A smurf attack is a denial-of-service exploit that spoofs ICMP ping messages to flood a system. This "slowing down" of the network can consist of hundreds of PCs sending an echo reply to each packet, and this can shut the network down completely.
At one time smurf attacks were a serious vulnerability in networks, as they affected not only the target network, but upstream connections and other connected networks suffered negative effects. Nowadays a smurf attack is not common because it's very easy to secure a network against it by turning off the forwarding of directed broadcasts on all router ports.
This was first published in September 2007