Crack is a password guessing program for Unix that is designed to locate insecure passwords. It works by scanning the password file, checking for passwords which match a long list of insecure password patterns, such as login names used on the system or words in the dictionary. It can also detect passwords created by a simple modification of a dictionary word, such as reversing it or adding a number on the end.

Crack can work with unusual password encryption algorithms, or with a password database managed by NIS (just ypcat the database to a file and run Crack on the file). It is also possible for the sysadmin to modify Crack's dictionaries or transformation rules. Typically Crack is left running in the background at low priority, but it can also be set up to run only during certain hours, or paused and restarted manually by the sysadmin.