Hundreds of the companies, including defense contractors, had been infected with the same malicious software, or malware, according to the source who was briefed upon this matter.

In past, machines running on Windows OS were prime targets of the hackers, though they have gradually turned their attention to Apple products over the past couple of years as the company gained market share over Microsoft Corp.

"This is the first really big attack on Macs," said the source, who opted to stay anonymous, "Apple has more on its hands than the attack on itself."

An Apple spokesman declined to specify how many companies had been breached in the campaign targeting Macs, saying he could not elaborate further on the statement it provided.

"Apple has identified malware which infected a limited number of Mac systems through a vulnerability in the Java plug-in for browsers. The malware was employed in an attack against Apple and other companies, and was spread through a website for software developers," the statement said.

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