IP Spoofing
Attack described in a 1985 paper by Morris.
First known use against Tsutomu Shimomura --but it's hard to detect.
Attacks are escalating; the problem did not vanish with Mitnick's arrest.
At least two different implementations in use.
Cryptographic authentication is a strong defense, but is rarely used.
A simpler defense has been developed, but it is not yet widely deployed.