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NOTE:This question is referring to a system which is Logical/Technical, so it is in the context of a system that you must choose the right answer.This is very important to read the question carefully and to identify the context whether it is in the Physical world or in the Technical/Logical world. RFC 2828 (Internet Security Glossary) defines fail safe as a mode of system termination that automatically leaves system processes and components in a secure state when a failure occurs or is detected in the system. A secure state means in the Logical/Technical world that no access would be granted or no packets would be allowed to flow through the system inspecting the packets such as a firewall for example. If the question would have made reference to a building or something specific to the Physical world then the answer would have been different.In the Physical World everything becomes open and full access would be granted.See the valid choices below for the Physical context. Fail-safe in the physical security world is when doors are unlocked automatically in case of emergency.Used in environment where humans work around.As human safety is prime concern during Fire or other hazards. The following were all wrong choices: Fail-secure in the physical security world is when doors are locked automatically in case of emergency.Can be in an area like Cash Locker Room provided there should be alternative manually operated exit door in case of emergency. Fail soft is selective termination of affected non-essential system functions and processes when a failure occurs or is detected in the system. Fail Over is a redundancy mechanism and does not apply to this question. There is a great post within the CCCure Forums on this specific Q: saintrockz who is a long term contributor to the forums did outstanding research and you have the results below.The CCCure forum is a gold mine where thousands of Qs related to the CBK have been discussed. According to the Official ISC2 Study Guide (OIG): Fault Tolerance is defined as built-in capability of a system to provide continued correct execution in the presence of a limited number of hardware or software faults. It means a system can operate in the presence of hardware component failures. A single component failure in a fault-tolerant system will not cause a system interruption because the alternate component will take over the task transparently. As the cost of components continues to drop, and the demand for system availability increases, many non-fault-tolerant systems have redundancy built-in at the subsystem level. As a result, many non-fault-tolerant systems can tolerate hardware faults - consequently, the line between a fault-tolerant system and a non-fault-tolerant system becomes increasingly blurred. According to Common Criteria: Fail Secure - Failure with preservation of secure state, which requires that the TSF (TOE security functions) preserve a secure state in the face of the identified failures. Acc. to The CISSP Prep Guide, Gold Ed.: Fail over - When one system/application fails, operations will automatically switch to the backup system. Fail safe - Pertaining to the automatic protection of programs and/or processing systems to maintain safety when a hardware or software failure is detected in a system. Fail secure - The system preserves a secure state during and after identified failures occur. Fail soft - Pertaining to the selective termination of affected non-essential processing when a hardware or software failure is detected in a system. Acc. to CISSP for Dummies: Fail closed - A control failure that results all accesses blocked. Fail open - A control failure that results in all accesses permitted. Failover - A failure mode where, if a hardware or software failure is detected, the system automatically transfers processing to a hot backup component, such as a clustered server. Fail-safe - A failure mode where, if a hardware or software failure is detected, program execution is terminated, and the system is protected from compromise. Fail-soft (or resilient) - A failure mode where, if a hardware or software failure is detected, certain, noncritical processing is terminated, and the computer or network continues to function in a degraded mode. Fault-tolerant - A system that continues to operate following failure of a computer or network component. It`s good to differentiate this concept in Physical Security as well: Fail-safe -Door defaults to being unlockedDictated by fire codes Fail-secure -Door defaults to being locked Reference(s) used for this question: SHIREY, Robert W., RFC2828: Internet Security Glossary, may 2000.