o the well-accredited statement "It is very important to know what you are trying to protect against" [10]. For example, in a confined network you can generally trust a participant to share your data with, while in an open environment this can be only done through the employment of security mechanisms that establish an acceptable level of trustworthiness with other network entities. This is exactly the case for SIP-based VoIP services. PSTN, VoIP's predecessor, relies on a closed network architecture where risks are limited and well controlled [1]. On the other hand, SIP-based VoIP services are offered through an open network architecture, like the Internet, facing a hostile environment and thus having to cope with a variety of existing and new threats. The attackers, in order to accomplish their goals, try to either exploit a well-known Internet inherited vulnerability or some other vulnerability that stems directly from SIP's architecture. Among the most common threats that communication systems have to deal with is that of unauthorized access. In the PSTN, unauthorized access con- cerns only voice data and not signaling. Despite PSTN's simplistic nature, it is very difficult to achieve unauthorized access (to voice data) because it requires physical access to the medium. On the downside, in a SIP-based VoIP service unauthorized access, to both voice data and signaling, is considered relatively simple because: (a) aggressors can easily gain access to the com- munication channel, (b) SIP uses a text-based message structure and thus everything can be effortlessly tracked, and (c) there are restrictions regarding the employment of confidentiality protection mechanisms in SIP that mainly © 2009 by Taylor & Francis Group, LLC SIP Security: Threats, Vulnerabilities and Countermeasures arise from the fact that intermediate nodes must have access to specific SIP message headers in order to correctly process and route messages to their final destination. Clearly this makes eavesdropping attacks an easy task for maliciou