The vulnerability allows an unauthenticated user to trigger the resend‑confirmation API. The email link is constructed from the raw Host header of the request, so an attacker can forge the Host to point to a domain they control. When the invitee clicks the link, the one‑time confirmation token is sent back to the real tenant domain, which the attacker then uses to confirm the account and register a passkey under their control, effectively taking over the pending account before the legitimate user can confirm.

Affected only the open‑reception:appointment‑booking‑software product, all releases up to and including v1.1.0. The vendor released a fix in version 2.0.0; no patches exist for earlier releases. The risk depends on the deployment: if the public front proxy forwards arbitrary Host headers (as the default Caddy catch‑all accepts), the flaw can be exercised; deployments with per‑tenant proxy blocks are not vulnerable.

The CVSS score of 8.3 indicates a high severity. With no EPSS data, the exploitation probability is unclear, but the attack is simple and does not require authentication. Since the flaw is not listed in the KEV catalog, it has not yet been combated by controlled exploitation. Attackers who can reach the endpoint and control the Host header can quickly acquire a confirmation token and take over a pending account.