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

There's a zero-day vulnerability in RealPlayer and Internet Explorer that allows malicious code to be installed …

—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

Examples of zero-day in a Sentence

Recent Examples on the Web

While Google’s procession of critical security fixes and zero-day warnings makes headlines, the bigger threat to its 3 billion users is hiding undercover. —Zak Doffman, Forbes.com, 19 Aug. 2025

In 2023, a WinRAR zero-day was exploited for more than four months before the attacks were detected. —Dan Goodin, ArsTechnica, 12 Aug. 2025

This roughly simulates what would happen with a zero-day ransomware attack that slipped past the antivirus. —PC Magazine, 5 Aug. 2025

Hackers are actively exploiting a new zero-day bug in Microsoft's SharePoint Server software. —Kurt Knutsson, FOXNews.com, 3 Aug. 2025

Word History

First Known Use

2000, in the meaning defined above

Rhymes for zero-day

• aaa
• adonais
• alleyway
• alumnae
• anime
• anyway
• arete
• attache
• baa
• beaujolais
• bouvier
• breakaway