A method, a device, and a system for alerting against unknown malicious codes includes judging whether any suspicious code exists in the packet, recording a source path of the suspicious code and sending alert information that carries the source path to a monitoring device. The embodiments of the present disclosure report the source paths of suspicious codes proactively at the earliest possible time, which lays a foundation for shortening the time required for overcoming virus threats, and avoids the trouble of installing software on the terminal.