“Companies that rely on usernames and passwords have to develop a sense of urgency about changing this,” said Avivah Litan, a security analyst at Gartner, the research firm. “Until they do, criminals will just keep stockpiling people’s credentials.”

Websites inside Russia had been hacked, too, and Mr. Holden said he saw no connection between the hackers and the Russian government. He said he planned to alert law enforcement after making the research public, though the Russian government has not historically pursued accused hackers.

So far, the criminals have not sold many of the records online. Instead, they appear to be using the stolen information to send spam on social networks like Twitter at the behest of other groups, collecting fees for their work.