Threats

A threat is a potential violation of security. The violation need not actually occur for there to be a threat. The fact that the violation might occur means that those actions that could cause it to occur must be guarded against (or prepared for). Those actions are called attacks.

Threats can be divided into four broad classes: disclosure, or unauthorized access to information; deception, or acceptance of false data; disruption, or interruption or prevention of correct operation; and usurpation, or unauthorized control of some part of a system. These four broad classes encompass many common threats. Since the threats are ubiquitous, an introductory discussion of each one will present issues that recur throughout the study of computer security.

Snooping, the unauthorized interception of information, is a form of disclosure. It is passive, suggesting simply that some entity is listening to (or reading) communications or browsing through files or system information. Wiretapping, or passive wiretapping, is a form of snooping in which a network is monitored.

Modification or alteration, an unauthorized change of information. Active wiretapping is a form of modification in which data moving across a network is altered; the term " active" distinguishes it from snooping (" passive" wiretapping). An example is the man-in-the-middle attack, in which an intruder reads messages from the sender and sends (possibly modified) versions to the recipient, in hopes that the recipient and sender will not realize the presence of the intermediary.

Masquerading or spoofing, an impersonation of one entity by another. It lures a victim into believing that the entity with which it is communicating is a different entity. This may be a passive attack (in which the user does not attempt to authenticate the recipient, but merely accesses it), but it is usually an active attack (in which the masquerader issues responses to mislead the user about its identity). It is often used to usurp control of a system by an attacker impersonating an authorized manager or controller.

Repudiation of origin, a false denial that an entity sent (or created) something. Suppose a customer sends a letter to a vendor agreeing to pay a large amount of money for a product. The vendor ships the product and then demands payment. The customer denies having ordered the product and by law is therefore entitled to keep the unsolicited shipment without payment. The customer has repudiated the origin of the letter. If the vendor cannot prove that the letter came from the customer, the attack succeeds.

Delay, a temporary inhibition of a service. Typically, delivery of a message or service requires some time t; if an attacker can force the delivery to take more than time t, the attacker has successfully delayed delivery. This requires manipulation of system control structures, such as network components or server components, and hence is a form of usurpation.