Zero day: a software bug that allows a hacker to break in and scamper through the world’s computer networks invisibly until discovered. One of the most coveted tools in a spy’s arsenal, a zero day has the power to tap into any iPhone, dismantle safety controls at a chemical plant and shut down the power in an entire nation – just ask the Ukraine. Zero days are the blood diamonds of the security trade, pursued by nation states, defense contractors, cybercriminals, and security defenders alike. In this market, governments aren’t regulators; they are clients – paying huge sums to hackers willing to turn over gaps in the Internet, and stay silent about them. This book is cybersecurity reporter Nicole Perlroth’s discovery, unpacked. An intrepid journalist unravels an opaque, code-driven market from the outside in – encountering spies, hackers, arms dealers, mercenaries and a few unsung heroes along the way.