A flexible hybrid firewall architecture is disclosed. A system implementing such an architecture includes an access control register, a memory having at least a region to which access is controllable by the access control register, the access control register including first field that contains a privilege identifier (ID) and a plurality of additional fields, each additional field containing control bits corresponding to a respective one of a plurality of permission levels, and control circuitry that, in response to receiving a transaction containing a transaction privilege ID, a security indicator, and a privilege indicator, controls access to the region when the transaction privilege ID matches the privilege ID contained in the first field by using the control bits of a field of the additional fields that corresponds to a security level indicated by the security indicator and a privilege level indicated by the privilege indicator of the transaction.