An issue was discovered in Contiki Operating System 3.0. A Persistent XSS vulnerability is present in the MQTT/IBM Cloud Config page (aka mqtt.html) of cc26xx-web-demo. The cc26xx-web-demo features a webserver that runs on a constrained device. That particular page allows a user to remotely configure that device's operation by sending HTTP POST requests. The vulnerability consists of improper input sanitisation of the text fields on the MQTT/IBM Cloud config page, allowing for JavaScript code injection.