A verification device operates to evaluate consistency of input values from a client device used by a server to execute a task. In an example the task may be installation of software into the client device, and the consistency evaluation of the input values may involve evaluating whether identifiers used to obtain attribute values of components of the client device for the installation apply to the same client device. To initiate execution of the task, the client device commands a plurality of data source servers to supply the attribute values. The data source servers return response messages defining the attribute values associated with the identifiers supplied by the client device. The client device supplies the response messages to the server for use in the execution of the task. Before executing the task, the server supplies information from the response messages to a verification device, the information defining a combination of the identifiers of the response messages, as the combination of data source servers that have defined attribute values associated with the identifiers. The verification device performing a consistency evaluation on said combination of identifiers, dependent on whether said combination of identifiers is consistent or inconsistent with previously stored combinations of identifiers for said combination of data source servers stored for previously executed tasks and/or whether values of a corresponding further attribute associated with the identifiers in the data source servers in the combination of data source servers are consistent. The server selecting between executing the task or not dependent on a result of the consistency evaluation.