The US government has told thousands of companies to beef up protection of computers which oversee power plants and other utilities.
The action comes after a survey revealed that thousands of these systems can be found online.
The survey was carried out via a publicly available search engine that pinpointed computers controlling critical infrastructure.
In total, the survey uncovered more than 500,000 potential targets.
The survey was carried out by Bob Radvanovsky and Jacob Brodsky of security consultancy InfraCritical who investigated the potential threat to so-called Scada systems.
Scada (Supervisory Control and Data Acquisition) is the industry term for the computers behind the machinery in power plants, water treatment centres, traffic controls and other utilities.
"The biggest thing is we are trying to assign a number - a rough magnitude - to a problem plaguing the industry for some time now," said Mr Radvanovsky in a blogpost,
The pair wrote a series of scripts, small computer programs, that interrogated the Shodan search engine. Shodan was created to log machines connected to the internet in the same way Google logs webpage contents.
In their search scripts the pair used 600 terms compiled from lists of Scada manufacturers and the names and product numbers of the control systems they sell.
Armed with a list of 500,000 potential targets, they approached the US Department of Homeland Security who pared it down to the most important 7,200 targets. The DHS is now in the process of contacting the firms who own these computers to warn them they can be found online.
In many cases, said the pair, convenience had led companies to connect such important systems to the web.
"A lot of these guys want to fix things at 3am without driving three hours in each direction," wrote Mr Brodsky.
Mr Radvanovsky and Mr Brodsky did not test the computers they found to see how well they were protected. However, other researchers have found many weaknesses in the software used to control Scada systems via the net.
While attacks on critical infrastructure are relatively rare, recent months have seen viruses and other malicious programs hit control systems at oil treatment plants and other facilities.