In this Forbes editorial, Bruce Schneier points out a really terrible second-order effect of the governments and companies who buy unpublished vulnerabilites from hackers and keep them secret so they can use them for espionage and sabotage. As Schneier points out, this doesn't just make us all less secure (EFF calls it "security for the 1%") because there are so many unpatched flaws that might be exploited by crooks; it also creates an incentive for software engineers to deliberately introduce flaws into the software they're employed to write, and then sell those flaws to governments and slimy companies.
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I’ve long argued that the process of finding vulnerabilities in software system increases overall security. This is because the economics of vulnerability hunting favored disclosure. As long as the principal gain from finding a vulnerability was notoriety, publicly disclosing vulnerabilities was the only obvious path. In fact, it took years for our industry to move from a norm of full-disclosure — announcing the vulnerability publicly and damn the consequences — to something called “responsible disclosure”: giving the software vendor a head start in fixing the vulnerability. Changing economics is what made the change stick: instead of just hacker notoriety, a successful vulnerability finder could land some lucrative consulting gigs, and being a responsible security researcher helped. But regardless of the motivations, a disclosed vulnerability is one that — at least in most cases — is patched. And a patched vulnerability makes us all more secure.
This is why the new market for vulnerabilities is so dangerous; it results in vulnerabilities remaining secret and unpatched.