OpenEMR is a free and open source electronic health records and medical practice management application. Prior to version 8.0.0, in "portal/portal_payment.php", the patient id used for the page is taken from the request ("$pid = $_REQUEST['pid'] ?? $pid" and "$pid = ($_REQUEST['hidden_patient_code'] ?? null) > 0 ? $_REQUEST['hidden_patient_code'] : $pid") instead of being fixed to the authenticated portal user. The portal session already has a valid "$pid" for the logged-in patient. Overwriting it with user-supplied values and using it without authorization allows a portal user to view and interact with another patient's demographics, invoices, and payment history—horizontal privilege escalation and IDOR. Version 8.0.0 contains a fix for the issue.