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Voicemail and VOIP

Commonly overlooked security risks

Introduction

Every company has a phone system of some type, and just like with smartphones, these systems often offer so much more than just basic PABX functionality — technology such as VoIP, video conferencing, Unified Communications platforms, cloud based PABXs are all becoming par for the course. Most, if not all of these systems, also include integrated voicemail functionality.

This article considers some of the avenues through which attackers may look to compromise the security of a company’s phone systems.

Overview

There are many reasons attackers may be interested in a company’s phone system, including:

  • Using it to make fraudulent calls;
  • Aiding in social engineering attacks;
  • Eavesdropping on sensitive calls;
  • Harvesting sensitive information from voicemails;
  • Compiling internal directories of company staff; and
  • Attempting to obtain call detail records for market intelligence and industrial espionage.

Complicating matters is that, unlike dedicated VoIP systems and Video Conferencing systems which are often audited and maintained by the IT Security teams, traditional PaBX, IVR and Voicemail systems are often overlooked or fall outside the management purview of the IT and Security teams.

Voicemail is still relied upon for everyday use by many organisations, and sensitive information is commonly left in voicemail messages. A motivated attacker targeting a company is highly likely to be able to gain valuable information by listening to the voicemail of system administrators or executives over a period of time.

With cheap VoIP services being readily available it is also simple for attackers to automate and scale up their attacks to make multiple simultaneous calls and complete them faster. There are many ways to automate phone attacks, and it is easy for an attacker to write a script or use existing software to automate a range of attacks, including those outlined below.

Types of Voicemail and VOIP Attacks

Brute force attacks

Having a four-digit PIN as the de facto standard for voicemail authentication means an attacker will have a reasonable chance at successfully guessing the PIN through manual avenues such as by just using the phone keypad. Even with longer PINs, common patterns and repeating numbers are so commonly used by staff that an attacker is likely to work out the PIN using a PIN list instead of having to manually enter in every possible combination of digits.

In combination with most voice mail and phone systems not supporting account lockouts, nor having other security controls such as logging and alerting, which are commonly applied to other IT systems, undertaking a successful PIN brute force attack to gain access to a staff member’s voicemail box is unlikely to prove difficult to the determined attacker. Additionally, there are also methods to bypass lockout timers in those instances where they are in place. One technique that is almost always successful on large voicemail systems is attempting two or three common PINs against every extension or user account. This would prevent triggering any account lockouts, if implemented.

Once an attack run is complete the attacker may well have access to several voicemail accounts.

Directory reconnaissance

The same automation that can be applied to PIN brute force attacks can also be used for other attacks against phone systems. One example is the creation of internal directories. An attacker can use the “find extension” feature of modern PABXs and voicemail systems to make a list of names, extensions and sometimes job titles within a company. They can also do this by calling and making a note of every greeting for every extension if the PABX doesn’t have a names directory feature.

Call-forwarding attacks

Another use attackers have for voicemail is the call forwarding feature which can be used for free phone calls or to aid in social engineering attacks.

Getting free phone calls is the simplest example. An attacker who has compromised a voicemail box sets up call forward to a mobile phone, overseas or other toll number they want to call, then calls back the local DDI number or the company’s main toll-free number and enters the extension number and waits for the call-forward to connect them to their pre-programmed toll number.


This attack can be reversed as well — an attacker can also use a compromised voicemail box to receive incoming calls using the call forwarding feature to forward calls to a VOIP or pre-paid phone controlled by the attacker.

They then will have control of an extension on that company’s phone system which they can give to external parties to call back on to appear like they are inside the company’s office, which can be an invaluable means to facilitate the commission of a further social engineering attack.

Leveraging Caller ID

Most phone systems, when call-forwarding is used, display the caller-ID of the voicemail extension rather than the number of the originating party.

An attacker may be able to leverage the call-forward feature to masquerade as a known external party, such as appearing to be a known vendor or to be calling from inside the target company’s building. This can gain greater traction for social engineering purposes.


On a penetration testing engagement some years ago, my colleagues and I took over a manager’s extension at corporate headquarters and set up a call forward to the security desk at one of their rural sites. We called ahead to the security desk to add our names to the visitor register. The security desk asked very few questions because their phone displayed the call as originating from the manager’s phone at the organisation’s headquarters.

When we arrived, the security desk was expecting us and allowed us to enter without any restrictions.

Recommendations

The attacks scenarios above relate to simple voicemail systems, which most people overlook, considering them to a straightforward way to store and retrieve messages. However, when you include customer facing IVRs, VoIP systems, PABX systems, Teleconferencing and Video Conferencing systems, Unified Communications systems, call-queue management systems and the endless other applications of modern phone systems, then the possible vulnerabilities that can be exploited by attackers are almost endless.

Large companies often own a block of numbers, generally in lots of ten thousand. It is a good idea to periodically audit these number blocks to classify all lines, then performing an audit of possible attack vectors of all the phone systems connected to them.

Practicing the same security hygiene for voicemail that you do for other systems is critical, for example:

  • Disabling default accounts;
  • Auditing voicemail boxes for common PIN’ or disallowing common or simple PINs;
  • Setting a minimum PIN length of six digits;
  • Setting unique temporary PINs when provisioning new voicemail boxes;
  • Setting up lockout timers that don’t lose their state over multiple calls;
  • Disabling call forwarding features;
  • Restricting allowed forwarding numbers to local mobiles and landlines only;
  • Deactivating unused voicemail boxes; and
  • Applying logging and alerting if possible.

With some phone systems this is often easier said than done. We hope this article gets you thinking about how your company uses and manages phone systems and how they might be abused by attackers if appropriate vigilance is not exercised.


Article by John McColl, Principal, Hivint

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