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Wikipedia

A zero-day (also known as a 0-day) is a vulnerability or security hole in a computer system unknown to its owners, developers or anyone capable of mitigating it. Until the vulnerability is remedied, threat actors can exploit it in a zero-day exploit, or zero-day attack.The term "zero-day" originally referred to the number of days since a new piece of software was released to the public, so "zero-day software" was obtained by hacking into a developer's computer before release. Eventually the term was applied to the vulnerabilities that allowed this hacking, and to the number of days that the vendor has had to fix them. Vendors who discover the vulnerability may create patches or advise workarounds to mitigate it. Still users need to deploy the mitigation to eliminate the vulnerability in their systems. Zero-day attacks are severe threats.