  Modern corporations physically separate their sensitive computational
infrastructure from public or other accessible networks in order to prevent
cyber-attacks. However, attackers still manage to infect these networks, either
by means of an insider or by infiltrating the supply chain. Therefore, an
attacker's main challenge is to determine a way to command and control the
compromised hosts that are isolated from an accessible network (e.g., the
Internet).
  In this paper, we propose a new adversarial model that shows how an air
gapped network can receive communications over a covert thermal channel.
Concretely, we show how attackers may use a compromised air-conditioning system
(connected to the internet) to send commands to infected hosts within an
air-gapped network. Since thermal communication protocols are a rather
unexplored domain, we propose a novel line-encoding and protocol suitable for
this type of channel. Moreover, we provide experimental results to demonstrate
the covert channel's feasibility, and to calculate the channel's bandwidth.
Lastly, we offer a forensic analysis and propose various ways this channel can
be detected and prevented.
  We believe that this study details a previously unseen vector of attack that
security experts should be aware of.
