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Network Security Controls
The list of security attacks is long, and the news media carry frequent accounts of serious security incidents. From these, you may be ready to conclude that network security is hopeless. Fortunately, that is not the case. Previous chapters have presented several strategies for addressing security concerns, such as encryption for confidentiality and integrity, reference monitors for access control, and overlapping controls for defense in depth. These strategies are also useful in protecting networks. This section presents many excellent defenses available to the network security engineer. Subsequent sections provide detailed explanations for three particularly important controlsfirewalls, intrusion detection systems, and encrypted e-mail.
Security Threat Analysis
Recall the three steps of a security threat analysis in other situations. First, we scrutinize all the parts of a system so that we know what each part does and how it interacts with other parts. Next, we consider possible damage to confidentiality, integrity, and availability. Finally, we hypothesize the kinds of attacks that could cause this damage. We can take the same steps with a network. We begin by looking at the individual parts of a network:
· local nodes connected via
· local communications links to a
· local area network, which also has
· local data storage,
· local processes, and
· local devices.
The local network is also connected to a
· network gateway which gives access via
· network communications links to
· network control resources,
· network routers, and
· network resources, such as databases.
These functional needs are typical for network users. But now we look again at these parts, this time conjuring up the negative effects threat agents can cause. We posit a malicious agentcall him Hectorwho wants to attack networked communications between two users, Andy and Bo. What might Hector do?
· Read communications. The messages sent and received are exposed inside Andy's machine, at all places through the network, and inside Bo's machine. Thus, a confidentiality attack can be mounted from practically any place in the network.
· Modify communications from Andy to Bo. Again, the messages are exposed at all places through the network.
· Forge communications allegedly from Andy to Bo. This action is even easier than modifying a communication because a forgery can be inserted at any place in the network. It need not originate with the ostensible sender, and it does not require that a communication be caught in transit. Since Andy does not deliver his communications personally and since Bo might even never have met Andy, Bo has little basis for judging whether a communication purportedly sent by Andy is authentic.
· Inhibit communications from Andy to Bo. Here again, Hector can achieve this result by invading Andy's machine, Bo's machine, routers between them, or communications links. He can also disrupt communications in general by flooding the network or disrupting any unique path on the network.
· Inhibit all communications passing through a point. If the point resides on a unique path to or from a node, all traffic to or from that node is blocked. If the path is not unique, blocking it shifts traffic to other nodes, perhaps overburdening them.
· Read data at some machine C between Andy and Bo. Hector can impersonate Andy (who is authorized to access data at C). Bo might question a message that seems out of character for Andy, but machine C will nevertheless apply the access controls for Andy. Alternatively, Hector can invade (run a program on) machine C to override access controls. Finally, he can search the network for machines that have weak or improperly administered access controls.
· Modify or destroy data at C. Here again, Hector can impersonate Andy and do anything Andy could do. Similarly, Hector can try to circumvent controls.
We summarize these threats with a list:
· intercepting data in traffic
· accessing programs or data at remote hosts
· modifying programs or data at remote hosts
· modifying data in transit
· inserting communications
· impersonating a user
· inserting a repeat of a previous communication
· blocking selected traffic
· blocking all traffic
· running a program at a remote host
Why are all these attacks possible? Size, anonymity, ignorance, misunderstanding, complexity, dedication, and programming all contribute. But we have help at hand; we look next at specific threats and their countermeasures. Later in this chapter we investigate how these countermeasures fit together into specific tools.
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