Via the MIT Technology Review, Tom Simonite writes:
A freshly discovered weakness in a popular piece of software, known in the trade as a “zero-day” vulnerability, can be cashed in for prices in the hundreds of thousands of dollars from defense contractors, security agencies and governments. This trade in zero-day exploits is poorly documented, but it is perhaps the most visible part of a new industry that in the years to come is likely to swallow growing portions of the U.S. national defense budget.
It became clear that this type of assault would define a new era in warfare in 2010, when security researchers discovered a piece of malicious software known as Stuxnet. Now [known] to have been a project of U.S. and Israeli intelligence, Stuxnet was carefully designed to infect multiple systems needed to access and control industrial equipment used in Iran’s nuclear program.
No U.S. government agency has gone on the record as saying that it buys zero-days.