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The bulletin stated that the skill level associated with Anonymous attacks to date - like those involving the penetration of Web and email servers of state and local law enforcement - was low. The bulletin said it was on a par with the skill level of “script kiddies” - young, untrained hackers.

Yet hackers with more rudimentary skills can quickly exploit security flaws like those identified by Mr. Beresford. “Once the vulnerabilities make their way into open source, that lowers the [skill] bar down to a ‘script kiddie’ level,” said the Homeland Security official.

Mr. Weiss said the exact level of skill required to hack an ICS system would depend on the setup at the facility and the kind of attack the hackers wanted to carry out.

“If you just want to stop the facility, that’s one thing,” he said. “If you want to destroy the machinery [as Stuxnet did], that’s harder.”