SIP can be susceptible to DoS (denial of service) attacks, especially DDoS (distributed DoS) attacks. A SUBSCRIBE request can generate a healthy stream of notifications as long as there is a dynamic source of presence information available. An attacker can take advantage of this by sending SUBSCRIBE requests for large numbers of users and placing the target URI in the contact-header field, where the presence information notifications are to be sent.
Still, SIP can foil DDoS attacks by disregarding NOTIFY requests that are not acknowledged by the subscriber or and unwanted by the original subscriber. SIP's authentication and authorization schemes for potential subscribers also eliminate DDoS attacks.
DoS attacks can be mounted against PAs to disrupt presence information for multiple subscribers, too, but SIP protects itself against these SYN-attacks by using a four-way handshake with digest authentication. If a shared secret isn't available for a potential subscriber who is not on the local domain, SIP can still verify the source of the request using an "anonymous" user mechanism. SIP also enables server-side messages to clients to "back off" from sending requests using a 503 response code, which avoids a flood of SUBSCRIBE requests as long as the SIP client generating the flood adheres to the protocol standard.
SIP is not only an easy protocol for signaling and presence information, it also can protect itself from security threats. If you're ready to implement a presence-management strategy, SIP can help with applications such as IM, VoIP and videoconferencing, as well as document management and Web portals.
Sean Doherty is a senior technology editor and lawyer based at our Syracuse University Real-World Labs®. A former project manager and IT engineer at Syracuse University, he helped develop centrally supported applications and storage systems. Write to him at [email protected].
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