OpenEMR through 8.2.0 contains a stored cross-site scripting vulnerability in the patient portal template system that allows authenticated administrators to inject arbitrary HTML and JavaScript by storing malicious payloads through the template save mode, which only filters literal PHP open tags. Attackers can exploit the lack of output encoding at the template retrieval endpoint combined with missing HttpOnly cookie attributes to exfiltrate session tokens via document.cookie access, enabling full session hijacking of any admin, clinician, or portal patient who views a poisoned template.