Coordinated cyberattacks launched from Europe and China breached computers at firms and government agencies worldwide in the past 18 months, The Wall Street Journal reported.
The Journal quoted computer security firm NetWitness as saying the attacks made mountains of data vulnerable to mining by hackers, although the damage had yet to be fully assessed.
Information bared to hackers ranged from credit card transactions to intellectual property of slightly more than 2,400 victims, including 10 US government agencies, according to the Journal.
"More than 75,000 computer systems at nearly 2,500 companies in the United States and around the world have been hacked in what appears to be one of the largest and most sophisticated attacks by cyber criminals discovered to date," The Washington Post adds.
The hacking operation began in late 2008 in Germany and has yet to be stopped, NetWitness said.
Workers at companies were tricked into visiting websites or opening email attachments that promised to clean viruses from computers but instead infected machines.
Malicious code used in the attacks allowed hackers to seize control of computers remotely.
Evidence cited by NetWitness indicated the culprits may be Eastern European gangsters.
The report came in the wake of Google revealing it was targeted by a sophisticated cyberattack aimed at the US firm's source code and Gmail accounts of Chinese human rights activists around the world.
Computer industry specialists subsequently said more than 30 companies were hit by those attackers.
The apparent online espionage prompted Google to vow it would stop bowing to Chinese censors and shut down its China search service if it cannot operate unfettered.
Google continues to filter searches in accordance with Chinese law while trying to negotiate a compromise with officials there.