What is Zero-Day?

1 min read Updated 2026-02-05

A vulnerability that is unknown to the software vendor and has no available patch. Zero-day exploits are particularly dangerous because there's no defense against them initially.

Understanding Zero-Day

Zero-day vulnerabilities are unknown to the vendor, meaning there's been "zero days" to fix them. They're highly valuable on black markets and to nation-states. Defense requires layered security, behavior monitoring, and rapid response capabilities when discovered.

Examples

  • Log4Shell was a zero-day before December 2021
  • Nation-state actors hoarding zero-days for cyber operations
  • Zero-day brokers paying millions for iOS/Android exploits

How to Prevent

  • Implement defense in depth (don't rely on any single control)
  • Use behavior-based detection, not just signature-based
  • Keep systems updated to minimize attack surface
  • Monitor for anomalous behavior
  • Have an incident response plan ready
  • Consider bug bounty programs to find vulnerabilities first

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