Signaling Attacks

The goal of any phone system is to establish and manage communication sessions for transmitting voice data, or sound, in general. Additionally, transmitting other data formats, such as video, text, or images, may also be supported. In any case, a stable and reliable transmission has to be maintained throughout the entire conversation, and the communication session needs to be closed when either party decides to end the call. To achieve that, two classes of protocols are used by VoIP technology in a similar manner to traditional telephony: signaling protocols and media transport protocols.

In general, before any voice can be sent, a call must be placed. In a classic PSTN network, a caller dials the digits of the desired phone number, which are then processed by the telephone company's system to ring the called party. With VoIP, the user dials the number (in the form of an actual number dialed on a telephone keypad or of a URI), and after that a complex series of packet exchanges occur, based on a VoIP signaling protocol, to connect the call.

In addition to SIP and H.323, two other standards are in use: Media Gateway Control Protocol (MGCP) and Megaco/H.248. These standards may be used in large deployments for gateway decomposition to ease message handling with media gateways (MGs) and media gateway controllers (MGCs). A number of other signaling protocols also exist. Here is a list of the most popular ones.

VoIP Signaling Protocol

Description

H.323

Defined by the ITU-T

Session Initiation Protocol (SIP)

Defined by the IETF; newer than H.323

Megaco (or H.248) and MGCP

Both media gateway control protocols

Inter-Asterisk eXchange protocol

Used by the Asterisk open-source PBX

(IAX2)

Skinny Client Control Protocol

Proprietary protocol from Cisco

(SCCP)

Skype

Proprietary peer-to-peer protocol

H.325

New signaling protocol proposal by ITU-T

Attacks targeting the signaling protocols are partially related to the vulnerabilities on the traditional phone networks that made the headlines in the 1970s under the common definition of phreaking. Several signaling attacks can be performed with minimal resources and have disastrous consequences. Since most currently available security testing tools are SIP-oriented, the following examples are mainly related to SIP signaling. However, the described attack classes can also be applied to other protocols with minor changes.

Before outlining the attack vectors specifically related to VoIP signaling, we will briefly introduce the best noncommercial testing tools available today.

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