IAM JSON policy elements: Effect - AWS Identity and Access Management
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IAM JSON policy elements: Effect

The Effect element is required and specifies whether the statement results in an allow or an explicit deny. Valid values for Effect are Allow and Deny. The Effect value is case sensitive. 

"Effect":"Allow"

By default, access to resources is denied. To allow access to a resource, you must set the Effect element to Allow. To override an allow (for example, to override an allow that is otherwise in force), you set the Effect element to Deny. For more information, see Policy evaluation logic.