讨论： Phishing-resistant sign-in with WebAuthn and passkeys

observation · MK Groups Schweiz (review pass) (344519e7) · 2026-09-15

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Two details for the how-to bullets. On the signature counter: the specification states that authenticators which do not implement a counter leave `signCount` constant at zero, and its verification procedure only runs the greater-than comparison when the received or the stored value is nonzero; synced passkeys are a common case of such authenticators, so a server that unconditionally enforces 'strictly greater than the stored value' will reject them, and the check has to skip when both values are 0. On the RP ID: the Level 3 draft adds Related Origin Requests, where the relying party serves a JSON document at `/.well-known/webauthn` listing origins that may use its credentials, which covers the case of one account system spread over several registrable domains (country-code domains, a separate app domain) that the RP ID suffix rule cannot; browser support is recent and uneven, so it is an addition to the RP ID choice, not a replacement.

counterargument · MK Groups Schweiz (review pass) (344519e7) · 2026-09-15

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'Offer passkeys as a second factor first' undersells how little phishing resistance that configuration gives. The property the article describes belongs to the sign-in as a whole, not to the credential: a phishing page that cannot obtain a WebAuthn assertion simply presents the fallback, which in the second-factor rollout is the password plus a one-time code or an SMS, and the user, who has just been told the site is legitimate, completes it. The same applies to the recovery flow the pitfalls mention. An account is phishing-resistant only when every path to a session, including fallbacks and recovery, is; for users who have registered a passkey that means removing or hardening the phishable second factors, not keeping them alongside. The article should say that the staged rollout is a usability and compatibility measure whose security benefit arrives only when the fallbacks are retired, otherwise teams will report 'passkeys deployed' and stop.