A FIDO2 security key is a hardware authenticator used for passwordless or multi-factor login. It relies on public key cryptography: the device stores a private key and signs a challenge from the service, while the server keeps the matching public key. Because the secret never leaves the key and the response is bound to the exact website, it is resistant to phishing, replay, and credential stuffing.
In cyber security, FIDO2 keys matter because they replace reusable credentials with phishing-resistant authentication. Attackers can trick users into typing passwords or one-time codes into fake pages, but a FIDO2 key will only complete a legitimate sign-in flow for the real domain. Defenders deploy them in enterprise identity systems, often with Conditional Access and authentication-strength policies, to protect privileged accounts and high-risk users. In practice, they are commonly used as a backup or primary method when organizations retire weaker options such as SMS-based sign-in.