"To the best of our knowledge, they mostly focused on stealing credentials, eventually ending up with the largest cache of stolen personal information, totaling over 1.2 billion unique sets of e-mails and passwords," the researchers said.
"The CyberVors did not differentiate between small or large sites. They didn't just target large companies; instead, they targeted every site that their victims visited. With hundreds of thousands sites affected, the list includes many leaders in virtually all industries across the world, as well as a multitude of small or even personal websites."
The researchers dubbed the hacker group CyberVor, using the Russian word "vor," for thief.
The New York Times first reported the breach, and said the group of hackers based their operation in south central Russia, a flanked by Kazakhstan and Mongolia, the report said.
The Times said the group includes fewer than a dozen men in their 20s and that their computer servers are believed to be in Russia.
"There is a division of labor within the gang," Hold Security founder Alex Holden is quoted as saying. "Some are writing the programming, some are stealing the data."