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
That same advisory also famously warns users to switch their phones off and on again each week to help protect against phishing attacks and zero-day exploits.
—Zak Doffman, Forbes.com, 16 Sep. 2025
Bitdefender Total Security 2025 offers protection against viruses, worms, Trojans, ransomware, spyware, and zero-day exploits, ensuring that you're covered against any digital threats.
—PC Magazine, 22 Aug. 2025
The zero-day the group used is now being tracked as CVE-2025-8088.
—Dan Goodin, ArsTechnica, 12 Aug. 2025
Hackers are actively exploiting a new zero-day bug in Microsoft's SharePoint Server software.
—Kurt Knutsson, FOXNews.com, 3 Aug. 2025
See All Example Sentences for zero-day
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Zak Doffman, Forbes.com, 16 Sep. 2025
Bitdefender Total Security 2025 offers protection against viruses, worms, Trojans, ransomware, spyware, and zero-day exploits, ensuring that you're covered against any digital threats.
—PC Magazine, 22 Aug. 2025
The zero-day the group used is now being tracked as CVE-2025-8088.
—Dan Goodin, ArsTechnica, 12 Aug. 2025
Hackers are actively exploiting a new zero-day bug in Microsoft's SharePoint Server software.
—Kurt Knutsson, FOXNews.com, 3 Aug. 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 29 Sep. 2025.
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