Zero-day

A zero-day is a software vulnerability that is unknown to the vendor or has no available fix yet, giving defenders zero days of warning before attackers can exploit it.

The term can describe the vulnerability itself or an attack that uses it, often called a zero-day exploit. Because no patch exists, traditional defences that depend on known signatures may not catch it. Zero-days are sometimes used in targeted attacks, but once one becomes public, criminal groups often race to exploit it widely before organizations can apply the fix.

Small businesses are not usually the first target of a rare zero-day, but they are frequently caught in the mass exploitation that follows public disclosure, particularly on internet-facing devices like firewalls, VPNs and file transfer tools. The window between disclosure and patching is when many organizations get hit.

Practical tip: you cannot patch what has no patch, so rely on layered defences. Minimize what is exposed to the internet, use behaviour-based endpoint detection, segment your network and keep reliable offline backups. When an emergency patch or vendor mitigation is released, make sure your IT provider applies it quickly.