Hackers breaking into businesses and government agencies with targeted attacks have not only stolen intellectual property, in some cases they have corrupted data too, the head of the US Federal Bureau of Investigation said.
The United States has been under assault from these targeted spear-phishing attacks for years, but they received mainstream attention in January, when Google admitted that it had been hit and threatened to pull its business out of China - the presumed source of the attack - as a result.
FBI Director Robert Mueller called these attacks a threat to the nation's security, speaking at the RSA Conference in San Francisco.
"Just one breach is all they need in order to open the floodgates," he said, speaking about the hackers behind these intrusions. "We have seen not only a loss of data, but also a corruption of that data."
Mueller did not say exactly what he meant by corruption of data, but security experts worry that if attackers are able to alter source code, they might put back-doors or logic bombs in the software they gain access to.
"If hackers made subtle, undetected changes to your code, they could have a permanent window into everything you do," Mueller said.
"Some in industry have likened this to death by 1,000 cuts. We are bleeding data, intellectual property, information, source code, bit by bit, and in some cases terabyte by terabyte."
Researchers investigating the Google attack - thought to have affected at least 100 companies including Intel, Adobe and Symantec - say that prime targets of the hackers were the source code management systems used by software developers to build code.
Companies often fail to put basic security controls on these systems, meaning that once an engineer or quality assurance tester's workstation has been hacked, the company's crown jewels are often accessible.
In some cases, hackers moved valuable intellectual property overseas using their victim's wide area networks, and then moved the data from branch offices to outside servers via the Internet, researchers say.
"We are playing the cyber equivalent of cat-and-mouse, and unfortunately the mouse seems to be one step ahead most of the time," Mueller said.