A method, a device, and a system for alerting against unknown malicious codes are disclosed. The method includes: detecting characteristics of a packet; judging whether any suspicious code exists in the packet according to a result of the detection; recording a source address of the suspicious code if the suspicious code exists in the packet; and sending alert information that carries the source address to a monitoring device. The embodiments of the present invention can report source addresses of numerous suspicious codes proactively at the earliest possible time, lay a foundation for shortening the time required for overcoming virus threats, and avoid the trouble of installing software on the client.