Basic Computer Security

Posted by arlene

Computer security is always changing. New hardware and new software arrive constantly, and the rapid evolution of Internet technologies only compounds the problem. New systems, and even changes in existing systems, require new security analyses. Beyond the technology evolution, changing business requirements demand system changes as well.

Key Security Issues

Computer security has many enemies, including complexity, flexibility, and people. Complex software has bugs, which can be exploited by an adversary. Very flexible software is difficult to configure correctly, which leads to errors. An excessive number of users leads to diffusion of the responsibility for security.

Complexity

Software has bugs, and more complex software has more bugs. Sometimes bugs are fairly harmless. When they are not, they may open paths of attack. This kind of bug often occurs when software is given unexpected inputs. Although the software may work when the input is within bounds, it may fail in dangerous ways when an adversary drives it outside its intended regime. For example, a network application may fail to check the length of character strings submitted over the network. An adversary sends a very long string, which overwrites the application’s stack, causing it to execute code sent by the attacker. The adversary now has all the privileges of that application, which may include access to the entire system.

Flexibility

Complex systems are often difficult to configure correctly. Configuration entries may appear innocuous but have great consequences for security. Systems that are changed frequently are particularly vulnerable to such problems, and frequent change is common for Internet systems responding to changing business requirements. For example, suppose that a user asks that a file be made readable to everyone temporarily. When the immediate need is over, no one remembers to change it back, leaving a part of the system unprotected.

People

Generally speaking, the more people who have access to a computer, the less secure it will be. Although most multiuser computers go to some trouble to isolate users from one another, that isolation is effective only if carefully administered.

Security Principles

When constructing applications for networked computers, the following principles are useful to help ensure the security of the system.

• Keep the security system very simple.

Complex applications may be unavoidable, along with their attendant bugs. One way to help protect such a system is to isolate it from some kinds of network access with a firewall. A firewall limits the kinds of network traffic allowed through to the end system. In practice, a firewall system should be simple enough so that the correctness of the firewall implementation and configuration can be easily evaluated. If you have some complex software, protect it from the bad guys with some simple software.

• Limit changes in system configuration.

Every change in the configuration of the system is a potential source of security problems. Obviously, a mistake in configuration may open a hole, but sometimes even a seemingly correct change may cause a problem in combination with other configuration options. In any event, careful records of changes are essential. If you do not understand the configuration of the system, you do not understand its security.

• Consider new versions carefully.

New versions of software may offer attractive new features, but they may also have unknown security problems. You may need some time to learn how to operate them securely. Of course, it is important to track and install patches for security problems as quickly as possible, but other changes should receive cautious deliberation. New software is seductive, but it may be dangerous.

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