The invention detects changes in one or more parameter values sent by a server through user space. In one embodiment, a Web server communicates with a client over the Internet. Before sending the parameter value or values to the client, the server performs a pre-processing step, creating a formatted data string. The server then transmits the formatted data string to the client in a URL or a cookie. When the client returns the formatted data string and other data to the server, the server performs a post-processing step to verify that the parameter value or values have not been tampered with. This round trip technique is a departure from approaches that merely detect tampering of data as it passes between two nodes of a network.