Preventing Emerging Threats Act of 2018 This bill amends the Homeland Security Act of 2002 to authorize the Department of Homeland Security (DHS) and the Department of Justice (DOJ) to authorize their personnel to act to mitigate a credible threat that an unmanned aircraft system or unmanned aircraft (i.e., drones) poses to the safety or security of facilities or assets, through a risk-based assessment. DHS may take actions to: detect, identify, monitor, and track the drone, without prior consent; warn the drone's operator; disrupt control of the drone, without prior consent; seize or exercise control of the drone; confiscate the drone; or use reasonable force, if necessary, to disable, damage, or destroy the drone. Any drone seized by DHS or DOJ is subject to forfeiture to the United States. DHS shall: (1) evaluate the threat from drones to U.S. critical infrastructure and to domestic large hub airports; and (2) assess the threat of vehicular terrorism and brief Congress on its findings and on a strategy to improve efforts to support emergency response providers and the private sector to prevent, mitigate, and respond to such threat.