Security for Private Branch Exchange Systems
By Richard Kuhn
Computer Security Division
Information Technology Laboratory
National Institute of Standards and Technology


Hacker attacks on computer networks are well known, but Private Branch Exchange 
(PBX) systems are also vulnerable. In one case, a hacker penetrated the Private 
Branch Exchange (PBX) system operated by a hospital in Escondido, California. 
For nearly two years, on various occasions, he blocked calls to and from the 
hospital, connected hospital operators to spurious numbers (including the county 
jail), and placed bogus emergency calls that appeared to be coming from inside 
the hospital. 
Unfortunately, the hospital’s experience is not unique. Failure to secure a PBX 
system can result in exposing an organization to toll fraud, theft of 
proprietary, personal, and confidential information, loss of revenue, or legal 
entanglements. Depending on how the organization’s network is configured and 
administered, information leading to intrusions of data networks may be 
compromised as well. A PBX is a sophisticated computer-based switch that can be 
thought of as essentially a small, in-house phone company for the organization 
that operates it. Protection of the PBX is thus a high priority. This bulletin 
introduces some of the vulnerabilities of PBX switches and describes some 
countermeasures that can be used to increase the security of your PBX. For a 
more detailed treatment of these issues, see NIST Special Publication (SP) 
800-24, PBX Vulnerability Analysis (see http://csrc.nist.gov).
Introduction
Digital PBXs are widespread throughout government and industry, having replaced 
their analog predecessors. Today, even the most basic PBX systems have a wide 
range of capabilities that were previously only available only in large-scale 
switches. These new features have opened up many new opportunities for an 
adversary to attempt to exploit the PBX, particular mostly by using the features 
for a purpose that was never intended. The threats to PBX telephone systems are 
many, depending on the goals of attackers. Threats include:

