Zero-day vulnerability

A zero-day (also known as a 0-day) is a vulnerability or security hole in a computer system unknown to its developers or anyone capable of mitigating it.^[1] Until the vulnerability is remedied, threat actors can exploit it in a zero-day exploit, or zero-day attack.^[2][3]

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.^[4][5][6] Vendors who discover the vulnerability may create patches or advise workarounds to mitigate it – though users need to deploy that mitigation to eliminate the vulnerability in their systems. Zero-day attacks are severe threats.^[7]