Security
User must have SELECT permission on an external table to read the data. External tables access underlying Azure storage using the database scoped credential defined in data source.
Note: A database scoped credential is a record that contains the authentication information that is required to connect to a resource outside SQL Server. Most credentials include a Windows user and password.
Reference:
https://docs.microsoft.com/en-us/azure/synapse-analytics/sql/develop-tables-external-tables
https://docs.microsoft.com/en-us/sql/t-sql/statements/create-database-scoped-credential-transact-sql