Flashback was arguably the biggest threat to hit Macland. Hitting the headlines in April, the infection volume landed somewhere around 600,000 machines. And though the bug was not completely unheard of among PC users, the vast majority of victims were running OS X, according to security vendor Kaspersky Lab. Masquerading as a browser plug-in, the bug typically leveraged Java vulnerabilities to steal passwords and other sensitive information. Apple issued various patches over subsequent weeks that appear to have done the trick.