This authentication method posed a severe security risk for Exchange. Hackers could "sniff" the connection between the servers and work out the credentials from there. This meant that for previous Exchange implementations, you also had to apply IPSec security to the communications between servers to encrypt the information being sent between them. Often, administrators overlooked this security concern, leaving many organizations unaware that there was a potential security risk.
With the introduction of Exchange 2000, NTLM was used as the default authentication protocol between servers. The primary reason for not using Kerberos was the lack of support for the protocol when using clustered servers.
Since Windows 2000 Server SP3, Kerberos authentication is now fully supported for single and clustered servers, meaning that any information or credentials that are passed between servers are secure. This eliminates the vulnerability of "sniffing" or "listening" in on the traffic between the two servers. By default, Kerberos is enabled whenever you add multiple servers to your Exchange topology.
For more information on how Kerberos authentication works, check out http://www.microsoft.com/security.
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About the authors:
David McAmis is an enterprise architect and partner in a consulting firm in Sydney, Australia. David has written a number of books and more than 100 articles that have appeared in magazines and journals.
Don Jones, MCSE, CTT+, is an independent consultant and founding partner of BrainCore.Net. Don is the author of more than a dozen books and the creator and series editor of Sams Publishing's Delta Guide series. He is also a contributing editor and columnist for Microsoft® Certified Professional Magazine, the Microsoft technology columnist for CertCities.com, and a speaker at technology conferences.