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In Chapter 1 the concept of defence-in-depth was discussed, where a defence system has many layers of defence. Unfortunately, as in military systems, it is not always possible to protect using front-line defences, even if there are multiple layers of them, against breaches in security (Figure 2.2). This can be because an intruder has found a weakness within the security barriers, or because the intruder has actually managed to physically locate themselves within the trusted areas. Thus all the gateway firewalls and DMZ’s cannot protect against an intruder once they have managed to base themselves physically or locally within a network. Along with this, most security systems can only guard against known types of attacks, such as in detecting known viruses. A particular problem is when new types of attacks occur, as these are more difficult to defend against. Thus a key factor is identifying threats, and how to mitigate against them. Many organisations are now rehearsing plans on how they cope with these threats, and have contingency plans. Unfortunately many other organisations have no plans for given threats, and these are the ones which are in most danger of a damaging attack. As in military systems, an allied force would setup spies whose task it is to detect intrusions, and any covert activities. Figure 2.3 illustrates this concept, where intrusion detection agents are used to listen to network traffic, and network/user activity to try and detect any breaches in security.
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