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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