This post assumes you're already familiar with the basics of WebAuthn.

When creating a WebAuthn credential, you can specify whether it should be discoverable using the residentKey option.

const credential = await navigator.credentials.create({
	publicKey: {
		authenticatorSelection: {
			residentKey: "required",
			// ...
		},
		// ...
	},
});

However, the relying party cannot control when or how the credential can be discovered. You may want it to become discoverable only after user verification and hide the account’s existence from snooping users. This can be especially important for security keys, where unlike devices or password managers that usually require initial authentication, physical possession alone is often sufficient to reveal registered credentials. To address this, the CTAP 2.1 specification defines a new credential protection extension, available through the credentialProtectionPolicy extension input. CTAP is the specification that defines how platforms (devices/browsers) and roaming authenticators (security keys) interact.

const credential = await navigator.credentials.create({
	publicKey: {
		extensions: {
			credentialProtectionPolicy: "userVerificationRequired",
			enforceCredentialProtectionPolicy: true,
		},
		// ...
	},
});

If the default value userVerificationOptional is used, the credential can be discovered and used without user verification. If userVerificationOptionalWithCredentialIDList is used, the credential cannot be discovered without user verification, but it can still be discovered and used without user verification if the credential ID is provided by the relying party. This matches the security of non-discoverable credentials. Finally, userVerificationRequired indicates that the credential cannot be discovered or used without user verification.

It’s important to highlight that the extension controls credential discovery within the authenticator. It is still up to the relying party to verify whether the assertion was made with user verification if they require it.

The related enforceCredentialProtectionPolicy extension input configures what should happen if the authenticator does not support credential protection policy. If set to true, the operation will fail if it cannot create a credential at the specified security level. Note that if you use a non-roaming authenticator that does not support credentialProtectionPolicy but the browser does, the request will be rejected. As such, this should only be set to true if you want to allow roaming authenticators.

As for browser support for the extension inputs, Chrome and Firefox support them, while Safari does not and will simply ignore them.

Browsers can also silently apply a default value if the relying party does not specify one, which is specifically the case in Chrome.

If residentKey is preferred or required, Chrome uses userVerificationOptionalWithCredentialIDList. As noted, this helps prevent someone with physical access to the authenticator from seeing which accounts are registered on it.

If residentKey is required and userVerification is preferred, Chrome will use userVerificationRequired instead. The confusing part is that this is not related to credential discovery, but rather serves as a safety measure to enforce user verification. Chrome assumes the credential is likely to be used as a single authentication step, since preferred is commonly used even for passkey authentication. However, because user verification is still optional and it is up to the relying party to check for it, if the server does not properly enforce user verification during authentication, someone could sign in as the user with only physical access to the authenticator.

The specific behavior is documented in the Chromium documentation.