Most Internet users have been targeted at some point by hackers using malicious emails, the head of the Department of Homeland Security said Wednesday.

Speaking in the context of the breach at the Democratic National Committee, DHS Secretary Jeh Johnson told listeners that they should be aware of the fact they are being targeted, often by state-backed hackers. "Everybody in this room, I suspect, [through] your employer, has received an email that is an attempt at spearphishing. Simply by not opening the email or the attachment to the email that looks suspicious to you, or you don't recognize, can make a huge difference.

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"The most devastating, intrusive attacks, by the most sophisticated actors, often originate with the simple act of spearphishing," Johnson said, adding that "employee-employer awareness education can raise that bar and make a huge difference."

Phishing is a method by which hackers convince a target to open an attachment or link in an email containing malicious code. One recent example of a target includes Hillary Clinton, whose private server received phishing emails from state-backed hackers in countries such as China, Germany, Russia and South Korea.

Johnson emphasized that he was not confirming how the DNC had been hacked or the identity of the perpetrator behind the breach. "We are not at this point prepared to attribute it to a particular actor or actors. That investigation continues," Johnson said.

He added that the feds were doing a "number of things" requiring a "whole of government approach" to figure out how to counteract future cyberattacks.