DNS and registration poisoning are clever attacks, which use your SIP server infrastructure to send unauthorized calls. Actually, the attack is against the authorization process rather than authentication. Once a user has a valid account, it can send unauthorized calls to PSTN. Let's start explaining DNS poisoning, which is simpler. It exploits the possibility in a service provider to make calls to foreign domains. We will describe the following attack steps:
Get a valid account.
Make a legitimate call to PSTN and get the gateway's IP in the Contact header.
Change your DNS server to point a valid fully qualified domain name (FQDN) to the gateway's IP.
Initiate a call to the valid FQDN. In many places such as universities, the system allows you to make free calls to external domains, for example, calling to
mit.edufromsip.edu.
Let's suppose that you want to make a call to an external international number such as +4423456789:
Once you have a valid account...