Zero-Day Vulnerability

A zero-day is a security flaw in a piece of software that the people who made it do not know about yet, so there is no fix available. The name comes from the idea that the maker has had zero days to deal with it, which leaves everyone using that software exposed until a repair is ready.

It is like a faulty lock the manufacturer has never heard of: they cannot warn you or send a replacement, and anyone who knows the trick can walk straight in. These flaws are especially dangerous because normal security tools have nothing to recognise the attack by. Once the maker finds out and releases an update, the race is on to install it before attackers take advantage of the now public weakness.