1. A computer-implemented method, comprising: receiving, at a computer security server system located between the Internet and a client computing device that makes requests over the Internet, a request for content directed to a particular content server system; forwarding the received request, with the computer security server system and over the Internet, to the particular content server system; receiving, over the Internet, code from the particular content server system in response to the request; applying a security countermeasure to the received code to create transformed code, wherein the security countermeasure comprises re-coding portions of the received code that are to be executed on the client computing device, into a second version that is to be executed on the client computing device and is different than a version received from the particular content server system; providing the transformed code to the client computing device over a network; receiving a communication from the client computing device over the network; and determining from the received communication from the client computing device that software on the client computing device has attempted to interact with the received code rather than with the transformed code.