A nuclear plant was attacked, and all networks were air gapped. A subsequent investigation revealed a worm as the source of the issue. What is the most likely explanation?

The introduction of a malicious USB by an unsuspecting employee is the most likely explanation for the worm's presence in an air-gapped environment. Air gaps are designed to isolate networks and prevent external access, but they can be compromised through physical media. Malicious USB devices are known methods for delivering malware, particularly in secure facilities like nuclear plants, where direct network connections are restricted. If an employee, perhaps unaware of the USB's malicious nature, plugs it into a system within the air-gapped network, it could lead to the infection of various systems with the worm.

While other options also present potential security risks, they do not align as closely with the conditions described in the scenario. Outdated ICS firmware could pose a risk but would not be the direct cause of how the malware entered the network. A remote access tool (RAT) implies an ongoing connection to an external source, conflicting with the air-gapped status. The connection of HVAC systems to a maintenance vendor may introduce vulnerabilities but doesn't directly explain how the worm infiltrated a secured environment. The context of the attack aligns most successfully with the scenario of an unsuspecting employee unintentionally introducing malware through physical media.