Sjur Usken

Views on new technologies and business opportunities from Sjur Usken

Monthly Archives: September 2008

VoIP attacks are escalating

There has been numerous VoIP attacks from very different sources the latest months. In this article we will go through attacks towards two different companies in Norway. I will explain how they did it and how you can protect yourself against it.

Who was the attackers?

The attackers IP adresses:


The first two IP adresses belongs to “Piradius” network in Malaysia.

The second two IP addresses belongs to a VoIP company in Bulgaria, These people has not been successful with their actions, but they had about 1000 SIP INVITES while trying to get through.

There was also some port scannings on port 5060 from an American IP, so we contacted the firm and it was most likely a break-in on the local servers.

The attackers business models

There seems to have been to way to make money. One way was to directly use it as an outbound gateway. This is quite risky since you have an existing business to protect.

The other way was to sell these minutes to another provider. One company specializing in discovering and making the gateway ready, then selling this access to prepaid phone card providers.

There are several others, like calling expensive numbers in other countries and then charging the terminating fees.

How did they find the VoIP gateways

The Piradius network (or the people hiring place in this network) did their port scanning from First they search just for open ports, port 5060 (UDP and TCP!) and 1720 (h323). If you have a Cisco PSTN gateway, remember that the Cisco gateways can do both SIP UDP/TCP and H323. The first machine tried all different numbers similar to this.525551690000.


  • 00525551690000
  • 000=23525551690000
  • 00100525551690000

They tried a lot of different variations of this, and after a while they went over to a brute force way, just counting their way upwards

  • 26100525551690000
  • 26200525551690000
  • 26300525551690000
  • 26400525551690000

The always used caller ID 5199362832664 on all calls. They probably had an Asterisk on the other phone number, noticing when it rang and which gateway that then had been used.

What was configured wrong?

One of the customers with an Asterisk had included the “outbound” context to the SIP provider within the reach for the inbound context. Calls coming in and there were no matching numbers internally was routed out to the SIP provider. This is such a bad idea…

Another customer had a Cisco gateway. Cisco gateways just routes VoIP traffic the same as IP based on the dial-peers. It was configured properly with dial-peers but not with the correct access lists. The Cisco just needs 1 dial-peer configured to bounce traffic like this.

The motives

These two attacks were directed to get free calling. The calls were going to expensive countries like Cuba and Jamaica. There has been no directly breach on the system with username/passwords to gain access and get information. The objectives were to send free telephony traffic through the unsecured PBXs.

How to protect your VoIP equipment

  • Always have a firewall or session border controller (SBC is just a specialized VoIP firewall) between you and the Internet.
  • Limit your access to your VoIP servers from the rest of the world
  • Do not let inbound contexts in Asterisk have access to outbound.
  • Be careful with dial-in features and get a new dial-tone based on a password.
  • Update the software regularly.
  • Use VPN tunnels to protect the VoIP traffic going over the Internet
  • Use SIP TLS and SRTP if possible (we are waiting on hardware manufacturers here as well)
  • Shutdown all services on equipment that is not in use. Example H323 on a Cisco gateway used only for SIP