T 5.136 Abuse of freely accessible telephone extensions

Telephones are often used without being assigned to a specific user. Some of these telephones, for example telephones in printer rooms, can only be accessed by a limited group of people. However, telephones are also often found in parking ramps, access control systems, or areas accessible to visitors.

If these telephones contain an electronic telephone book containing internal telephone numbers, there is the risk of such internal telephone numbers being exposed to outsiders.

When VoIP telephones are used in freely accessible areas, additional aspects must be taken into consideration. VoIP telephones consist primarily of software and are often operated in data networks that are also used for other IT applications. An attacker could therefore attempt to exploit vulnerabilities in the VoIP software or install malware when he/she has direct access to the device. Softphones in particular are also at risk, because an attacker may attempt to obtain administrator rights on the terminal device or on other IT systems in the same network with the help of a bootable CD-ROM, for example.

VoIP telephones need to be connected to a data network. An attacker could connect a portable computer to this network connection and may then be able to access the network under some circumstances even though it is protected from the outside by a firewall. He/she could then exploit this access to initiate attacks on the confidentiality, integrity, and availability. Even an insider could attempt to misuse these connections without having to initiate the attacks from his/her own workstation computer (where such activity would be recorded).