What action should you implement to respond effectively to an insider attack?

Implementing a response to an insider attack requires a focused approach to contain the identified threat while minimizing further risk to the organization. Placing malicious users in a quarantine network is the most effective action as it isolates them from the rest of the network, preventing them from accessing sensitive data or further compromising critical resources.

This containment strategy is vital in responding to insider threats because it allows an organization to investigate the actions of the malicious user without the risk of additional harm. In quarantine, the user can be monitored and assessed for their actions, which aids in forensic analysis and learning how to protect against similar incidents in the future.

In contrast, placing all users in a quarantine network could disrupt normal business operations and is not a targeted approach to address the specific threat. Allowing malicious users access to sensitive information poses significant risks and undermines the organization's security posture. Leaving an insider’s computer open in the network could lead to an escalation of the attack, enabling the insider to continue malicious activities undetected.

Thus, the choice to place malicious users in a quarantine network balances the need to secure the environment while allowing appropriate investigative measures to be taken.