Zero-day

A vulnerability attackers know about or exploit before the vendor has a fix, so defenders have had zero days to patch.

Once a fix is published the bug is no longer a zero-day, but the race starts: attackers compare the patch with the old version and build exploits, sometimes within hours. That is why time to patch matters more than the label. Until a patch exists, mitigations are configuration changes, firewall rules or switching the feature off.

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Related terms

CVE (Common Vulnerabilities and Exposures): A public catalog that gives each known security vulnerability one ID, such as CVE-2025-55182, so everyone talks about the same bug.
WAF (web application firewall): A filter in front of a web app that blocks malicious HTTP requests before they reach your code.
Dependency updates (Dependabot, Renovate): Keeping libraries and frameworks current, usually with bots that open pull requests for new versions.

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