Hackers have stolen usernames and passwords for nearly two million accounts at Facebook, Google, Twitter, Yahoo and others, according to a report released this week.

The massive data breach was a result of keylogging software maliciously installed on an untold number of computers around the world, researchers at cybersecurity firm Trustwave said. The virus was capturing log-in credentials for key websites over the past month and sending those usernames and passwords to a server controlled by the hackers.

On Nov. 24, Trustwave researchers tracked that server, located in the Netherlands. They discovered compromised credentials for more than 93,000 websites, including:

• 318,000 Facebook accounts
• 70,000 Gmail, Google+ and YouTube accounts
• 60,000 Yahoo accounts
• 22,000 Twitter accounts
• 9,000 Odnoklassniki accounts (a Russian social network)
• 8,000 ADP accounts
• 8,000 LinkedIn accounts

Trustwave notified these companies of the breach. They posted their findings publicly on Tuesday.

Facebook, LinkedIn and Twitter told CNNMoney they have since reset passwords for all of its compromised users. Google declined to comment. Yahoo and ADP did not provide immediate responses, CNNMoney said.

Among the compromised data are 41,000 credentials used to connect to File Transfer Protocol (FTP, the standard network used when working from home) and 6,000 remote log-ins, said CNNMoney.

The hacking campaign started secretly collecting passwords on Oct. 21, and it might be ongoing: Although Trustwave discovered the Netherlands proxy server, Miller said there are several other similar servers they haven't yet tracked down, the report said.

Of all the compromised services, Miller said he is most concerned with ADP. Those log-ins are typically used by payroll personnel who manage workers' paychecks. Any information they can see can be viewed by hackers, CNNMoney reported.

"They might be able to cut checks, modify people's payments," Miller speculated