Of the 2,066 organizations with more than US$1 million in annual revenue surveyed, 87 percent reported some type of computer security breach within a year, ranging from internal theft to viruses to Web site defacement, the FBI reported.

IT managers and system administrators reported spyware and viruses were the most common problem, followed by port scans, sabotage of data or networks, and then adult pornography. While not necessarily illegal, adult pornography is against the policy of most organizations, the study noted.

More than 50 percent of hacking attempts came from within the US and from China, as many organizations were able to trace where intrusion attempts originated. But hackers are using computers that are under their control but located in other countries, combined with the use of proxies to make detection more difficult.

The FBI said a Romanian hacker could use a proxy computer in China to gain access to a compromised computer in the US, leading to a false conclusion that the attack originated in the US.

"Difficulty tracking IP (Internet Protocol) addresses and prosecution in China combined with other economic, military and political concerns make this an unusually troubling statistic, especially when considering the potential impact of industrial espionage and state-sponsored cyberwarfare efforts," the survey said.