The all I know about this problem is :

Create new user in security database, this user has to have the full access to database tables as Your application needs.

Then log as this user and create role named SYSDBA, not by create role command, use direct access to RDB$ROLES and INSERT command. Name Your user as an owner of this role.

Now the only one user that can access database is Your user, SYSDBA cannot connect nor using embedeed server.

It's very usefull to use Your own users, than SYSDBA, because You can fully controll the access of these users to database.

Tom