Thousands of wind and solar power systems across Europe were exposed to the internet, potentially allowing hackers to gain control of critical energy infrastructure, Dutch researchers said Tuesday.

The researchers identified 8,547 internet-facing systems connected to solar parks and wind farms across 35 European countries, according to Reuters. They believe attackers could have gained full control at roughly 181 sites.

One exposed wind turbine interface displayed its location and live operating data alongside “Start, Stop and Reset” controls. Some of the systems controlled multiple turbines or an entire wind farm.

“What we can map in hours, an attacker can map in hours too,” the researchers warned.

The researchers used machine learning to sort and group internet data and connect the exposed systems to specific energy sites. Of the systems they identified, 7,942 were linked to solar parks and 605 to wind farms.

Most were administrative pages with login screens rather than systems that offered direct operational control.

Spain had the largest number of exposed solar systems identified by the researchers, with 2,766, while Germany had the most exposed wind systems, with 212.

The researchers urged operators to remove administrative interfaces from the public internet immediately, warning that systems tied to cities, airports and other infrastructure could pose a particular risk.

“If you can turn off the energy within the city or the airport, imagine that at a larger scale,” researcher Soufian El Yadmani told Reuters.