The Global Discovery Server (GDS) is a special OPC UA Server that provides Discovery services for a plant or entire system. In addition it can provide certificate management functionality (See OPC 10000-12)

There are multiple methods of accessing a GDS:

  1. Servers can register with the Discovery Server
  2. Clients can query the GDS for available Servers
  3. Clients can pull certificates from the GDS
  4. Servers can pull certificates from the GDS
  5. The GDS can push certificates to a Server
  6. The GDS can access other discovery Servers to build a list of available Servers.

Several types of threats need to be discussed with regard to the available access methods:

  1. Threats where a rogue GDS is in a system.
  2. Threats against the GDS, including the presence of rogue Clients or Servers
  3. Threats against the certificate management functionality provided by a GDS.

The following guidelines are important to remember when dealing with a GDS:

As described in OPC 10000-4, the FindServersOnNetwork Service can be used without security and is therefore vulnerable to denial of service (DOS) attacks. A Discovery Server should minimize the amount of processing required to send the response for this Service. This can be achieved by preparing the result in advance.

The GDS only accept Server registrations from Servers that are trusted or have appropriate administrative access rights. This will help ensure that a rogue Server does not become registered with a GDS.

A GDS, that also provides certificate management, supports User Access security as described in OPC 10000-12. This includes restricting all certificate management functionality to administrators. Furthermore, the list of Clients that are allowed to access management functionality may be limited.

Certificate management includes a provisioning phase and run time phase. The provisioning phase is when the GDS is providing initial certificate(s) to Clients or Servers that are just entering the system. The runtime phase is the day to day operation of system and includes providing updated CRLs, certificate renewals and updated trust lists.

The provisioning of systems is inherently not secure, but can be very useful in providing a greatly simplified deployment of a complex system. Provisioning in a GDS is not enabled by default, but requires an administrative action to enable. It is also recommended that the provisioning feature, when enabled, will only stay enabled for a limited time.

The runtime phase of GDS certificate operations can be performed in a very secure manner, since all Servers and Clients already have certificates to ensure a secure connection. For the push model of certificate management, the GDS establishes a secure channel using the highest security level available in the target Server. It does not provide updated CRLs, Certificates or TrustLists via an endpoint that has a lower security level than the security level of the updates. For example if a 4096 certificate is to be updated it cannot be updated using a 2048 channel, but a 2048 certificate can be updated using a 4096 channel. If a new higher level certificate needs to be deployed, it is handled in the same manner as the provisioning of a new server.