Most computer users worry about clicking on suspicious links, visiting unknown websites, and opening strange documents.

But the common thumb drive is probably the biggest security threat.

How did the United States derail Iran's effort to build a nuclear bomb? Thumb drive.

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How did Edward Snowden obtain U.S. government secrets? Thumb drive.

Many people, even IT professionals who should know better, will plug in any old thumb drive they find lying around into their computer.

In survey by AhnLab Inc., a South Korean security firm, 78 percent of IT security professionals admitted to plugging in USB flash drives they found abandoned or lying around.

Over 68 percent had been involved in a security breach — either at home, work or personally.

Many of those breaches could be related to infected USB drives.

"I am utterly shocked at these figures, in particular, the 78 percent number," said Brian Laing, AhnLab's vice president of marketing and business development, in a statement.

IT security professionals are clearly ignoring basic rules, he said. A flash drive could infect networks, and convert a PC to a bot for use by cyber criminals, leading to stolen intellectual property and financial information.

In a Department of Homeland Security test, staffers dropped flash drives in the parking lots of government buildings and private contractors, reported CIOInsight.

Of those who found the drives, 60 percent plugged them into office computers. If the drive had an official logo, the number jumped to 90 percent.

"Even with the knowledge of the potential outcome, curiosity can indeed kill the cat," security expert Brian Laing told CIOInsight. "Policies are useful, but without enforcement, they are not a successful measure."

The government has rules in place that are supposed to stop people from installing thumb drives, but policy statements on paper didn't stop Snowden.

National Security Agency officials "were laying down on their job if they didn't disable the USB port," a security specialist told The Washington Times.

The Pentagon has granted many exceptions, possibly thousands, to allow staff members to use flash drives and other portable devices, a Pentagon spokesman told Reuters. Although the waivers may make IT systems more vulnerable, they are needed for the Pentagon's computer network to operate efficiently, officials said.

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