Information about the current users of a database and about their permissions is stored in the database system tables and system views.
Most system tables are owned by the special user ID SYS. It is not possible to connect to the SYS user ID.
The DBA has SELECT access to all system tables, just as to any other tables in the database. The access of other users to some of the tables is limited. For example, only the DBA has access to the SYS.SYSUSERPERM table, which contains all information about the permissions of users of the database, as well as the passwords of each user ID. However, SYS.SYSUSERPERMS is a view containing all information in SYS.SYSUSERPERM except for the password, and by default all users have SELECT access to this view. All permissions and group memberships set up in a new database for SYS, PUBLIC, and DBA can be fully modified.