For Client-Server, an attacker can send a large volume of Messages, or a single Message that contains a large number of requests, with the goal of overwhelming the OPC UA Server or dependent components such as CPU, TCP/IP stack, operating system, or the file system. Flooding attacks can be conducted at multiple layers including OPC UA, HTTP or TCP.

Message flooding attacks can also target a Client, although this is less of a risk, since the Client chooses who to connect to. A Client might receive a flood from a compromised Server which might disrupt the Application.

Message flooding attacks can use both well-formed and malformed Messages. In the first scenario, the attacker could be a malicious person using a legitimate Client to flood the Server with requests. Two cases exist, one in which the Client does not have a Session with the Server and one in which it does. Message flooding may impair the ability to establish OPC UA Sessions or terminate an existing Session. In the second scenario, an attacker could use a malicious Client that floods an OPC UA Server with malformed Messages in order to exhaust the Server’s resources.

For PubSub, an attacker can send a large volume of dataset messages with the goal of overwhelming the subscriber, the middleware or dependent components such as CPU, TCP/IP stack, operating system, or the file system. Flooding attacks can be conducted at multiple layers including OPC UA, UDP, AMQP, MQTT.

As in Client-Server, PubSub message flooding attacks can use both well-formed and malformed Messages. For well-formed Messages, the attacker could be one in which the publisher is not a member of the SecurityGroup and one in which it is a member. For malformed Messages, an attacker could use a malicious Publisher that floods a network with malformed Messages in order to exhaust the system’s resources.

In general, Message flooding may impair the ability to communicate with an OPC UA entity and result in denial of service.