zero-day
adjective
ze·ro-day
ˈzir-(ˌ)ō-ˈdā 
ˈzē-(ˌ)rō-
computing
: of, relating to, or being a vulnerability (as in a computer or computer system) that is discovered and exploited (as by cybercriminals) before it is known to or addressed by the maker or vendor
Thus, in the parlance of the trade, these vulnerabilities are known as "zero-day exploits," because it has been zero days since they have been uncovered and fixed.—
James Bamford
James BamfordThere's a zero-day vulnerability in RealPlayer and Internet Explorer that allows malicious code to be installed …—Mark Hopkins
Mark Hopkins… most of what's on the black market doesn't have that zero-day freshness. Criminals typically target older vulnerabilities, for which fixes have already been released …—Lev Grossman
Lev GrossmanExamples of zero-day in a Sentence
Recent Examples on the Web
The Microsoft Threat Intelligence Center and Microsoft Security Response Center previously found Play ransomware being deployed after threat actors used a zero-day security vulnerability in the Windows Common Log File System.
—Davey Winder, Forbes.com, 7 June 2025
Read more By Matthew Gault In the near future one hacker may be able to unleash 20 zero-day attacks on different systems across the world all at once.
—Wired News, 4 June 2025
The attack could also leak private code and confidential issue data, such as zero-day vulnerability details.
—Dan Goodin, ArsTechnica, 23 May 2025
The breach didn’t involve an exotic zero-day exploit.
—Faizan Mustafa, Forbes.com, 23 May 2025
See All Example Sentences for zero-day
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Davey Winder, Forbes.com, 7 June 2025
Read more By Matthew Gault In the near future one hacker may be able to unleash 20 zero-day attacks on different systems across the world all at once.
—Wired News, 4 June 2025
The attack could also leak private code and confidential issue data, such as zero-day vulnerability details.
—Dan Goodin, ArsTechnica, 23 May 2025
The breach didn’t involve an exotic zero-day exploit.
—Faizan Mustafa, Forbes.com, 23 May 2025
See All Example Sentences for zero-day
Cite this Entry
“Zero-day.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/zero-day. Accessed 12 Jun. 2025.
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