“Each NAC product works a little differently, but in every case, we found the means to bypass it,” says Ofir Arkin, CTO and co-founder of Insightix. For example, most NAC technology assumes that users will be granted access to the network via Dynamic Host Control Protocol (DHCP), which keeps IP addresses in a pool and hands them out as each user is authenticated. Through DHCP, NAC systems can restrict user access and recognize unauthorized attempts to gain entry to sensitive information. However, an insider with access to the corporate network often has the option to configure his PC with a static IP address, Arkin observes.
“That means if you can find the address of the router, which is contained in TCP/IP settings on most PCs, you can link directly to the router and enter the network undetected,” Arkin says.
NAC systems are also at risk because they normally work entirely through IP addresses, without collecting information on where devices are located or how they are connected to the network, Arkin states. NAC systems generally cannot detect activity between nodes on the same subnet, particularly if the client avoids broadcast transmissions.
Users could also gain access through unauthorized devices or old, forgotten systems and connections that don’t show up in a standard DHCP address discovery.
http://www.darkreading.com/document.asp?doc_id=98626&WT.svl=news1_2