The SEIS (Sicherheit in eingebetteten IP-basierten Systemen / Security in embedded IP-based Systems) project intends to investigate the viability of using the Internet Protocol (IP) for the whole car communication with special focus on the resulting security issues. Some modern cars contain up to 70 ECUs (Electronic Control Units) that communicate over various dedicated bus systems. While IP promises to provide a homogeneous and cheap interconnection solution, new security challenges may arise -- yet vehicular safety must be maintained. This paper sketches an initial attacker model, discusses some of these security challenges, and starts to explore the possible solution space.