Failing badly

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Failing badly and failing well are concepts in systems security and network security describing how a system reacts to failure. The terms have been popularized by Bruce Schneier, a cryptographer and security consultant.^[1]

A system that fails badly is one that fails catastrophically once failure occurs. A single point of failure can thus bring down the whole system. Examples include:

Databases (such as credit card databases) protected only by a password. Once this security is breached, all data can be stolen.
Buildings depending on a single column or truss, whose removal would cause a chain reaction collapse under normal loads.
Security checks which concentrate on establishing identity, not intent (thus allowing, for example, suicide attackers to pass).
Internet access provided by a single service provider. If the provider's network fails, all Internet connectivity is lost.

A system that fails well is one that compartmentalizes or contains failure. Examples include:

Databases that do not allow downloads of all data in one attempt, limiting the amount of compromised data.
Structurally redundant buildings conceived to resist loads beyond those expected under normal circumstances, or resist loads when the structure is damaged.
Concrete structures, which show fractures long prior to breaking under load, thus giving early warning.
Armoured cockpit doors on airplanes, which confine a potential hijacker within the cabin even if they are able to bypass airport security checks.
Internet connectivity provided by more than one vendor or discrete path, known as multihoming.