At a manufacturer, the plant historian is powered on, yet operators cannot open it during the shift because the service refuses connections. Which CIA pillar failed?

Availability, the plant historian refuses connections so operators cannot open it during the shift is the pillar that failed. Availability means authorized people can use the service when they need it. The historian host is powered on, but the service refuses connections, so operators cannot open it during the shift. A box that has power and a service that will not accept sessions is still a loss of use. Confidentiality, historian data was copied out to a party that should not have received it would mean historian data was copied to a party that should not have it. Nothing in the scenario describes data leaving the historian. The complaint is that operators cannot get in. Integrity, operator setpoints were edited and the stored values can no longer be trusted would mean setpoints were edited and the stored values can no longer be trusted. No edit is described. The service will not take the connection, which is a different failure from a value that was changed. Non-repudiation, operators deny they tried to open the historian during the shift would matter if the question were whether operators could deny that they tried. Their denial does not explain the refused connections, and non-repudiation is not the CIA pillar that covers loss of use. Exam caveat: a host that is powered on can still fail availability when the service people need will not accept a connection. Operational check: from an operator station, attempt the historian session, confirm the refusal, and then check whether that service is running on the host.