If a hacker finds a bug at Microsoft or Google, they could report it to the company, get it patched up, and get a nice reward. OR, they could sell it to a government for WAY more money. That government would keep it secret, and might use it to spy on other countries, or its own people.

New York Times cybersecurity reporter Nicole Perlroth takes us into the scariest marketplaces for Zero Day exploits, and explains why it's big business for young hackers.

Visit https://darknetdiaries.com/episode/98/ for a list of sources, full transcripts, and to listen to all episodes.
...
https://www.youtube.com/watch?v=rCK2lB8ahU8