The Goobi viewer is a web application that allows digitised material to be displayed in a web browser. From version 25.08 to before version 26.05, the IIIF Auth 2.0 endpoints auth/login and auth/token in AuthorizationFlowResource accept an origin query parameter without validation and then (1) embed it into the Content-Security-Policy: frame-ancestors response header and (2) embed it into an HTML response body as a JavaScript single-quoted postMessage argument. Because the attacker controls both the login and token calls that initiate the flow, the unescaped value yields reflected XSS. This is