Public Privileges in Oracle

There is a pseudo-user in oracle called PUBLIC.

Any privileges granted to PUBLIC have, in effect, been granted to every user; every account you create will have access to these privileges.

By default, the public user has a large number of privileges. In particular, he has execute
permission on a number of PL/SQL utility packages.

To know on which table PUBLIC has the select privilege execute the query,

select table_name from dba_tab_privs where grantee='PUBLIC' and privilege='SELECT';

Workaround Example:
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1)Create user and connect with the user and it will fail.

SQL> conn arju/arju
Connected.
SQL> create user t identified by t;
User created.

SQL> conn t/t
ERROR:
ORA-01045: user T lacks CREATE SESSION privilege; logon denied
Warning: You are no longer connected to ORACLE.

2)Grant create session to Public and you will see conn t/t works.

SQL> conn arju/arju
Connected.

SQL> grant create session to public;
Grant succeeded.

SQL> conn t/t;
Connected.

3)Revoke create session from t and then again try to connect as t user.

SQL> conn arju/arju
Connected.

SQL> revoke create session from public;
Revoke succeeded.

SQL> conn t/t;
ERROR:
ORA-01045: user T lacks CREATE SESSION privilege; logon denied
Warning: You are no longer connected to ORACLE.

As the create session privilege was granted to PUBLIC so conn t/t worked and again whenever it revoked conn t/t fails.

You should explicitly grant create session privilege to the user rather giving through PUBLIC. Like,
GRANT CREATE SESSION TO T;