CVE-2026-48076: OpenReception's bootstrap booking flow allows unauthenticated booking on isPublic=false channels

OpenReception's appointment booking software provides an end-to-end encrypted appointment booking platform. The new-client booking flow in versions 1.0.1 and prior consists of three calls: bootstrap-challenge (returns a 16-bit PoW challenge with difficulty=4 leading hex zeroes), bootstrap-verify (validates the PoW and issues a Bearer booking access token), and create-new-client (consumes the token and creates the tunnel and first appointment). The token correctly binds to tenantId, tunnelId, clientPublicKey, and emailHash, but never to channelId. The bootstrap-challenge request schema does not even accept a channelId, and the issued token's payload contains no channel information. Independently, the service function createNewClientWithAppointment checks only channel.archived = false. The channel.isPublic check that protects addAppointmentToTunnel is missing in the new-client path. The combination means: an attacker completes the bootstrap flow normally (16-bit PoW, completes in well under one second on commodity hardware, no rate limiting beyond the throttle store), receives a valid booking access token, and then submits the create-new-client payload with channelId pointing to a private (isPublic = false) channel. The booking lands as CONFIRMED if the target channel has requiresConfirmation = false (the default), otherwise as NEW. The patient-facing UI does not list private channels in its picker (/api/public/channels correctly filters isPublic = true), so the channel ID must be obtained out of band. The companion finding V-10 (schedule endpoint discloses private channels) provides exactly that: a single unauthenticated GET reveals every private channel ID for any tenant. V-10 plus V-11 together make private channels fully reachable to anonymous attackers. As of time of publication, no known patched versions are available.