The attacker simply took advantage from the fact that DefensiveThinking administrator forgot to set up the policies for Frontpage extensions.
To deface Mitnick's site it was enough to go on your windows desktop, create a new webfolder and name it with www.defensivethinking.com. The lack of security set to Frontpage allowed the attacker to view the entire DefensiveThinking websystem as a folder of the attacker windows computer. To deface the webpage it was enough to create an HTML file with the defacement message and drag&drop it into the newly created webfolder. As simple as told. This configuration mistake allowed the attacker to view, browse, read all the files in DefensiveThinking's web structure. We'd like also to say that the hacker didn't have the previlege on DefensiveThinking to remove or rename the default.asp home page so he just uploaded a hacked.html file. When Mitnick, interrogated by friends at The Register stated: -"The compromised computer is a public system on a network separate from production systems at Defensive Thinking. No customer information was released nor was in danger of being compromised"-, we really hope it went in that way.