Ofer Shezaf highlights one of the fundamental ways of categorizing security tools in his post Black Cat, White Cat | InfoSec aXioms.
Black listing, sometimes called negative security or “open by default”, focuses on catching the bad guys by detecting attacks. Security controls such as Intrusion Prevention Systems and Anti-Virus software use various methods to do so. The most common method to detect attacks matching signatures against network traffic or files. Other methods include rules which detect conditions that cannot be expressed in a pattern and abnormal behavior detection.
White listing on the other hand allows only known good activity. Other terms associated with the concept are positive security and “closed by default” and policy enforcement. White listing is commonly embedded in systems and the obvious example is the authentication and authorization mechanism found in virtually every information system. Dedicated security controls which use white listing either ensures the build-in policy enforcement is used correctly or provide a second enforcement layer. The former include configuration and vulnerability assessment tools while the latter include firewalls.
Unfortunately, when manufactures apply the term “Next Generation” to firewalls, they may be misleading the marketplace. As Ofer says, a firewall, by definition, performs white listing, i.e. policy enforcement. One of the key functions of a NGFW is the ability to white list applications. This means the applications that are allowed must be defined in the firewall policy. On the other hand, if you are defining applications that are to be blocked, that’s black listing, and not a firewall.
Also note that Next Generation Firewalls also perform Intrusion Prevention, which is a black listing function. So clearly, NGFWs perform white listing and black listing functions. But to truly earn the right to call a network security appliance a “Next Generation” Firewall, the device must enable application white listing. Adding “application awareness” as a blacklist function is nice, but not a NGFW. For more information, I have written about Next Generation Firewalls and the difference between UTMs and NGFWs.
