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Threat actors exploited Windows 0-day for more than a year before Microsoft fixed it
(arstechnica.com)
This is a most excellent place for technology news and articles.
Zero Day just means that you have zero days to fix it before it becomes a problem. Doesn't mean that you actually take zero days to fix it.
What? No it doesn't, it means that the exploit has been known for zero days, aka it's an unknown exploit.
From wiki
My understanding, zero day means when the exploit was discovered it was already being used in the wild. This is different from an exploit discovered by a bounty program or by security researchers.