Abstract: A network system for completing a transaction session includes a merchant subsystem storing a first data set unrelated to the transaction and a secure subsystem storing second data related to the transaction. The second data includes at least one or more user interface (UI) components and one or more client-side application programs. The network system acts for sending the first data set from the merchant subsystem to a client computing device; sending the second data from the secure subsystem to the client computing device for displaying the one or more UI components on the client computing device; executing the one or more client-side application programs on the client computing device for collecting transaction-related data from the user of the client computing device; and sending collected data to a transaction processing subsystem for completing the transaction session.