Important terminology and information for making the most of this section

Important terminology and information for making the most of this section

Before we move on to consider specific issues of network security, I need to introduce some important terms that I shall use when describing how data is stored, processed or transmitted to other locations. These are:

  • Confidentiality, in terms of selecting who or what is allowed access to data and systems. This is achieved through encryption and access control systems. Even knowledge of the existence of data, rather than the information that it contains, may be of significant value to an eavesdropper.

  • The integrity of data, where modification is allowed only by authorised persons or organisations. The modifications could include any changes such as adding to, selectively deleting from, or even changing the status of a set of data.

  • The freshness of data contained in messages. An attacker could capture part or all of a message and re-use it at a later date, passing it off as a new message. Some method of incorporating a freshness indicator (e.g. a time stamp) into messages minimises the risk of this happening.

  • The authentication of the source of information, often in terms of the identity of a person as well as the physical address of an access point to the network such as a workstation.

  • The availability of network services, including security procedures, to authorised people when they are needed.

In general, attacks on data networks can be classified as either passive or active

Figure 2 Forms of attack (Source: based on Stallings, 2001,

Passive attacks

A passive attack is characterised by the interception of messages without modification. There is no change to the network data or systems. The message itself may be read or its occurrence may simply be logged. Identifying the communicating parties and noting the duration and frequency of messages can be of significant value in itself. From this knowledge certain deductions or inferences may be drawn regarding the likely subject matter, the urgency or the implications of messages being sent. This type of activity is termed traffic analysis. Because there may be no evidence that an attack has taken place, prevention is a priority.

Traffic analysis, however, may be a legitimate management activity because of the need to collect data showing usage of services, for instance. Some interception of traffic may also be considered necessary by governments and law enforcement agencies interested in the surveillance of criminal, terrorist and other activities. These agencies may have privileged physical access to sites and computer systems.

Active attacks

An active attack is one in which an unauthorised change of the system is attempted. This could include, for example, the modification of transmitted or stored data, or the creation of new data streams.  four sub-categories here: masquerade or fabrication, message replay, message modification and denial of service or interruption of availability.

Masquerade attacks, as the name suggests, relate to an entity (usually a computer or a person) taking on a false identity in order to acquire or modify information, and in effect achieve an unwarranted privilege status. Masquerade attacks can also incorporate other categories.

Message replay involves the re-use of captured data at a later time than originally intended in order to repeat some action of benefit to the attacker: for example, the capture and replay of an instruction to transfer funds from a bank account into one under the control of an attacker. This could be foiled by confirmation of the freshness of a message.

Message modification could involve modifying a packet header address for the purpose of directing it to an unintended destination or modifying the user data.

Denial-of-service attacks prevent the normal use or management of communication services, and may take the form of either a targeted attack on a particular service or a broad, incapacitating attack. For example, a network may be flooded with messages that cause a degradation of service or possibly a complete collapse if a server shuts down under abnormal loading. Another example is rapid and repeated requests to a web server, which bar legitimate access to others. Denial-of-service attacks are frequently reported for internet-connected services.

Because complete prevention of active attacks is unrealistic, a strategy of detection followed by recovery is more appropriate.

Activity 4

What example of a replayed message could lead to a masquerade attack?

In this unit I shall not deal with the detailed threats arising from computer viruses, but just give a brief explanation of some terms. The word ‘virus’ is used collectively to refer to Trojans and worms, as well as more specifically to mean a particular type of worm.

  • A Trojan is a program that has hidden instructions enabling it to carry out a malicious act such as the capture of passwords. These could then be used in other forms of attack.

  • A worm is a program that can replicate itself and create a level of demand for services that cannot be satisfied.

  • The term virus is also used for a worm that replicates by attaching itself to other programs.


How might you classify a computer virus attack according to the categories in Figure 2 (see Section 3.2)?


An attack may also take the form of a hoax. A hoax may consist of instructions or advice to delete an essential file under the pretence, for instance, of avoiding virus infection. How would you categorise this type of attack?

Threats to network security are not static. They evolve as developments in operating systems, application software and communication protocols create new opportunities for attack.

During your study of this unit it would be a good idea to carry out a web search to find the most common forms of network attack. A suitable phrase containing key words for searching could be:

  • most common network security vulnerabilities

Limit the search to reports within a year. Can you relate any of your findings to the general categories discussed above? What areas of vulnerability predominate? When I searched in early 2003, the most commonly reported network attacks were attributable to weaknesses in software systems (program bugs) and protocol vulnerabilities. Poor discipline in applying passwords rigorously and failure to implement other security provision were also cited. Another particular worry was the new opportunities for attack created by wireless access to fixed networks.