The vulnerability arises because the booking access token issued during the new-client booking flow does not bind to the channelId, and the createNewClientWithAppointment function only verifies that the channel is not archived, omitting the isPublic check that protects other appointment creation paths. An attacker can complete the proof-of-work challenge to obtain a valid token and then submit a create-new-client request with a private channelId, resulting in confirmed or new appointments in private channels. The private channel IDs are disclosed by an unauthenticated schedule endpoint, enabling attackers to target private channels despite the UI filtering them out. This combination fully exposes private channels to unauthorized appointment creation.

The vulnerability allows unauthorized users to create appointments in private channels, potentially disrupting private scheduling and causing unauthorized data or service access. The impact includes integrity loss (unauthorized appointment creation) and availability impact (potential disruption of appointment schedules). Confidentiality is not directly affected. The CVSS score is 6.5 (medium severity) with network attack vector, low attack complexity, no privileges required, no user interaction, unchanged scope, no confidentiality impact, low integrity impact, and low availability impact.

As of the publication date, no official patch or fix is available. Users should monitor the vendor advisory for updates. Mitigation may require restricting access to the schedule endpoint to prevent enumeration of private channel IDs and implementing proper authorization checks binding tokens to channelId and verifying channel.isPublic in the new-client booking flow. Until a fix is released, consider limiting exposure of the affected endpoints and channels where possible.