Two Sides of Vulnerability Scanning

There are two approaches to network vulnerability scanning, active and passive. The active approach encompasses everything an organization does to foil system breaches, while the passive (or monitoring) approach entails all the ways the organization oversees system security. When making buying decisions for your organization, it's a mistake to think that you have to choose between the two types of protection.

The passive approach allows security personnel to monitor which operating systems are in use; what is being sent to, from and within the system; which services are available; and where parts of the system may be vulnerable to security threats. The active approach, on the other hand, offers much information about system and application vulnerabilities.

Active scanning tools are used where constant vigilance is required. They have a specific area of focus that the product is programmed to monitor. (And they are sometimes configured to prevent particular situations as well, such as the use of USB key chains on a network.) Their core monitoring functionality is generally very rigid and can't be easily customized or extended.

When an organization uses the passive approach in scanning its LAN, the information obtained will normally include data pertaining to the hosts in the network -- which ports are open, which software versions are being maintained and which services are running.

There is a huge potential with passive analysis because it allows you to assess the vulnerability of your software without interfering with the client or server. This technology facilitates IT asset management, since it allows an IT manager to instantly get a list of which users are running vulnerable copies of certain software programs.

When combined with passive vulnerability scanning, an active scan can help provide a more complete picture of the software load-out on client-side systems, as well as on servers. In short, the two types of scanners complement each other.

When it comes to selecting the right passive scanning product for your organization, there is no shortage of options. Tenable Network Security, for example, offers a product called NeVO. The NeVO vulnerability monitor can determine what's happening on your network without having to actively scan it. NeVO runs 24/7 and helps uncover whether any new hosts, ports, services or vulnerabilities have suddenly appeared since the last active scan of the network was performed. Although NeVO uses its own pattern matching and signature language to detect potential threats, Tenable does publish new NeVO signatures regularly, allowing you to easily keep this product up to date.

Guardian Digital's flagship operating platform, EnGarde Secure Linux, is another example of a passive security tool with intrusion-detection capabilities to assist users in pinpointing security threats. Guardian also offers the Internet Defense and Detection System, which the company claims is the first open-source IDS application to provide both enhanced intrusion-detection and -prevention capabilities in one system.

Highly customizable software such as GFI Software's LANguard Network Security Scanner is another example of a passive scanner that can unearth a wide range of security issues on your computer network. GFI also produces an active scanner called the LANguard Portable Storage Control, which is best applied to plug holes in very specific areas that have been identified by the passive scanner tools.

When deciding which approach to use on your network, remember that the key difference between the two approaches to security is action. Passive security involves providing notification of potential security issues, yet it allows those issues to continue until the administrator takes action. An active security system, on the other hand, alerts administrators of any issues in question and also takes measures to prevent them from causing damage, such as blocking the offending IP address or closing off the port.

The bottom line is that passive scanning in systems can expose a lot of information about all aspects of the system in normal communications without intruding upon operations. Active scanning has the potential to discover more information, and when combined with passive scanning, it gives a more complete picture. The wise IT manager will use both.

Douglas Schweitzer is an Internet security specialist. Contact him at


Copyright © 2005 IDG Communications, Inc.

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