If they have achieved a high level of access, they can take a copy of the file with them and run a password-cracking program on the file. Crackers A cracking program encrypts a long list of character strings, such as all words in a dictionary, and checks it against the encrypted file of passwords. If it finds even one match, the intruder has access to the system. This sort of attack doesn't require a high degree of skill on the part of the hacker. All sorts of password-cracking programs are available on the Internet, many from security Web sites promoting regular password checks by system administrators.
Some systems can defend against cracking programs by keeping the password file under tight security.
The bigger problem is sniffers. The problem becomes worse for data sent over the Internet. Routers are big computers that act as traffic cops, directing the flow of traffic from one crowded room to another. A sniffer installed on a router has the potential to pick off thousands of passwords. And like password-cracking programs, sniffers are everywhere.