The present invention generally relates to detecting malicious network activity coming from network devices such as routers and firewalls. Specifically, embodiments of the present invention provide for detecting stealth malware on a network device by comparing inbound and outbound network traffic to discover packets originating from the network device and packets that violate configuration rules. When combined with a network traffic monitor server configured to monitor actual network traffic reports and to receive known network traffic reports from host computers, the system can detect stealth network traffic originating from both network devices and host computer systems.