We wouldn't blame you for worrying about the US government's willingness to remain silent on internet vulnerabilities in the name of national security; no one wants to be left open to a preventable attack. However, the White House sees these disclosures as a complicated issue, and has posted an explanation of its reasoning in an attempt to assuage fears. The administration argues that it has a "disciplined, rigorous and high-level" decision system that balances the risks to the public against the value of any intelligence. Agencies are more likely to share details of security flaws if there's a great potential for damage, or if it's likely that someone will use the exploits. At the same time, officials are more likely to stay hush-hush when there's a high-priority target, or if it's relatively safe to use an exploit for a short while.