Guest permissions on user databases
Applies to: 
SQL Server
This rule determines whether the guest user has permission to access the database. This rule applies to user databases only.
Best practices recommendations
Revoke the guest user permission to access the database if it isn't required.
The guest user can't be dropped, but guest user can be disabled by revoking its CONNECT permission by executing REVOKE CONNECT FROM GUEST within any database other than master, tempdb, or msdb.