Part I: Introduction—Marketing Buzz:The last two years have seen a big new marketing-buzz named "Admission Control" or "Endpoint Compliance Enforcement" and most major network and security players have developed a product-suite to secure their share of the cake. As the market is still evolving and one framework has been quite successful on the market: "Cisco Network Admission Control". NAC is a pivotal part of Cisco’s "Self Defending Network" strategy and supported on the complete range of Cisco network- and security-products. From a security point of view “NAC” is a very interesting emerging technology which deservers some scrutiny. We are able to hack the Cisco NAC-solution by exploiting a fundamental design flaw.