Definition
The name means zero days of a public fix. A flaw in an operating system, a browser, a messaging app, or a library is a zero-day while attackers can use it and the vendor has not shipped a patch. Disclosure can later give that flaw a CVE. The ID names it. The patch is what closes it. Until a device installs that patch, the flaw still works on the current version.
Relevance
A Unity editor version, a package, or a native plugin can carry a zero-day the same way a system library can. An update closes flaws the vendor has already fixed. A zero-day is the window where the version you ship is still the one the attack works on.
Example
A native plugin in your Android build parses a file the game downloads. The parser has a flaw, and no plugin update exists yet. A crafted file runs code inside the game process. When the plugin author ships a fixed build, you upgrade, rebuild, and that file no longer works.
Connected
Synonyms
0-day