  Theft of service–i.e., toll fraud, probably the most common of motives for 
  attackers. 
  Disclosure of information- data disclosed without authorization, either by 
  deliberate action or by accident. Examples include both eavesdropping on 
  conversations and unauthorized access to routing and address data.
  Data modification- data altered in some meaningful way by reordering, 
  deleting, or modifying it. For example, an intruder may change billing 
  information or modify system tables to gain additional services.
  Unauthorized access- actions that permit an unauthorized user to gain access 
  to system resources or privileges.
  Denial of service- actions that prevent the system from functioning in 
  accordance with its intended purpose. A piece of equipment or entity may be 
  rendered inoperable or forced to operate in a degraded state; time-dependent 
  operations may be delayed.
  Traffic analysis- a form of passive attack in which an intruder observes 
  information about calls (although not necessarily the contents of the 
  messages) and makes inferences, e.g., from the source and destination numbers 
  or frequency and length of the messages. For example, an intruder observes a 
  high volume of calls between a company’s legal department and the Patent 
  Office and concludes that a patent is being filed. 
PBX Characteristics
PBXs are sophisticated computer systems, and many of the threats and 
vulnerabilities associated with operating systems are shared by PBXs. There are 
two important ways, however, in which PBX security is different from 
conventional operating system security:
  External access/control. Like larger telephone switches, PBXs typically 
  require remote maintenance by the vendor. Instead of relying on local 
  administrators to make operating system updates and patches, organizations 
  normally have updates installed remotely by the switch manufacturer. This of 
  course requires remote maintenance ports and access to the switch by a 
  potentially large pool of outside parties.
  Feature richness. The wide variety of features available on PBXs, particularly 
  administrative features and conference functions, provides the possibility of 
  unexpected attacks. An attacker may use a feature in a manner that was not 
  intended by its designers. Features may also interact in unpredictable ways, 
  even when implemented correctly, leading to system compromise even if each 
  component of the system conforms to its security requirements and the system 
  is operated and administrated correctly.
Maintenance
Maintenance procedures are among the most commonly exploited functions in 
networked systems. The problem is even more acute with PBXs because PBX 
maintenance frequently requires the involvement of outside personnel. Ways in 
which an adversary could exploit vulnerabilities in maintenance features to gain 
unwanted access to the switch follow.
Special Manufacturers Features
There may be features that the manufacturer will rely on in the event a 
customer’s PBX becomes disabled to such a point that on-site maintenance 
personnel cannot resolve the problems. The manufacturer could instruct the 
maintenance personnel to configure and connect a modem to the maintenance port. 
The manufacturer may then be able to dial in and use certain special features to 
resolve the problems without sending a representative to the customer’s 
location. The potential cost savings is a primary reason for adding such special 
features. A switch manufacturer would not want the special features to be well 
known because of their potential for misuse. These types of features may be 
accessible via login IDs and passwords held privately by the manufacturer. Some 
possible special features are listed below:
  Database upload/download utility: Such a utility allows the manufacturer to 
  download the database from a system that is malfunctioning and examine it at 
  their location to try to determine the cause of the malfunction. It would also 
  allow the manufacturer to upload a new database to a PBX in the event that the 
  database became so corrupted that the system became inoperable. Compromise of 
  such a utility could allow an adversary to download a system’s database, 
  insert a Trojan horse, or otherwise modify it to allow special features to be 
  made available to the adversary, and upload the modified database back into 
  the system.
  Database examine/modify utility: Such a utility allows the manufacturer to 
  remotely examine and modify a system’s database to repair damage caused by 
  incorrect configuration, design bugs, or tampering. This utility could also 
  provide an adversary with the ability to modify the database to gain access to 
  special features.
  Software debugger/update utility: This type of utility gives the manufacturer 
  the ability to remotely debug a malfunctioning system. It also allows the 
  manufacturer to remotely update systems with bug fixes and software upgrades. 
  Such a utility could grant an adversary the same abilities. This is perhaps 
  the most dangerous vulnerability because access to the software would give an 
  adversary virtually unlimited access to the PBX and its associated 
instruments.
Dial-Back Modem Vulnerabilities
Unattended remote access to a switch clearly represents a vulnerability. Many 
organizations have employed dial-back modems to control access to remote 
maintenance facilities. This access control method works by identifying the 
incoming call, disconnecting the circuit, and dialing the identified person or 
computer at a predetermined telephone number. Although helpful, this form of 
access control is weak because methods of defeating many dial-back modems are 
well known.
Countermeasures
  Ensure that remote maintenance access is normally blocked unless unattended 
  access is required. Whenever possible, require some involvement of local 
  personnel in opening remote maintenance ports.
  Install two-factor (i.e., two different mechanisms) strong authentication on 
  remote maintenance ports. Smart card-based systems or one-time password 
  tokens, in addition to conventional login/password functions, make it much 
  more difficult for attackers to breach your system’s security.
  Keep maintenance terminals in a locked, restricted area.
  Turn off maintenance features when not needed, if possible.
Administrative Databases
Administrative databases represent “the keys to the kingdom” for a PBX. Among 
the most critical security tasks for PBX owners are administration of the PBX, 
the creation and modification of its user databases, and the operating software 
controlling the switch.
Passwords
Most PBXs grant administrative access to the system database through an 
Attendant Console or a generic dumb terminal. Username/password combinations are 
often used to protect the system from unwanted changes to the database. If 
remote access to the maintenance features is available, some form of password 
protection usually restricts it. There may be a single fixed maintenance 
account, multiple fixed maintenance accounts, or general user-defined 
maintenance accounts. The documentation provided with the PBX should state what 
type of maintenance access is available.
Passwords may also be set to factory default values that can be changed by the 
user. Default values are typically published in the documentation provided with 
the PBX. If there are multiple maintenance accounts and maintenance personnel 
use only one, the others may remain at their published factory settings. Anyone 
who knew the factory default settings could then gain access to the switch.
Physical Security
Physical access to the PBX hardware grants access to the software, the 
configuration database, and all calls going in and out of the PBX. With access 
to the PBX, an adversary could exploit practically any conceivable 
vulnerability.
The type of media on which the software and databases are stored is important to 
a PBX’s physical security. If these are stored on ROM-type devices or on an 
internal hard disk, it is more difficult to gain access to them than if they are 
stored on floppy disks or CD-ROM. ROM devices are mounted on circuit boards and 
may be soldered rather than socketed, making removal and replacement difficult. 
Likewise, an internal hard disk is probably mounted internally and bolted to the 
chassis, making removal and replacement difficult. However, floppy disks are 
easily removable and replaceable. An adversary with access to the floppy disks 
could easily conceal a disk containing modified software/databases, gain access 
to the PBX, and replace the original disk with the modified disk. Similarly, 
CD-ROMs can be easily removed and replaced. Since equipment for creating CD-ROMs 
is readily available, an adversary may find it equally easy to copy and modify a 
CD-ROM-based system.
If the PBX supports configuration and maintenance via a dumb terminal, the 
terminal may be located near the PBX. If the terminal is not at the same 
location as the PBX, the terminal port is still available and could be used by 
an adversary with a PC acting as a terminal.
Some PBXs may be configured as a central system unit with peripheral units at 
remote locations. The remote peripheral units may also support 
configuration/maintenance via a dumb terminal and therefore have the same 
vulnerabilities as the system unit’s terminal. Also, all calls routed through a 
particular peripheral unit are accessible to someone with physical access to the 
peripheral unit.
Attendant Consoles may offer access to PBX maintenance and configuration 
software. Special features may also be available to Attendant Consoles such as 
Override, Forwarding, and Conferencing. If any of these features are available 
to the user of an Attendant Console, physical access to it should be restricted 
to prevent giving an adversary access to these features.
Most PBXs have an attached system printer. Various information may be output to 
the printer including source and destination of calls that are made or received 
(possibly every call), access codes used to access certain features, account or 
authorization codes used for making special calls, etc. Access to these 
printouts could provide information enabling toll fraud or other compromises.
Remote Access
A very useful but potentially vulnerable feature of many PBXs is remote 
administrative access. The PBX may allow an administrator to make changes to the 
system configuration database through an Attendant Console or from a terminal 
that is not physically located near the PBX, perhaps over a dial-in line with a 
modem.
Remote Access via an Attendant Console

