Page 1303 - (ISC)² CISSP Certified Information Systems Security Professional Official Study Guide

power, and critical systems are plugged into the UPS system. If power
fails, the battery backup will provide continuous power to the systems

for a short period of time. Line-interactive UPS are becoming popular,
and they provide additional services beyond a basic UPS. They include
a variable-voltage transformer that can adjust to the overvoltage and
undervoltage events without draining the battery. When power is lost,
the battery will provide power to the system for a short period of time.

Generators provide power to systems during long-term power outages.

The length of time that a generator will provide power is dependent on
the fuel, and it’s possible for a site to stay on generator power as long
as it has fuel and the generator remains functional. In the lengthy
aftermath of Hurricane Irma in Puerto Rico in 2017, generators were
called on to operate for extended periods and began to fail after weeks
and months of continuous operation.

Generators also require a steady fuel supply—they commonly use

diesel fuel, natural gas, or propane. In addition to making sure that
you have sufficient fuel on hand, you should also take steps to ensure
that you can be delivered fuel on a regular basis in the event of an
extended emergency. Remember, if the disaster is widespread, there
will be significant demand for a limited fuel supply. If you have
contracts in place with suppliers, you’re much more likely to receive
fuel in a timely manner.

Trusted Recovery

Trusted recovery provides assurances that after a failure or crash, the
system is just as secure as it was before the failure or crash occurred.
Depending on the failure, the recovery may be automated or require
manual intervention by an administrator. However, in either case
systems can be designed to ensure that they support trusted recovery.

Systems can be designed so that they fail in a fail-secure state or a fail-
open state. A fail-secure system will default to a secure state in the
event of a failure, blocking all access. A fail-open system will fail in an
open state, granting all access. The choice is dependent on whether
security or availability is more important after a failure.

For example, firewalls provide a significant amount of security by