User Administration and Authentication 
Before you can work with the database system, you must log on to the database with a database tool or via an interface. You log on with your user name. You enter your password and the database system authenticates your identity using this password.
Hazards
Unauthorized access to the database using another person's user data
A person who is not authorized to access the database learns the user name and password of an authorized person and logs on to the database using these credentials. Particularly at risk are standard users with standard passwords that are generally known.
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Database Administration, Managing Users