s and criteria for their evaluation, in respect to SPIT. Actually assess- ment and evaluation of the existing anti-SPIT mechanisms is spread in many domains, such as efficiency (i.e., whenever it avoids SPIT false alarms or iden- tifies the actual SPIT successfully), performance (i.e., overhead in the network, number of calls examined per minute, etc.), and finally, qualitative (i.e., scala- bility issues, adoption, availability, etc.). Another important dimension when evaluating existing or even forthcoming anti-SPIT mechanism is related to the legal framework and any anti-SPIT mechanism should be compliant to EC or national legislation. 19.3 SPIT Vulnerability Analysis In the VoIP context, a SPITter might perform various SPIT attacks by first building a set of active SIP URIs which contain callees' SIP address. Moreover, any call (i.e., INVITE) automation method can be useful for SPITters, as well. As a conclusion, the identified SIP threats are separated into two main categories (a) creating a list of valid SIP addresses [1-7], and (b) when SPIT automation is required [8-10]. Sending ambiguous requests to proxies. A proxy server needs to decide the next step destination of a call or a message. The decision is made by looking at a specific part of the header of the request message or by contact- ing a service which is implemented by the SIP registrar server. If the header request/URI part does not offer adequate information then the proxy service is used. The proxy server offers an answer via a 485 SIP message. This kind of response message can contain a contact header field with a list of new URIs available for trial. For example if there are many URIs which contain a specific sequence of characters, then the 485 message would contain all the possible URIs with this string. The worst case arises where a special character can evoke a 485 message with all the registrars of the service's URI database. Therefore, this is an important weakness because a SPITter can easily collect a set of valid URIs. © 2009 by Tayl