PLAINFIELD, N.J. (CBSNewYork) – Authorities are warning about the growing scam known as “ransomware.”

That’s when hackers disable personal computers – even entire networks – and then demand a ransom to give users their files back.

Hackers have even demanded ransoms from municipalities. The town of Plainfield had its computer systems taken hostage.

“The hijacker has requested or demanded a ransom,” Mayor Adrian O. Mapp told CBS2’s Demarco Morgan.

Mapp said the computer system was hijacked when an employee clicked on an infected link.

City officials scrambled to pull the computer servers offline, but three were compromised, leaving emails and other city files inaccessible, Morgan reported.

“We have about 10 years of documents that we are not able to access,” Mapp said.

The hijackers holding the files demanded roughly €650 paid in Bitcoin.

The “ransomware” often gets into a computer system by something as simple as a click.

“They prey on a user’s willingness to click on the latest viral videos,” said Ryan Naraine of Kapersky Lab. “They prey on people’s willingness to click on Facebook links. They’re even sending spam through email and in addition to using Twitter.”

Once a computer is infected, it encrypts all files or locks the user out until they pay for the key.

“Everyone’s computer, it’s full of pictures. In many cases, family photos,” Naraine explained. “The ransomware is communicating with a server. The server is sending instructions to start encrypting all of these files.”

In just minutes, the computer is compromised.

“If I try to look at all my photos from my last vacation, it’s nothing, it’s garbage,” Naraine said. “Imagine an average business, this happening in the background. Not only this computer, but encrypting every computer within the network at the same time.”

The ransom demands are often relatively small – hundreds to a few thousand dollars, Morgan reported. But the loss to an individual or business can be huge.

In 2014, the FBI received over 1,800 complaints about ransomware, an estimated loss of more than $23-million. In 2015, the FBI received over 2,400 complaints, and victims lost over $24-million.

Those are just the cases being reported.

Experts say it’s important to back up your files, and avoid clicking on suspicious links.

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