The prevention of authorized access to a system resource or the delaying of system operations and functions. This can occur from a number of different attacks vectors including message flooding, resource exhaustion and application crashes. Each of these are described separately.

Denial of Service impacts Availability.

See 5.1.2 for the reconciliation of this threat.

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.

An attacker can send a limited number of messages that obtain a resource on the system. The commands are typically valid, but they each use up a resource resulting in a single Client obtaining all resources blocking valid Clients from accessing the Server. For example, on a Server in which only 10 Sessions are available a malicious person using a legitimate Client, might obtain all 10 Sessions. Or a malicious Client might try to open 10 secure channels, without actually completing the process.

Resource exhaustion attacks do not occur in the same manner for PubSub communications since no session or resources are allocated. For PubSub communication, the Publisher is not susceptible. In broker-less PubSub communication, the Subscriber can, with the use of filters, bypass any resource exhaustion issues. In broker case, both the Publisher and Subscriber must connect to the broker. Although the Publisher and Subscriber are not directly susceptible (as in the broker-less case), the broker is susceptible. The details for broker communication is not part of OPC UA but is defined by the broker protocol.

An attacker can send special message that will cause an application to crash. This is usually the result of a known problem in a stack or application. These system bugs can allow a Client to issue a command that would cause the Server to crash, as an alternate it might be a Server that can respond to a legitimate message with a response that would cause the Client to crash. The attacker could also be a Publisher that issues a Message that would cause Subscribers to crash.