VoIP Security

We can barely scratch the surface of the complex matter of VoIP security in this appendix; therefore, before we dig in, we want to steer you in the direction of the VoIP Security Alliance (http://www.voipsa.org). This fantastic resource contains an excellent mailing list, white papers, howtos, and a general compendium of all matters relating to VoIP security. Just as email has been abused by the selfish and criminal, so too will voice. The fine folks at VoIPSA are doing what they can to ensure that we address these challenges now, before they become an epidemic. In the realm of books on the subject, we recommend the most excellent Hacking Exposed VoIP by David Endler and Mark Collier (McGraw-Hill Osborne Media). If you are responsible for deploying any VoIP system, you need to be aware of this stuff.

Spam over Internet Telephony (SPIT)

We don’t want to think about this, but we know it’s coming. The simple fact is that there are people in this world who lack certain social skills, and that coupled with a kind of mindless greed, means that these folks think nothing of flooding the Internet with massive volumes of email. These same types of characters will think little of doing the same with voice. We already know what it’s like to get inundated with telemarketing calls; try to imagine what might happen when those telemarketers realize they can send voice spam at almost no cost. Regulation has not stopped email spam, and it will probably not stop voice spam, so it will be up to us to prevent it.

Encrypting Audio with Secure RTP

If you can sniff the packets coming out of an Asterisk system, you can extract the audio from the RTP streams. This data can be fed offline to a speech processing system, which can listen for keywords such as “credit card number” or “PIN” and present the data it gathers to someone who has an interest in it. The stream can also be evaluated to see if there are DTMF tones embedded in it, which is dangerous because many services ask for passwords and credit card information to be input via the dialpad. In business, strategic information could also be gleaned from captured audio.

Using Secure RTP can combat this problem by encrypting the RTP streams. More information about SRTP is available in the section called “Encrypting SIP calls”.


In the traditional telephone network, it is very difficult to successfully adopt someone else’s identity. Your activities can (and will) be traced back to you, and the authorities will quickly put an end to the fun. In the world of IP, it is much easier to remain anonymous. As such, it is no stretch to imagine that there are hordes of enterprising criminals out there who will be only too happy to make calls to your credit card company or bank, pretending to be you. If a trusted mechanism is not discovered to combat spoofing, we will quickly learn that we cannot trust VoIP calls.

What Can Be Done?

The first thing to keep in mind when considering security on a VoIP system is that VoIP is based on network protocols, and needs be evaluated from that perspective. This is not to say that traditional telecom security should be ignored, but we need to pay attention to the underlying network.

Basic network security

One of the most effective things that can be done is to secure access to the voice network. The use of firewalls and VLANs are examples of how this can be achieved. By default, the voice network should be accessible only to those things that have a need. For example, if you do not have any softphones in use, do not allow client PCs access to the voice network.

Segregating voice and data traffic

Unless there is a need to have voice and data on the same network, there may be some value in keeping them separate (this can have other benefits as well, such as simplifying QoS configurations). It is not unheard of to build the internal voice network on a totally separate LAN, using existing CAT3 cabling and terminating on inexpensive network switches. This configuration can even be less expensive.


Placing your VoIP system in a demilitarized zone (DMZ) can provide an additional layer of protection for your LAN, while still allowing connectivity for relevant applications. Should your VoIP system be compromised, it will be much more difficult to use it to launch an attack on the rest of your network, since it is not trusted. Regardless of whether you deploy within a DMZ, any abnormal traffic coming out of the system should be considered suspect.

Server hardening

Hardening your Asterisk server is critical. Not only are there performance benefits to doing this (running nonessential processes can eat up valuable CPU and RAM resources), but the elimination of anything not required will reduce the chance that an exploited vulnerability in the operating system can be used to gain access and launch an attack on other parts of your network.

Running Asterisk as non-root is an essential part of system hardening. See Chapter 3, Installing Asterisk for more information.


Asterisk 1.8 includes the ability to use both SIP TLS for the encryption of signaling and SRTP for the encryption of the media between endpoints. More information about encrypting SIP calls can be found in the section called “Encrypting SIP calls”. Asterisk has also supported encryption between endpoints using IAX2 since version 1.4). Information about enabling encryption across IAX2 trunks can be found in the section called “IAX encryption”.

Physical security

Physical security should not be ignored. All terminating equipment (such as switches, routers, and the PBX itself) should be secured in an environment that can only be accessed by authorized persons. At the user end (such as under desks), it can be more difficult to deliver physical security, but if the network responds only to devices that it is familiar with (e.g., restricting DHCP to devices whose MAC addresses are known), the risk of unauthorized intrusions can be mitigated somewhat.