Several clients have recently been asking about "Virtual Network Segmentation" products that claim to segment networks to reduce PCI compliance. They may use ARP or VLANs to control access to various network segments. These type of controls work at Layer 2 and the hacker community is well versed at using tools such as Ettercap or Cain & Abel to bypass those controls. We've recently written about Network Segmentation for PCI as part of the PCI X-Ray series.
While rereading the PCI Wireless Guidance document, I came across this nugget that puts a nail in the coffin of using VLANs as a security control:"Relying on Virtual LAN (VLAN) based segmentation alone is not sufficient. For example, having the CDE on one VLAN and the WLAN on a separate VLAN does not adequately segment the WLAN and take it out of PCI DSS scope. VLANs were designed for managing large LANs efficiently. As such, a hacker can hop across VLANs using several known techniques if adequate access controls between VLANs are not in place. As a general rule, any protocol and traffic that is not necessary in the CDE, i.e., not used or needed for credit card transactions, should be blocked. This will result in reduced risk of attack and will create a CDE that has less traffic and is thus easier to monitor."