OpenReception EUVD-2026-54173

OpenReception's appointment booking software provides an end-to-end encrypted appointment booking platform. Prior to version 1.0.2, the registration handler at POST /api/auth/register/{userId} validates the relationship between the WebAuthn challenge and the registration cookie's email but never validates that the userId in the URL belongs to that email. An unauthenticated attacker requests a challenge for their own email, generates a registration response with their own authenticator, and submits it against any victim user's URL. The challenge-vs-cookie email match passes, the WebAuthn ceremony validates, and addPasskey writes the attacker's credential into the victim's user_passkey rows. The next victim-email login accepts a passkey assertion from the attacker's authenticator and issues a session as the victim. User IDs are not strictly secret on this platform, but the exact set of exposure surfaces should be assessed by the maintainers. Staff-list endpoints return user IDs to authenticated tenant members per the route signature; live verification of all exposure surfaces (whether user IDs leak through any unauthenticated route, through invite-confirmation URLs, or through other administrative views) is part of the pending live PoC. Where the attacker knows the victim's email and userId, the analysis below becomes account takeover. Version 1.0.2 fixes the issue.