Security assurance is a discipline aiming to demonstrate that a target system holds some non-functional properties and behaves as expected. These techniques have been recently applied to the cloud, facing some critical issues especially when integrated within existing security processes and executed in a programmatic way. Furthermore, they pose significant costs when hybrid systems, mixing public and private infrastructures, are considered. In this paper, we a present an assurance framework that implements an assurance process evaluating the trustworthiness of hybrid systems. The framework builds on a standard API-based interface supporting full and programmatic access to the functionalities of the framework. The process provides a transparent, non-invasive and automatic solution that does not interfere with the working of the target system. It builds on a Virtual Private Network (VPN)-based solution, to provide a smooth integration with target systems, in particular those mixing public and private clouds and corporate networks. A detailed walkthrough of the process along with a performance evaluation of the framework in a simulated scenario are presented.