Heating, ventilation, and air conditioning (HVAC) systems can be used as a means to bridge air-gapped networks with the outside world, allowing remote attackers to send commands to malware placed inside a target’s isolated network. […]

from https://www.bleepingcomputer.com/news/security/attackers-can-use-hvac-systems-to-control-malware-on-air-gapped-networks/