The degree of the vulnerability created by remote access via an Attendant 
Console is determined by several factors: password access, physical connection 
of the Attendant Console to the PBX, and availability of administrative features 
through the Attendant Console.
Remote Access via a Terminal 
If a standard dumb terminal can be used for access to the administrative 
features, more opportunities become available for an adversary to gain unwanted 
access. A modem could be connected to a terminal port and an outside dial-in 
line allowing easy access for the PBX administrator to do remote configuration 
and maintenance. Unfortunately, it also gives easy remote access to an 
adversary. By setting up remote access in this manner, a poor password 
protection system, the existence of “backdoors” (e.g., a special key sequence 
that would bypass required authorization levels), or the use of easy-to-guess 
passwords would seriously undermine the security of the system.
Software Loading and Update Tampering
When software is initially loaded onto a PBX and when any software 
updates/patches are loaded, the PBX is particularly vulnerable to software 
tampering. An adversary could intercept a software update sent to a PBX 
administrator. The update could be modified to allow special access or special 
features to the adversary. The modified update would then be sent to the PBX 
administrator who would install the update and unknowingly give the adversary 
unwanted access to the PBX.
Countermeasures
  Perhaps the most important task for password security is to make passwords 
  resistant to cracking by automated tools. A password generator that creates 
  random passwords can go a long way in defeating password crackers. Both free 
  and commercial random password generation tools are available. Commercial 
  products are available that can generate passwords of user-selectable length 
  that are very resistant to cracking.
  Many software packages use error detection codes to protect against 
  transmission or disk copying errors. Conventional error detection codes such 
  as checksums or cyclical redundancy checks (CRC) are not sufficient to ensure 
  tamper detection. Strong error detection based on cryptography must be used. 
  These methods use cryptographic algorithms that guarantee detection of even a 
  single bit modification.
  Because of the potential for exploitation by intruders, PBX boot disks and 
  utilities must be given more protection than usually afforded typical office 
  software such as word processing packages. Strong physical security should be 
  provided for PBX software. Audit reports from the PBX should be shredded or 
  destroyed in the same way as sensitive memos or financial information.
  To ensure the security of printouts, they must be shredded when discarded.
User Features
An adversary may be able to exploit vulnerabilities in a system's features and 
the way in which features can interact. As with many aspects of information 
technology, the proliferation of features that make PBXs easy to configure and 
use has led to an expansion of vulnerabilities. Many of these are inherent in 
the features themselves or arise out of feature interactions, making them 
difficult to avoid. This discussion illustrates some of these vulnerabilities so 
that administrators will be able to weigh the risks of features against their 
benefits.
Attendant Console

