Zero day: a software bug that allows a hacker to break into your devices and move around undetected. One of the most coveted tools in a spy's arsenal, a zero day has the power to silently spy on your iPhone, dismantle the safety controls at a chemical plant, alter an election, and shut down the electric grid. For decades, under cover of classification levels and non-disclosure agreements, the United States government became the world’s dominant hoarder of zero days. U.S. government agents paid top dollar―first thousands, and later millions of dollars― to hackers willing to sell their lock-picking code and their silence. Then the United States lost control of its hoard and the market. Now those zero days are in the hands of hostile nations and mercenaries who do not care if your vote goes missing, your clean water is contaminated, or if our nuclear plants melt down.