Security researchers say they have uncovered clandestine attacks across three continents on the routers that direct traffic around the Internet, potentially allowing suspected cyberspies to harvest vast amounts of data while going undetected.

In the attacks, a highly sophisticated form of malicious software, dubbed SYNful Knock, has been implanted in routers made by Cisco, the world's top supplier, U.S. security research firm FireEye said on Tuesday.

Routers are attractive to hackers because they operate outside the perimeter of firewalls, anti-virus, behavioral detection software and other security tools that organizations use to safeguard data traffic. Until now, they were considered vulnerable to sustained denial-of-service attacks using barrages of millions of packets of data, but not outright takeover.

"If you own (seize control of) the router, you own the data of all the companies and government organizations that sit behind that router," FireEye Chief Executive Dave DeWalt told Reuters of his company's discovery.

"This is the ultimate spying tool, the ultimate corporate espionage tool, the ultimate cybercrime tool," DeWalt said.

The attacks have hit multiple industries and government agencies, he said.

Cisco confirmed it had alerted customers to the attacks in August and said they were not due to any vulnerability in its own software. Instead, the attackers stole valid network administration credentials from targeted organizations or managed to gain for themselves physical access to the routers.

"We’ve shared guidance on how customers can harden their network, and prevent, detect and remediate this type of attack," Cisco said in a statement.