Certain embodiments of this disclosure describe techniques for detecting a spoofed network device and preventing the serving of content, such as advertisements, to the spoofed network device. In certain embodiments, a network security system is provided. The network security system can include hardware and/or software programmed to prevent the provision of content to a spoofed client device. The network security system can provide a mechanism for certifying to content providers, such as advertisers, whether or not a client is a legitimate mobile device or a spoofed device. Accordingly, content providers can prevent the delivery of content to fraudulent devices instead of relying on imprecise solutions that detect fraudulent activity after it has occurred.