1. A method comprising: in a server comprising one or more processors and a memory system adapted to store one or more computer programs for execution by the one or more processors: receiving, at a taint engine of the server, a request from a web application of a client system via a communications link, the request comprising a request header and a request body; generating a standard network transfer protocol response comprising a response header and a response body in response to the request from the client system; evaluating a request taint value in the request header to determine whether the request is a standard network transfer protocol request or a multipart network transfer protocol request; if the request is a multipart network transfer protocol request, constructing a multipart network transfer protocol response based on the generated standard network transfer protocol response; and communicating the multipart network transfer protocol response to the client system, wherein the client system is configured to detect tainted content based on the multipart network transfer protocol response.