You can query several system views to get detailed information about exactly which privileges and roles users have and how they come to have them. This can help you to understand why a user is or is not authorized to perform particular actions, or access particular data.
You must have the system privilege CATALOG READ to query the following views.
System Tables and Monitoring Views
System tables and monitoring views query information about the system using SQL commands. The results appear as tables in the SYS schema.
The system view M_CONNECTIONS contains additional information about the authentication method:
SELECT USER_NAME, AUTHENTICATION_METHOD FROM M_CONNECTIONS. By default, users can only query information about themselves.
Display Roles Granted to a User
The system view EFFECTIVE_ROLES displays the roles of the currently logged-on user. It shows both the roles that were granted directly to the user, and the roles that were inherited from other roles. This system view complements the system view EFFECTIVE_PRIVILEGES.