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.