SQL> select * from RDB$DATABASE;
Statement failed, SQLSTATE = 28000
no permission for read/select access to TABLE RDB$DATABASE

select *
  from RDB$USER_PRIVILEGES p
 where p.RDB$RELATION_NAME = 'RDB$DATABASE'

grant all on RDB$DATABASE to public;

PUBLIC	SYSDBA	S	0	RDB$DATABASE
PUBLIC	SYSDBA	I	0	RDB$DATABASE
PUBLIC	SYSDBA	U	0	RDB$DATABASE
PUBLIC	SYSDBA	D	0	RDB$DATABASE
PUBLIC	SYSDBA	R	0	RDB$DATABASE

select *
  from RDB$USER_PRIVILEGES p
 where p.RDB$RELATION_NAME starting with 'RDB$'

SQL> select * from RDB$DATABASE;
Statement failed, SQLSTATE = 28000
no permission for read/select access to TABLE RDB$DATABASE

ACL version 1
	person: SYSDBA, privileges: (PCDWR)
	all users: (*.*), privileges: (R)

update RDB$SECURITY_CLASSES
   set RDB$ACL = ( select RDB$ACL
                     from RDB$SECURITY_CLASSES
                    where RDB$SECURITY_CLASS = 'SQL$1' )
 where RDB$SECURITY_CLASS <> 'SQL$1'

SQL> connect TEST user SYSDBA password masterkey;
Database:  TEST, User: SYSDBA
SQL> revoke all on RDB$DATABASE from public;
Warning: Select on RDB$DATABASE is not granted to PUBLIC.
-Warning: Insert on RDB$DATABASE is not granted to PUBLIC.

SQL> connect TEST user TEST password test;
Database:  TEST, User: TEST
SQL> select count(*) from RDB$DATABASE;

       COUNT
============
           1