1. A computer for use in a network of computers which interact by passing messages between them in order to perform a plurality of distributed applications, said computer being arranged in operation to: i) provide two or more local-component-hosting virtual machines, and a shared message-handling virtual machine, each with its own access control mechanism; ii) in said local-component-hosting virtual machine, execute one or more local components of a corresponding distributed application; iii) in said shared message-handling virtual machine, store separately for each local-component-hosting virtual machine, an enforcement configuration file specific to said local component, which file specifies a set of one or more security actions to be carried out on said messages to and/or from said local-component-hosting virtual machine; iv) intercept messages between said local-component-hosting virtual machines and other computers; and v) on intercepting a message, execute, in said shared message-handling virtual machine, a message handling program to perform one or more security actions on said message, as specified in the enforcement configuration file applicable to messages to and/or from the local-component-hosting virtual machine involved in the communication of the intercepted message.