2 cissp ® Official Study Guide Eighth Edition

Control Redundancy and Diversity
As with any security solution, relying on a single security mechanism is unwise. Defense in 
depth uses multiple types of access controls in literal or theoretical concentric circles or lay-
ers. This form of layered security helps an organization avoid a monolithic security stance. 
A monolithic mentality is the belief that a single security mechanism is all that is required 
to provide sufficient security. By having security control redundancy and diversity, a static 
environment can avoid the pitfalls of a single security feature failing; the environment has 
several opportunities to deflect, deny, detect, and deter any threat. Unfortunately, no secu-
rity mechanism is perfect. Each individual security mechanism has a flaw or a workaround 
just waiting to be discovered and abused by a hacker.
Essential Security Protection 
Mechanisms
The need for security mechanisms within an operating system comes down to one simple 
fact: software should not be trusted. Third-party software is inherently untrustworthy, no 
matter who or where it comes from. This is not to say that all software is evil. Instead, this 

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Chapter 9 
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Security Vulnerabilities, Threats, and Countermeasures
is a protection stance—because all third-party software is written by someone other than 
the OS creator, that software might cause problems. Thus, treating all non-OS software as 
potentially damaging allows the OS to prevent many disastrous occurrences through the use 
of software management protection mechanisms. The OS must employ protection mecha-
nisms to keep the computing environment stable and to keep processes isolated from each 
other. Without these efforts, the security of data could never be reliable or even possible.
Computer system designers should adhere to a number of common protection mecha-
nisms when designing secure systems. These principles are specific instances of the more 
general security rules that govern safe computing practices. Designing security into a 
system during the earliest stages of development will help ensure that the overall security 
architecture has the best chance for success and reliability. In the following sections, we’ll 
divide the discussion into two areas: technical mechanisms and policy mechanisms.