The NSA discovering a bug but not publicising it leaves the world no different to if the NSA had never found it, or if there had been no NSA.
That the NSA is much better at finding software vulnerabilities than the open source community?
Plus: You don't know that the NSA knew about it two years ago. You know that a reporter says that two unnamed sources say that the NSA knew about it about two years ago.
http://www.nytimes.com/2014/04/13/us/politics/obama-lets-nsa-exploit-some-internet-flaws-officials-say.html
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Obama is such a great dude.