An exploitable arbitrary memory read vulnerability exists in the MQTT packet-parsing functionality of Cesanta Mongoose 6.13. It is a heap-based buffer over-read in mg_mqtt_next_subscribe_topic. A specially crafted MQTT SUBSCRIBE packet can cause an arbitrary out-of-bounds memory read potentially resulting in information disclosure and denial of service. An attacker needs to send a specially crafted MQTT packet over the network to trigger this vulnerability.