Blocking attacks on SIP VoIP proxies caused by external processing

Abstract As Voice over IP (VoIP) applications become increasingly popular, they are more and more facing se-curity challenges that have not been present in the tradi-tional Public Switched Telephone Network (PSTN). One of the reasons is that VoIP applications rely heavily on ex-ternal Internet-based infrastructures (e.g., DNS server, web server), so that vulnerabilities of these external infrastruc-tures 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 ex-ternal 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 experi-ments prove the feasibility of blocking attacks. Finally, we also discuss several defending methods and present an im-proved protection mechanism against blocking attacks