The message broker does not comply with the MQTT 3.1.1 specification during an authentication failure. The broker returns an incorrect error code should there be an authentication failure for the MQTT client.

The broker throws an EOFException with the wrong error code:

Connection lost (32109) - java.io.EOFException
    at org.eclipse.paho.client.mqttv3.internal.CommsReceiver.run(CommsReceiver.java:197)
    at java.base/java.lang.Thread.run(Thread.java:829)
Caused by: java.io.EOFException
    at java.base/java.io.DataInputStream.readByte(DataInputStream.java:272)
    at org.eclipse.paho.client.mqttv3.internal.wire.MqttInputStream.readMqttWireMessage(MqttInputStream.java:92)
    at org.eclipse.paho.client.mqttv3.internal.CommsReceiver.run(CommsReceiver.java:137)
    ... 1 more

Table 3.1 (Connect Return code values) in the MQTT 3.1.1 specification outlines the broker must return the value 4should there be an authentication failure.