noun

zero-day

ZEER-oh-day

Zero-day (noun) means a flaw in software that the people who make it do not yet know about, so no fix exists at the moment it starts being used against users. Example: “The browser maker shipped an emergency update after a zero-day was found being used against journalists.”

noun

computing A flaw in software that the people who make it do not yet know about, so no fix exists at the moment it starts being used against users.

"The browser maker shipped an emergency update after a zero-day was found being used against journalists."

"Security researchers sat on the zero-day until the vendor had a patch ready."

adjective

computing Describing an attack, exploit or vulnerability of this kind, one that works against software with no fix yet available.

"The intrusion used a zero-day exploit, so up-to-date machines were affected as well."

How to Use Zero-day

Learner’s notes

In plain EnglishA hole in a programme that its makers have not found or fixed yet.

When to use it

Hyphenated as zero-day in both noun and adjective use; sometimes written 0-day in security circles. Standard in technical and news writing.

Memory tip

Keep the hyphen. 'Zero day' without it reads as a day numbered zero, not a security flaw.

Trace the full origin ↓

Easily confused with

exploit → patch → backdoor vulnerability →

Common pairings

zero-day exploit zero-day vulnerability zero-day attack disclose a zero-day zero-day in the wild

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Fill the Gap

Can you complete this real example?

The browser maker shipped an emergency update after a _____ was found being used against journalists.

Etymology

From the count of days the maker of the software has had to fix the flaw, namely zero, because it is already being used against them by the time they learn of it.

Origin: English

Synonyms

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zero-day vulnerability zero-day flaw 0-day unpatched vulnerability unpatched previously unknown

Antonyms

patched vulnerability known vulnerability

Rhymes for zero-day

ay may day say ray hay lay pay jay way yay nay

See all rhymes for zero-day →

Frequently Asked Questions

What is the difference between a zero-day and an exploit?

A zero-day is the flaw itself, a mistake in the software that nobody has fixed yet. An exploit is the piece of code or technique that takes advantage of a flaw. A zero-day exploit is an exploit built for a flaw that has no fix available.

Why is it called zero-day?

The name counts how long the makers of the software have had to respond. Once a flaw is being used in the wild before the vendor knows of it, their head start is zero days.

Does keeping software updated protect against a zero-day?

By definition a zero-day has no update available at the time it is used, so updates cannot address it in that window. Updates matter for the much larger number of flaws that already have fixes, and they close a zero-day once a patch is finally released.