nanomq NULL Pointer Dereference in MQTTv5 Client CONNECT Decoder Leading to Remote DoS

In nanomq versions 0.24.11 and earlier, a NULL pointer dereference in nni_mqttv5_msg_decode_connect() allows a malicious MQTT broker to crash any connecting NanoMQ MQTTv5 client (including bridge mode) with a single packet, causing remote denial of service via SIGSEGV. In nni_mqttv5_msg_decode_connect() (mqtt_codec.c:1863), the code iterates over CONNECT properties using variable prop when it should use will_prop. When a CONNECT packet has no connect-level properties (prop == NULL) but has will properties (will_prop != NULL), dereferencing prop->next causes SIGSEGV at address 0x38 (NULL + offsetof(property, next)).

This affects both nanomq_cli and NanoMQ bridge mode (Core component), as both use the same mqtt_client.c receive path. This can lead to remote DoS if a malicious MQTT broker can crash the client process with a single 35-byte packet and persistent DoS if auto-reconnect causes infinite crash loop.