What is a vulnerability in IT security?

A vulnerability is a weakness that an attacker can use to do something they should not be able to do: run their own code, read data, or get past a login. It can be a bug in software, a default password nobody changed, a misconfigured service or an old protocol left switched on. Vulnerabilities in popular products get public identifiers and severity scores, so you can check whether you are affected. Not every vulnerability matters equally: one in an internet-facing VPN gateway is far more urgent than one in a program nobody can reach. Knowing what you run, and what is exposed, is the first step.

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