Keycloak 10.0.0 to 18.0.0 contains a cross-site scripting vulnerability via the client-registrations endpoint. On a POST request, the application does not sanitize an unknown attribute name before including it in the error response with a ‘Content-Type’ of text/HTML. Once reflected, the response is interpreted as HTML. This can be performed on any realm present on the Keycloak instance. Since the bug requires Content-Type application/JSON and is submitted via a POST, there is no common path to exploit that has a user impact.