Among other things, this document describes a computer-implemented security method such as for authenticated selection of security countermeasures and for reliable identification of computing devices. The method can include receiving, by a computing system, a request from a computing device for an electronic resource. The computing system can identify a security token received from the device that made the request. Based on the security token, particular security countermeasures can be selected that are to be applied to the electronic resource to be served in response to the request. The countermeasures can be operable to interfere with an ability of malware to interact with the served electronic resource when the served electronic resource is on the computing device. Portions of the electronic resource that are to be executed on the computing device can be re-coded using the selected particular security countermeasures.