Attendant Consoles typically have more function keys and a larger alphanumeric 
display than standard instruments to support the extra features available to the 
Attendant Console. The Attendant Console may be used for access to maintenance 
and administrative functions, but there are potential vulnerabilities of the 
Attendant Console with respect to maintenance and administration. Some typical 
features available with an Attendant Console are Override, Forwarding, and 
Conferencing.
Attendant Override

Attendant Override is intended to allow the Attendant to break into a busy line 
to inform a user of an important incoming call. An adversary with access to an 
Attendant Console could use this feature to eavesdrop on conversations. The PBX 
should provide for some protection against such uses of Override by providing 
visual and/or audible warnings that an Override is in progress.
Attendant Forwarding

A common feature granted to the Attendant is the ability to control forwarding 
of other instruments. An adversary with access to the Attendant Console could 
use this feature to forward any instrument's incoming calls to a long-distance 
number. The adversary could then call the target instrument and be forwarded to 
the long-distance number, thereby gaining free long-distance access.
Attendant Conferencing

Attendants may also have the ability to initiate a conference or join into an 
existing conference. If this feature is available, the potential exists for an 
adversary logged in as an attendant to eavesdrop on a conversation or add an 
additional party to a conference without the knowledge of the other parties.
Automatic Call Distribution (ACD)
ACD allows a PBX to be configured so that incoming calls are distributed to the 
next available agent (e.g., reservation clerk) or placed on hold until an agent 
becomes available. Agents may be grouped together with each group having a 
supervisor. The group of supervisors may then even have a higher-level 
supervisor. The number of supervisors and number of levels of supervisors is 
dependent on the type of PBX being used.
Most ACD systems grant a supervisor the ability to monitor the calls of the 
group they are supervising. Because of this feature, ACD systems are a potential 
vulnerability to the users of PBX. If an adversary could gain access to the 
configuration tools or the system database, they could become an ACD supervisor 
and set up an ACD group. The supervisor could then monitor the calls of any of 
the users in the group.
Account Codes/Authorization Codes
Account Codes are normally used for tracking calls made by certain people or 
projects so that bills can be charged appropriately. For example, a user may be 
required to enter an Account Code prior to placing a long-distance call. 
Depending on the configuration of the PBX, the Account Code may have to be on a 
list of approved codes for the call to be successful. If this is the case, the 
Account Code may be considered an Authorization Code because the user must dial 
a specific Account Code that is authorized for making long-distance calls.
Another important use for Access Codes is for Dial In System Access (DISA). DISA 
typically allows a user to dial in to the PBX system from an outside line and 
gain access to the normal features of the PBX, almost as if they were a 
subscriber on the PBX instead of an outside caller. This feature is typically 
used to allow employees to make long-distance calls from the corporate PBX while 
out of the office by dialing in to the switch, then entering a code to make 
long-distance calls. It is easily abused by anyone with the authorization code, 
possibly leading to large fraudulent long-distance charges.
Certain Account Codes may also be allocated for changing a user’s Class of 
Service (COS). When the COS is changed, the user may have access to a different 
set of features. For example, most instruments may be assigned a COS that does 
not permit the use of an Override feature, but a special COS that is only 
accessible by using an Account Code may be created that does permit the use of 
Override. By using the Account Code, an adversary could then gain access to the 
Override feature.
Since the Account Codes are used for billing, there are records kept of the 
calls that are made for the various Account Codes. These records generally 
include the source, destination, Account Code, and time/date of the call. The 
records may be stored as files on one of the system’s disks or they may be 
printed out on a system printer. If the records are printed, an adversary who is 
able to gain access to the printer will have access not only to traffic 
information, but also to the printed Account Codes. Once the codes are known, 
the adversary will be able to use the codes for toll fraud, additional feature 
access, etc.
Override (Intrude)
An Override or Intrude feature is common to many PBXs. Due to its potential 
vulnerability, it is commonly selectable as a feature that can be 
allowed/disallowed on a single instrument or a group of instruments. Override is 
intended to allow one user (perhaps a supervisor) to break into a busy line to 
inform another user (perhaps a subordinate) of an important message. This 
feature could be used by an adversary with access to any instrument permitted to 
use the Override feature to eavesdrop on conversations. The PBX should provide 
for some protection against such uses of Override by providing visual and/or 
audible warnings that an Override is in progress.
Diagnostics
In addition to the major diagnostic features available at a maintenance terminal 
or Attendant Console, many PBXs provide diagnostics that can be initiated from 
any instrument. These diagnostic features may permit a user to make connections 
through the PBX by bypassing normal call processing restrictions. An adversary 
with access to these diagnostic features may be able to deny service or make 
undetected connections allowing for the monitoring of other calls.
Feature Interaction
With the advent of the digital PBX and its wealth of features, the interaction 
between features presents a significant possibility for vulnerabilities. For 
example, in some systems the return-call and camp-on features can be manipulated 
to defeat caller-ID blocking. With the large number of features available in 
modern PBXs, it becomes difficult for the manufacturer to consider all of the 
ways in which different features may interact. Because of this, vulnerabilities 
may exist that were undetected by the manufacturer that allow an adversary 
unwanted access to the PBX and its instruments.
Since the actual Feature Interaction vulnerabilities found on a specific system 
depend heavily on the particular implementation of the features, it would be 
nearly impossible to describe every possibility for a generic system. NIST SP 
800-24 includes detailed examples of some feature interactions.
Countermeasures
  Vulnerabilities can be minimized if the Attendant Console connects to the PBX 
  with a different physical connection than that of the telephone instruments.
  If the Attendant console connects to the PBX in the same manner as the 
  telephone instruments, vulnerabilities can be reduced by having some sort of 
  line configuration feature. Such a feature could reduce vulnerabilities by 
  requiring that a line be specifically configured for use with an Attendant 
  Console. With such a configuration requirement, a telephone instrument could 
  not be easily replaced with an Attendant Console to gain access to the 
  administrative features.
  When implementing a Class of Service, feature interaction should be given much 
  thought. Many of the feature vulnerabilities discussed involve Feature 
  Interaction since several COS items or system options may have to be 
  enabled/disabled to allow them to occur.
  Because the vulnerabilities described in this section are inherent in feature 
  implementation, they are difficult to defend against. The most effective 
  strategy is to ensure that only essential features are activated. 
Computer Telephony
One of the biggest new developments in telecommunications is the advent of 
computer-based telephony systems (CT). As microprocessor speeds have increased 
and memory prices dropped, it has become possible to implement a PBX on little 
more than a high-end PC. A CT system typically requires only the addition of 
specialized voice processing boards to an ordinary office PC with 64 MB of 
memory, a 3 GB disk, and a 300 MHz processor. Some CT systems use specialized 
real-time operating systems, but the trend is toward commercial off-the-shelf 
systems such as Windows, Linux, or other versions of UNIX. This development has 
brought great reductions in the cost of PBX systems, but means the possibility 
of enormously increased security risks. Two factors in particular can increase 
exposure: greatly expanded integration of telephony with the computer network 
and implementation of PBX functions over operating systems with widely known 
vulnerabilities. Some of the features appearing in new CT systems include:
  Voice over IP,

