Two million logins and passwords from services such as Facebook, Google and Twitter have been found on a Netherlands-based server, part of a large botnet using controller software nicknamed 'Pony'.
Another company whose users' login credentials showed up on the server was ADP, which specialises in payroll and human resources software, said Daniel Chechik, a security researcher with Trustwave's SpiderLabs.
It is expected that cybercriminals will go after main online services, but "payroll services accounts could actually have direct financial repercussions," he said.
ADP moved $1.4 trillion in fiscal 2013 within the US, paying one in six workers in the country, according to its website.
Facebook had the most stolen credentials, at 318,121, followed by Yahoo at 59,549 and Google at 54,437. Other companies whose login credentials showed up on the command-and-control server included LinkedIn and two Russian social networking services, VKontakte and Odnoklassniki. The botnet also stole thousands of FTP, remote desktop and secure shell account details.
It wasn't clear what kind of malware infected victims' computers and sent the information to the command-and-control server.
Trustwave found the credentials after gaining access to an administrator control panel for the botnet. The source code for the control panel software, called 'Pony', was leaked at some point, Chechik said.
The server storing the credentials received the information from a single IP address in the Netherlands, which suggests the attackers are using a gateway or reverse proxy in between infected computers and the command-and-control server, he said.
"This technique of using a reverse proxy is commonly used by attackers in order to prevent the command-and-control server from being discovered and shut down - outgoing traffic from an infected machine only shows a connection to the proxy server, which is easily replaceable in case it is taken down," Chechik said.
Information on the server indicated the captured login credentials may have come from as many as 102 countries, "indicating that the attack is fairly global," he said.