OpenReception's appointment booking software provides an end-to-end encrypted appointment booking platform. Prior to version 1.0.5, the "add-to-tunnel" endpoint creates a new appointment row in any client tunnel without any caller authentication. A request that supplies any valid "tunnelId" and any valid "emailHash" (the two need not belong to the same tunnel) results in an inserted appointment with "status = "CONFIRMED"", attacker-controlled ciphertext fields, attacker-controlled date and duration, and an attacker-chosen agent. The endpoint validates only that some tunnel exists with the given "emailHash", then writes the appointment using the attacker-supplied "tunnelId" directly. The "emailHash" lookup is effectively an existence check on the tenant; it does not authenticate the caller as the owner of the supplied "tunnelId". Combined with the absence of any session, Authorization header, booking access token, or PoW, this makes the endpoint accept arbitrary appointment writes into arbitrary tunnels. By contrast, the sibling endpoint "create-new-client" (used to bootstrap a brand-new client tunnel) requires a Bearer bootstrap booking access token issued by the bootstrap-challenge / bootstrap-verify flow. The "add-to-tunnel" endpoint, intended for return-clients booking additional appointments, has no equivalent gate. The application's own middleware confirms this is intentional: "add-to-tunnel" is explicitly listed in the apiAuthHandle public-route allowlist alongside the bootstrap and challenge endpoints (which legitimately have no session). Version 1.0.5 fixes the issue.