1.1 Assessing the Threat

The first step toward developing an effective network security plan is to assess the threat that connection presents to your systems. RFC 1244 identifies three distinct types of security threats usually associated with network connectivity:

• Unauthorized access (a break-in by an unauthorized person).

• Disclosure of information (any problem that causes the disclosure of valuable or sensitive information to people who should not have access to the information).

• Denial of service (any problem that makes it difficult or impossible for the system to continue to perform productive work).

Assess these threats in relation to the number of users who would be affected, as well as to the sensitivity of the information that might be compromised. For some organizations, break-ins are an embarrassment that can undermine the confidence that others have in the organization. Intruders tend to target government and academic organizations that will be embarrassed by the break-in. But for most organizations, unauthorized access is not a major problem unless it involves one of the other threats: disclosure of information or denial of service.

Assessing the threat of information disclosure depends on the type of information that could be compromised. While no system with highly classified information should ever be directly connected to the Internet, systems with other types of sensitive information might be connected without undue hazard. In most cases, files such as personnel and medical records, corporate plans, and credit reports can be adequately protected by standard UNIX file security procedures. However, if the risk of liability in case of disclosure is great, the host may choose not to be connected to the Internet.

Denial of service can be a severe problem if it impacts many users or a major mission of your organization. Some systems can be connected to the network with little concern. The benefit of connecting individual workstations and small servers to the Internet generally outweighs the chance of having service interrupted for the individuals and small groups served by these systems. Other systems may be vital to the survival of your organization. The threat of losing the services of a mission-critical system must be evaluated seriously before connecting such a system to the network.

In his class on computer security, Brent Chapman classifies information security threats into three categories: threats to the secrecy, availability, and integrity of data. Secrecy is the need to prevent the disclosure of sensitive information. Availability means that you want information and information processing resources available when they are needed; a denial-of-service attack disrupts availability. The need for the integrity of information is equally obvious, but its link to computer security is more subtle. Once someone has gained unauthorized access to a system, the integrity of the information on that system is in doubt. Furthermore, some intruders just want to compromise the integrity of data. We are all familiar with cases where intruders gain access to a Web server and change the data on the server in order to embarrass the organization that runs the Web site. Thinking about the impact network threats have on your data can make it easier to assess the threat.

Network threats are not, of course, the only threats to computer security, or the only reasons for denial of service. Natural disasters and internal threats (threats from people who have legitimate access to a system) are also serious. Network security has had a lot of publicity, so it's a fashionable thing to worry about; but more computer time has probably been lost because of fires than has ever been lost because of network security problems. Similarly, more data has probably been improperly disclosed by authorized users than by unauthorized break-ins. This book naturally emphasizes network security, but network security is only part of a larger security plan that includes physical security and disaster recovery plans.

Many traditional (non-network) security threats are handled, in part, by physical security. Don't forget to provide an adequate level of physical security for your network equipment and cables. Again, the investment in physical security should be based on your realistic assessment of the threat.