The vulnerability (CVE-2026-48075) in OpenReception's appointment booking software arises from the `add-to-tunnel` endpoint lacking proper authorization checks. Before version 1.0.5, this endpoint accepts requests with any valid tunnelId and emailHash, which need not correspond to the same tunnel or user, and inserts an appointment with attacker-controlled details and status "CONFIRMED". The endpoint only verifies the existence of a tunnel with the given emailHash but does not authenticate the caller as the owner of the tunnelId, nor does it require any session, authorization header, access token, or proof-of-work. This allows arbitrary appointment creation in arbitrary tunnels. The sibling endpoint `create-new-client` requires a bootstrap booking access token, but `add-to-tunnel` is intentionally exempt from such checks. Version 1.0.5 addresses this issue by adding proper authorization.

An attacker can create arbitrary confirmed appointments in any client tunnel without authentication, potentially disrupting legitimate bookings or injecting malicious data. The integrity of appointment data can be compromised, but confidentiality and availability impacts are not explicitly stated.

Upgrade to OpenReception appointment booking software version 1.0.5 or later, where this authorization issue in the `add-to-tunnel` endpoint is fixed. Until then, restrict access to the endpoint or implement additional authorization controls if possible. Patch status is not explicitly confirmed in the advisory, but version 1.0.5 is stated to fix the issue.