instead of the one-way hash process. Digest authentication, an improved version of Basic, uses a hash instead of Base64 to protect passwords. However, Digest is only supported by Microsoft products. Thus, neither of these additional IIS 5.0 authentication methods allow for the widest range of clients on the Internet.

About the author
James Michael Stewart is a researcher and writer for Lanwrights, Inc.

This was first published in July 2002

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