Among the companies targeted in just the last month have been Feedly, a service that aggregates web content; Evernote, the productivity service; and Moz, which makes marketing analytics software. On Thursday, Move, a San Jose, Calif. start-up that provides online real estate services, was added to the list.
More from The New York Times:
U.S. Plans to Step Up Detention and Deportation of Migrants
In Chaos, Iraq's Kurds See a Chance to Gain Ground
Cochran Asking Blacks to Rescue Him in Republican Primary
At Moz, Anthony Skinner, the company's chief technology officer, said in an interview that initially the hacker — or a group of hackers who use the same email address — asked for $200 to stop attacks on their system. When the company refused to pay, Mr. Skinner said, the ransom demand increased to $2,000. Moz has since signed up with services like CloudFlare, a company that helps mitigate DDoS attacks by spreading traffic among systems around the world, but Mr. Skinner said the attacker has found new ways to attack their systems.
"If we move one way, they come after us a different way," Mr. Skinner said. "It's a game of cat-and-mouse at this point."
Mr. Skinner said the attackers (or attacker) have covered their tracks using anonymity software and flooding companies from thousands of different IP addresses in the United States. Those addresses are likely to be bots, or computers infected with malware that allow the hackers to apply them to an attack.
Read MoreCybersecurity firm says large hedge fund attacked
Moz is working with other affected companies, including Feedly, but said that because each company has different infrastructure, and because the attack methods differ, sharing information about the extortion attacks only provides so much help.
"We're all getting hit differently and so banding together can only do so much, beyond commiserating with your friends," Mr. Skinner said.
He said the attacks have been frustrating not just for Moz and its employees, but for the company's customers, who don't understand why their systems are being impacted.