The backdoor entry of the Oracle database! You can enter to the database through the back door using SQL * Plus tool with “Prelim” parameter Prelim, directly connects to the SGA but it does not open a session.
You can connect to the database with Prelim as following.
[oracle@maxerman bin]$ sqlplus /nolog
SQL*Plus: Release 22.214.171.124.0 Production on Wed Oct 18 10:42:55 2017
Copyright (c) 1982, 2014, Oracle. All rights reserved.
SQL> set_prelim on
SQL> conn / as sysdba
Prelim connection established