Building Secure Servers with Linux

Defenses

This is the shortest section in this chapter, not because it isn’t important, but because the rest of the book concerns specific tools and techniques for defending against the attacks we’ve discussed. The whole point of threat analysis is to determine what level of defenses are called for against the various things to which your systems seem vulnerable.

There are three general means of mitigating risk. A risk, as we’ve said, is a particular combination of assets, vulnerabilities, and attackers. Defenses, therefore, can be categorized as means of the following:

• Reducing an asset’s value to attackers

• Mitigating specific vulnerabilities

• Neutralizing or preventing attacks

Asset Devaluation

Reducing an asset’s value may seem like an unlikely goal, but the key is to reduce that asset’s value to attackers, not to its rightful owners and users. The best example of this is encryption: all of the attacks described in the examples earlier in this chapter (against poor ABC Corp.’s besieged email system) would be made largely irrelevant by proper use of email encryption software.

If stolen email is effectively encrypted (i.e., using well-implemented cryptographic software and strong keys and pass phrases), it can’t be read by thieves. If it’s digitally signed (also a function of email encryption software), it can’t be tampered with either, regardless of whether it’s encrypted. (More precisely, it can’t be tampered with without the recipient’s knowledge.) A “physical world” example of asset ...