Blocking attacks on SIP VoIP proxies caused by external processing

As Voice over IP (VoIP) applications become increasingly popular, they are more and more facing security challenges that have not been present in the traditional Public Switched Telephone Network (PSTN). One of the reasons is that VoIP applications rely heavily on external Internet-based infrastructures (e.g., DNS server, web server), so that vulnerabilities of these external infrastructures have an impact on the security of VoIP systems as well. This article presents a Denial of Service (DoS) attack on VoIP systems by exploiting long response times of external infrastructures. This attack can lead the whole VoIP system in a blocked state thus reducing the availability of its provided signalling services. The results of our experiments prove the feasibility of blocking attacks. Finally, we also discuss several defending methods and present an improved protection mechanism against blocking attacks.