Behavioral analytics tools monitor user actions and detect anomalies that may indicate insider threats, such as unauthorized access or unusual data exfiltration activities. These tools establish baselines for normal behavior and flag deviations.

Safety controls are security controls that are designed to protect human life and physical assets from harm or damage. Examples of safety controls include fire alarms, sprinklers, emergency exits, backup generators, and surge protectors. Safety controls should fail open, which means that they should remain operational or allow access when a failure or error occurs. Failing open can prevent or minimize the impact of a disaster, such as a fire, flood, earthquake, or power outage, on human life and physical assets. For example, if a fire alarm fails, it should still trigger the sprinklers and unlock the emergency exits, rather than remain silent and locked. Failing open can also ensure that essential services, such as healthcare, transportation, or communication, are available during a crisis. Remote access points, logging controls, and logical security controls are other types of security controls, but they should not fail open in a data center. Remote access points are security controls that allow users or systems to access a network or a system from a remote location, such as a VPN, a web portal, or a wireless access point. Remote access points should fail closed, which means that they should deny access when a failure or error occurs. Failing closed can prevent unauthorized or malicious access to the data center's network or systems, such as by hackers, malware, or rogue devices. Logging controls are security controls that record and monitor the activities and events that occur on a network or a system, such as user actions, system errors, security incidents, or performance metrics. Logging controls should also fail closed, which means that they should stop or suspend the activities or events when a failure or error occurs. Failing closed can prevent data loss, corruption, or tampering, as well as ensure compliance with regulations and standards. Logical security controls are security controls that use software or code to protect data and systems from unauthorized or malicious access, modification, or destruction, such as encryption, authentication, authorization, or firewall. Logical security controls should also fail closed, which means that they should block or restrict access when a failure or error occurs. Failing closed can prevent data breaches, cyberattacks, or logical flaws, as well as ensure confidentiality, integrity, and availability of data and systems. Reference: CompTIA Security+ Study Guide: Exam SY0-701, 9th Edition, page 142-143, 372-373, 376-377

Air-gapping is the most effective way to protect an application server running unsupported software from network threats. By physically isolating the server from any network connection (no wired or wireless communication), it is protected from external cyber threats. While other options like port security or a screened subnet can provide some level of protection, an air gap offers the highest level of security by preventing any network-based attacks entirely.
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CompTIA Security+ SY0-701 Course Content: Domain 03 Security Architecture.
CompTIA Security+ SY0-601 Study Guide: Chapter on Secure System Design.

Nessus is a vulnerability scanner that can identify insecure ports, legacy protocols, and other security issues on servers. It is designed to detect vulnerabilities and misconfigurations in systems.