English

Alternative forms

• zero day
• 0-day

Adjective

zero-day ‎(not comparable)

1. (computing, idiomatic) (of vulnerability) newly discovered, and therefore still not fixed and possibly exploited by hackers or other criminals
2. (computing, idiomatic) (of exploit, its threat, or attack) benefiting from newly found and yet unpatched or unmitigated flaw in software or hardware; using zero-day vulnerability
   • 2003, John Viega, Matt Messier, Secure Programming Cookbook for C and C++

     If your software is popular and has a high demand, you will want to defend against the "zero-day" cracker.

   • 2014, Michael Gregg, CASP CompTIA Advanced Security Practitioner Study Guide: Exam CAS-002

     Before discussing ways to counter zero-day attacks, let's begin with the definition of what a zero-day attack is. A zero-day attack is one that the vendor does not yet know about or hasn't been fixed.

   • 2005, Valdes et al, Recent Advances in Intrusion Detection

     Automatically creating reliable signatures of zero-day exploits is the focus of intense research efforts.

3. Used other than as an idiom: see zero,‎ day.

   Average residues of 1.44 and 2.18 mg/kg chlortetracycline were seen in liver and kidney, respectively, at zero-day withdrawal.

Noun

zero-day ‎(plural zero-days)

1. (computing, idiomatic) vulnerability that has been discovered recently, and is yet unpatched or unmitigated; zero-day vulnerability

   New Internet Explorer zero-day exploited in Hong Kong attacks

   These days, however, more zero days are being used and discovered.

   All the four zero-days originally were reported to Microsoft, affecting Internet Explorer on the desktop.

References

• Hacker Lexicon: What Is a Zero Day? (on Wired.com)
• 0-day (zero-day) (on About.com)
• What is a Zero-Day Vulnerability? (on PCTools.com)