A zero-day is a security flaw that the vendor has had zero days to fix because it was unknown or unpatched when attackers began using it. An exploit for it is called a zero-day exploit.
Why it matters
Zero-days are dangerous because normal patching cannot help until the vendor ships a fix. They are valuable to criminals and espionage groups and are often used in targeted attacks. Defenders limit damage through layered controls, monitoring, network segmentation and applying patches the moment they appear.
Example
A browser bug is found being used in the wild. The vendor releases an emergency update within days, and the vulnerability then receives a CVE identifier. Users who updated quickly are protected, while those who delayed remain exposed.
Common mistakes
- Thinking zero-days are the main cause of breaches. Most attacks use known, unpatched flaws.
- Delaying updates once a fix is released.
- Believing antivirus signatures can reliably catch unknown exploits.
Related
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