One who hijacks.

ETYM Latin pirata, Greek peiran to attempt, undertake.
1. A ship manned by pirates; SYN. pirate ship.
2. Someone who robs at sea or plunders the land from the sea without having a commission from any sovereign nation; SYN. buccaneer, sea robber, sea rover.

A person who overcomes the security measures of a computer system and gains unauthorized access. The goal of some crackers is to obtain information illegally from a computer system or use computer resources. However, the goal of the majority is only to break into the system. See also hacker (definition 2).