Security company McAfee, which uncovered the intrusions, said it believed there was one "state actor" behind the attacks but declined to name it, though one security expert who has been briefed on the hacking said the evidence points to China.
The long list of victims in the five-year campaign include the governments of the United States, Taiwan, India, South Korea, Vietnam and Canada; the Association of Southeast Asian Nations (ASEAN); the International Olympic Committee (IOC); the World Anti-Doping Agency; and an array of companies, from defense contractors to high-tech enterprises.
In the case of the United Nations, the hackers broke into the computer system of the UN Secretariat in Geneva in 2008, hid there unnoticed for nearly two years, and quietly combed through reams of secret data, according to McAfee.
"Even we were surprised by the enormous diversity of the victim organizations and were taken aback by the audacity of the perpetrators," McAfee's vice president of threat research, Dmitri Alperovitch, wrote in a 14-page report released on Wednesday.
"What is happening to all this data ... is still largely an open question.