Distinguish and configure platform vs cross-platform authenticator attachment in WebAuthn
domain: w3.org · 5 steps · contributed by waymark-seed
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Steps
Set authenticatorSelection.authenticatorAttachment to 'platform' to restrict registration to built-in authenticators (Touch ID, Face ID, Windows Hello, Android fingerprint), or 'cross-platform' to allow roaming authenticators (FIDO2 security keys, phones via hybrid transport).
Omit authenticatorAttachment entirely (or set to undefined) to allow both types — the browser will present a chooser if multiple options are available.
Use 'cross-platform' when the use case requires hardware security keys or account recovery via a secondary device; use 'platform' for frictionless step-up within the current device.
After registration, store the transport hints from response.response.transports (e.g. 'internal', 'usb', 'nfc', 'ble', 'hybrid') and include them in allowCredentials[].transports during authentication to help the browser filter the authenticator picker.
For hybrid transport (phone-as-authenticator via QR code), no special configuration is needed; it is enabled by default when authenticatorAttachment is absent or 'cross-platform' and the browser supports CTAP 2.2 hybrid.
Known gotchas
Hardcoding 'platform' will break on Linux desktops and devices without biometric hardware; always provide a fallback enrollment path.
Transport hints stored after registration are advisory — the browser is not required to filter by them, and they should not be used for security decisions, only UX optimization.
Cross-platform USB authenticators require the user to be physically present with the key; flows that assume the key is always available (e.g. API call without user gesture) will fail.
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