OPC UA supports a robust Alarm and Condition information model which includes the ability to disable alarms, shelve alarms, and to generally manage alarms. Alarm processing and management is an important part of maintaining efficient control of a plant. From a security point of view it is important that this avenue be adequately protected, to ensure that a rogue agent does not create a dangerous or financial situation. OPC UA provides the tools required for this protection, but the implementer needs to ensure that they are exercised correctly. All functions that allow changes to the running environment are able to generate Audit Events and are to be restricted to appropriate users.
The disabling of Alarms is one such function that should be restricted to personnel with appropriate access rights. Furthermore, any action that disables an alarm, whether it be initiated by personnel or some automated system, should generate an Audit Event indicating the action.
The shelving of alarms should follow similar guideline as the disabling of alarms with regard to access and Auditing, although it may be available to a wider range of users (operators, engineers). Also, the implementer should ensure that appropriate timeouts are configured for Alarm Shelving. These timeouts should ensure that an Alarm cannot be shelved for a period of time that could cause safety concerns.
Dialog Events could also be used to overload a Client. It would be a best practice for Servers that support dialogs to restrict the number of concurrent dialogs that could be active. Also, Dialogs should include some timeout period to ensure that they are not used to create a DOS. Client implementers should also ensure that any dialog processing cannot be used to overwhelm an operator. The maximum number of open dialogs should be restricted and dialogs should be able to be ignored (i.e. other processing should still be available).