This issue involves passkey registration failure when transitioning from localhost to a custom local domain using WebAuthn. The failure is caused by the browser rejecting the self-signed TLS certificate, which is required for secure WebAuthn operations. The user observes that including the port in the relying party ID (rp_id) allows registration, though this is against WebAuthn specifications. The root cause is the untrusted self-signed certificate and possibly incorrect rp_id/origin configuration during development.

The impact is limited to development and testing environments where self-signed certificates are used. It prevents successful passkey registration due to browser security restrictions on TLS certificate trust. There is no indication of a security vulnerability or exploit in production environments. This is a functional issue affecting developer experience rather than a security threat.

Use a valid, trusted TLS certificate for the domain to avoid certificate errors during passkey registration. Avoid including the port number in the relying party ID (rp_id), as this is against WebAuthn standards. For local development, consider using tools that provide trusted local certificates (e.g., mkcert) or configure browsers to trust the self-signed certificate explicitly. No official patch is applicable since this is a configuration and environment issue.