See 4.3.2 for a description of this threat. For discussion purposes denial of service is broken into three major categories message flooding, resource exhaustion and application crashes.
OPC UA minimizes the loss of Availability caused by Message flooding by minimizing the amount of processing done with a Message before the Message is authenticated. This prevents an attacker from leveraging a small amount of effort to cause the legitimate OPC UA Application to spend a large amount of time responding, thus taking away processing resources from legitimate activities.
GetEndpoints (specified in OPC 10000-4) and OpenSecureChannel (specified in OPC 10000-4) are the only services that the Server handles before the Client is authenticated. The response to GetEndpoints is only a set of static information so the Server does not need to do much processing. The response to OpenSecureChannel consumes significant Server resources because of the signature and encryption processing. OPC UA has minimized this processing, but it cannot be eliminated.
First, the Server could intentionally delay its processing of OpenSecureChannel requests once it receives more than some minimum number of bad OpenSecureChannel requests. It should also issue an alarm to alert plant personnel that an attack is underway that could be blocking new legitimate OpenSecureChannel calls.
Second, when an OpenSecureChannel request attempts to exceed the Server’s specified maximum number of concurrent channels the Server replies with an error response without performing the signature and encryption processing. Certified OPC UA Servers are required to specify their maximum number of concurrent channels in their product documentation as specified in OPC 10000-7.
In PubSub, the Subscriber filters messages that it processes based on header information, allowing it to quickly discard any messages that do not conform to its required filter. In addition, the message signature is checked to eliminate any message that is well formed, but not from the desired SecurityGroup. PubSub can also be configured for unicast instead of multicast, which allows the network infrastructure to block multicast flooding attacks.
OPC UA Auditing functionality provides the site with evidence that can help the site discover that flooding attacks are being mounted and find ways to prevent similar future attacks (see 4.14). As a best practice, Audit Events should be monitored for excessive connection requests.
OPC UA user and Client Authentication reduce the risk of a legitimate Client being used to mount a resource exhaustion attack. Additionally, Server Auditing allows the detection of the Client if a resource exhaustion attack was carried out by a legitimate Client. Servers are also required to recycle OpenSecureChannel request that have not been completed (specified in OPC 10000-4), this will eliminate attacks from non-legitimate Clients. Resource exhaustion attacks do not apply to PubSub Systems, since no sessions or resources are allocated.
OPC UA provides certification of OPC UA Applications. The lab testing and certification includes testing by injecting error and junk commands which might discover common faults. OPC Foundation stacks are also fuzz tested to ensure they are resilient to errors. Although a certified OPC UA Application does not guarantee fault free operation, the certified OPC UA Application is more likely to be resilient to application crashes caused by denial of service attacks.