  Browser-based call handling and administration,

  Integration of IP PBX with legacy PBXs and voicemail systems,

  Integration of wireless networks with office network systems, and

  Virtual private networks.

A complete exposition of the risks of CT systems is beyond the scope of this 
document. The safest course of action is to assume that most or all of the 
vulnerabilities described here apply to CT systems as well as traditional PBXs. 
CT systems may also have added vulnerabilities resulting from well-known 
weaknesses of PC operating systems. Future NIST publications may address CT 
security issues in more depth.
Recommendations
Not all of the security measures described in this bulletin will be applicable 
to every organization. The first step in improving PBX security is to assess the 
organization’s current telephony applications. This bulletin describes important 
areas to consider. Following this assessment, NIST SP 800-24 can be used in 
conducting a detailed evaluation. SP 800-24 also includes a set of baseline 
security considerations for PBXs and a more complete set of countermeasures for 
common vulnerabilities.
References
NIST SP 800-24, PBX Vulnerability Analysis, National Institute of Standards and 
Technology, 2000.
Online resources:
  NIST Computer Security Resource Clearinghouse: http://csrc.nist.gov
  DISA Information Assurance:
  http://www.disa.mil/infosec/iaweb/default.html
Disclaimer: Any mention of commercial products or reference to commercial 
organizations is for information only; it does not imply recommendation or 
endorsement by the National Institute of Standards and Technology nor does it 
imply that the products mentioned are necessarily the best available for the 
purpose.
