Maritime cyber security is emerging as a field as reports of cyber attacks against computerized maritime systems have started arriving. Modern vessels are equipped with computerized systems for navigation employing the Global Positioning System (GPS), known as Integrated Navigation Systems (INS) and Electronic Chart Display and Information Systems (ECDIS). This paper describes a proof-of-concept attack on an INS and its integrated ECDIS, and reports on a demonstration of the attack on a vessel. The attack includes malware that acts as a man-in-the-middle intercepting and manipulating GPS coordinates. Furthermore, the paper discusses the feasibility of the attack, as well as countermeasures.