Sam Stelfox

Thoughts from a software engineer, systems hacker and Linux gubernāre.


Sensitive information can be transmitted through phone systems, POTS lines can also be very very expensive if anonymous calls are allowed through the POTS line. Mitigations must be implemented in the network, network perimeter, servers, and phones. Mitigations in the network aim to deny access to attackers through virtual separation of data and phone networks using VLANs, and perimeter security devices such as firewalls and filtering routers.

Servers AND phones must be security hardened. Phones must require strong authentication and authorization of users to prevent identity spoofing. Phones should also encrypt voice streams through high risk environments such as the internet.


Devices and users should authenticate themselves separately to the PBX. Additionally devices should ensure the validity of the server before allow user authentication.

Network Separation


Dividing the network into multiple VLANs does not provide any benefit if the traffic between the VLANs is not restricted. VLANs are so trivial to implement that there really isn't any reason not to use them, and they do provide a solid defense against malicious traffic on other networks... As long as attackers can't craft packets to traverse the VLANs...

Filters need to configured at every available device to permit only traffic between phones and the telephony server.

Physical/Layer 2

An attacker who has physical access to the network may bypass any VLAN separation by simply unplugging an IP phone and introducing a malicious machine. A simple solution would be to only allow know MAC addresses, however, this too could be fooled quite trivially. The solution to this is 802.1X authentication. Additional trickery can be added using various switch port security features such as dhcp client sniffing.


Ideally, voice networks should have their own service providers for basic functionality like DNS, DHCP, and NTP. This might be a bit overkill, however, it's pretty simple to implement these services and they don't require much of a box to do it. What this gets you is a complete separation of voice and data networks preventing even leaks about the other networks from occuring on the other one.

In the event of a voice network that goes only to POTS lines and doesn't need any external SIP clients, or to receive voice calls from outside the local voice network, then having the services separated allows for total network isolation. Even with external SIP clients it can be made so that there is exactly one point where external data can get into the voice network and out of it. This provides a very nice and convienient place to drop an IDS.

Call Eavesdropping

Eavesdropping on unencrypted voice communication is more likely when conversations travel over a general use IP network. Media streams are easily reconstructed if packets can be captured. Security protections at the network layer can make call eavesdropping more difficult, but not prevent it. If VoIP traffic traverses public networks unencrypted, eavesdropping is a serious concern.

By eavesdropping on VoIP calls, attackers can potentially obtain sensitive information such as passwords, usernames, caller location, and addresses of other devices. They could also use traffic analysis to determine relationship patterns between unique callers on the system.

A switched network means that network traffic is not broadcast to all devices connected to a switch, thus making packet capturing more difficult. However, there are methods to subvert the switch. One method is to flood the switch with traffic, which overfills the MAC address table causing the switch to broadcast traffic on all ports. The other method of subverting a switched network is to implement a man-in-the-middle attack. In this case, an attacker pretends to be the victim device by spoofing the victim’s MAC address.

The best and only robust solution to eavesdropping is to encrypt all voice traffic on the network from end to end.

Signal Leakage

With many VoIP implementations signaling is done out-of-band. Signalling is meta-data about calls such as the destination number, extensions, caller identity information, etc.

If this information is sent plaintext an attacker can potentially build up a social graph of users of the phone system that may be later used in a social engineering attack.

If this signalling information is sent without a form of crytographic verification, plaintext or not, these messages would be suscetible to MITM attacks allowing an attacker to cause havoc such as change who you are calling tranparently, even potentially routing them to the attacker themselves to continue a social enginerring attack, effectively disrupting the integrity of a critical communications service.


Impersonating a caller can potentially allow an attacker access services on telephony servers reserved for the legitimate user, spoof caller ID information, bypass filters based on caller name, and eavesdrop on calls. While people are used to using a caller's voice as a form of authentication, this only works after answering the phone and with telephony applications that are voice-based.

If phones can be contacted from the Internet, this could lead to problems with SPAM over Internet Telephony (SPIT), because SPAMmers can cheaply automate sending messages or calling phones. In addition, by impersonating a user to the telephony server, an attacker can eavesdrop on calls from anywhere on the network using the registration hijacking attack. This attack allows an attacker to send spoofed messages to the telephony server and potentially reroute a user's calls.

Call Plan Security

Outgoing PIN

A neat little trick that could be accomplished to greatly increase the security of a PBX to prevent abuse from people making paid phone calls could be to require a PIN be entered whenever an outgoing call is made to a number that has never been dialed from an extension before.


Any fixed phone should have it's physical location stored and available to make 911 calls safely. Remote SIP clients or any other phone that moves regularily, however, should not be able to dial 911 and should rely on additional means for emergency services.

Call Log Auditing

Here, there comes a tradeoff. Keeping call logs (not recordings mind you) can be very useful in tracking down abuse, the other side of this coin is privacy for the users. Knowing the source number, destination number, call duration, and time the call occurred are important for this nothing more and nothing less.

If these logs were kept only for a month it would limit the privacy impact. If any call that lasted under 15 seconds never entered the call log, as these would have minimal importance, and probably wouldn't be particularily valuable.

The source number could be removed after a week, anonymising the call logs to a certain extent that would probably be acceptable to most users.

This is one of those security measures I wouldn't want to relax quite this much unless things such as the outgoing pin, a strong IDS, and firewall logs had been implemented.