1. A method of restricting a client processing system being compromised by a threat, wherein the method comprises: receiving, in a proxy server, response data from a remote processing system, according to a request from the client processing system to download data from the remote processing system; using, in one or more emulated operating systems of the proxy server, the downloaded data; monitoring behaviour of the use of the data in the one or more emulated operating systems; and in response to detecting malicious behaviour indicative of a threat, restricting the client processing system being compromised with the threat of the response data.