zero-day

noun

cyberattack that utilizes a recently-publicized computer software vulnerability on systems which are yet to be mitigated

Of warez, released less than a day after the official product release.
“Back then, having "zero day warez" (brand new cracked games that are not even one day old yet) on your BBS first was a big status symbol.”
Of a vulnerability, newly discovered, and therefore still not fixed and possibly exploited by hackers or other criminals.
“Although it's had tons of publicity, the IE zero-day vulnerability is so far only a real threat to people running IE6 on Windows XP, and you have to wonder about the competence of the companies actually affected by it[.]”
“Zero-day flaws are a burgeoning commercial field in the underground hacker economy where some gangs work to order for commercial or government clients which want to break into systems.”
Of an exploit or its threat or an attack, benefiting from a newly found and yet unpatched or unmitigated flaw in software or hardware; using a zero-day vulnerability.
“If your software is popular and has a high demand, you will want to defend against the "zero-day" cracker.”
“2005, Valdes et al, Recent Advances in Intrusion Detection Automatically creating reliable signatures of zero-day exploits is the focus of intense research efforts.”
Used other than figuratively or idiomatically: 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.”

A vulnerability that has been discovered recently, and is yet unpatched or unmitigated; a zero-day vulnerability.
“New Internet Explorer zero-day exploited in Hong Kong attacks”
“America’s National Security Agency (NSA) spends upwards of $25m in a year buying previously undisclosed security vulnerabilities – known as zero days, because that’s the length of time the target has had to fix them – but the large investment may not result in as much of a collection of hacking capabilities as is widely assumed.”
A day without obligations, where one does not worry about being productive.
“Whatever you do, do not cut out the “zero days” as suggested in Joel Snape’s article on productivity tips (Big, beautiful goals – but can’t be bothered? 11 great productivity tips for lazy people, 10 July). These are days where you have no obligations, where you do not worry about being productive.”