For accelerating the development of sophisticated driving-assist technologies such as automated driving, securing vehicles against cyberattacks is challenging. To promote the development of security-measurement methods, a company's electronic control unit (ECU) places tough restrictions on security analysis. In such circumstances, we need an environment that includes transparent ECUs with high adaptability. Ideally, anyone will be able to apply technology and evaluate that technology with ECUs. Simulating an actual vehicle through hardware is also required for assessing threats of cyberattacks. We need not only to provide an adaptable platform for developing measures for existing cybersecurity but also simulate any function in actual vehicles using white-box ECUs. In addition, to easily demonstrate and evaluate the applied security technique, it is important to make the environment portable. Considering these requirements, we propose the portable automotive security testbed with adaptability (PASTA), and give an example for evaluating it for proof of concept. PASTA has the possibility to contribute to a comprehensive development platform against vehicle cyberattacks.