A network security breach in which a network connected to the Internet is bombarded with replies to PING requests. A smurf attacker sends PING requests to an Internet broadcast address, a special address that broadcasts all received messages to the hosts connected to the subnet. Each broadcast address can support up to 255 hosts, so a single PING request can be multiplied 255 times. The return address of the request itself is spoofed as the address of the victim, so all the hosts receiving the PING request reply to the victim's address rather than the real sender's address.
A single attacker sending hundreds or thousands of these PING messages per second can fill the victim's T1 (or even T3) line with PING replies, bringing the entire Internet service to its knees. Smurfing falls under the general category of Denial of Service (DoS) attacks, which are security attacks that don't try to steal information but attempt to disable a computer or network.