Zero-Day Vulnerability

A zero-day vulnerability gets its name because defenders have had "zero days" to prepare a fix when it surfaces. Such flaws are highly prized: governments, brokers, and criminal groups pay large sums for working exploits, and they are routinely used by advanced persistent threat actors for stealthy intrusions. Defending against zero-days relies on layered controls — exploit mitigations (ASLR, CFG, sandboxing), behavioural EDR/XDR detection, network segmentation, virtual patching via WAFs/IPS, and rapid coordinated disclosure once researchers find them. Once a vendor publishes a fix, the issue becomes an n-day vulnerability.