A system and method is described for communicating requests from a Web mashup being executed within the context of a Web browser to third-party Web services in a manner that does not violate the Same Origin Policy (SOP) observed by many Web browsers. In one embodiment, a workflow engine operating in the context of a Web browser executes a plurality of executable components comprising a Web mashup. The workflow engine transparently redirects Web service requests generated by the executing components to a proxy server that acts as an intermediate between the workflow engine and third-party Web services. The proxy server, which is not subject to SOP, transmits the Web service requests to the third-party Web service and obtains requested data therefrom. The proxy server further returns the requested data in a manner that is useable by the Web mashup components.