If you use the P&ID Modeler web services in Revit 2018, you should apply this update. It fixes some vulnerabilities that are exposed when connecting to the P&ID Modeler web services.

You can download the update from your Autodesk Accounts Portal…

Or you can download and install from the Autodesk Desktop App…

For a list of all the issues fixed in the various Revit 2018, check out this link.