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.