Re: [webauthn] Add onlyCreate to prevent creation of a new key for existing user (#2313)

> * Call `get` with constant user ID and immediate mediation set.

This won't work, because [immediate mediation](https://github.com/w3c/webauthn/wiki/Explainer:-WebAuthn-immediate-mediation) does not allow to set `allowCredentials` (the closest equivalent of a "user ID" parameter) since that would enable malicious RPs to silently probe for credential existence and thus de-anonymize users without consent.

I don't think this is functionally different from #1568, because the hard part is how the client should decide whether or not a credential exists. The only difference between this (assuming it would only reveal credential existence after the user confirms consent) and #1568, then, is whether the client fails out or fails over to `get` when it decides that a credential does exist. This doesn't really solve any of the issues with #1568.

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Received on Friday, 11 July 2025 11:33:07 UTC