A previously unknown vulnerability with no available patch, giving defenders zero days to prepare.
A zero-day (or 0-day) is a vulnerability unknown to the software vendor—there's no patch because the vendor doesn't know it exists. Zero-day exploits targeting these vulnerabilities are particularly dangerous because traditional defences like patching are impossible. The term 'zero-day' refers to having zero days between vulnerability disclosure and exploitation—attackers strike before defenders can respond. Nation-states and sophisticated criminals hoard zero-days for targeted attacks. Defence relies on detection, layered security, and rapid response when zero-days are discovered.
Why It Matters
The DSC Perspective:
Zero-days explain why patching alone isn't sufficient. Defence in depth, behavioural detection, and incident response capabilities provide protection when patches don't exist. Not every organisation faces zero-day attacks, but critical infrastructure should plan for them.