The words “zero day” strike fear in military, intelligence and corporate leaders. The term is used by hackers and security specialists to describe a flaw discovered for the first time by a hacker that can be exploited to break into a system.

In recent years, there has been one stunning revelation after the next about how such unknown vulnerabilities were used to break into systems that were assumed to be secure.

One came in 2009, targeting Google, Northrop Grumman, Dow Chemical and hundreds of other firms. Hackers from China took advantage of a flaw in Microsoft’s Internet Explorer browser and used it to penetrate the targeted computer systems. Over several months, the hackers siphoned off oceans of data, including the source code that runs Google’s systems.

Another attack last year took aim at cybersecurity giant RSA, which protects most of the Fortune 500 companies. That vulnerability involved Microsoft Excel, a spreadsheet program. The outcome was the same: A zero-day exploit enabled hackers to secretly infiltrate RSA’s computers and crack the security it sold. The firm had to pay $66 million in the following months to remediate client problems.

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