In this article, I explain the underlying videoconferencing protocol, how it is vulnerable, and how the tool detects the vulnerability. I also discuss why we decided to release this tool rather than fully enabling it in QualysGuard at this time.
What is Auto-answer?
‘Auto-answer’ is the ability to accept incoming calls or requests automatically without or with very little prompting. ‘Auto-answer’ is enabled by default in most videoconferencing equipment, and can allow an attacker to ‘join’ a videoconference by stealth.
Videoconferencing and H.323
H.323 is a recommendation from the ITU Telecommunication Standardization Sector (ITU-T) that defines the protocols used to provide audio-visual communication sessions on any packet network. Most videoconferencing equipment uses the H.323 protocol stacks, which contain the protocols as shown below:
In the TCP/IP layering model, H.323 sits at the application layer and uses the TCP/UDP services provided to transmit H.323 packages in a TCP/IP based network. H.323 uses Q.931, the ITU standard ISDN connection control signaling protocol, to establish connections. When the Q.931 connection has been successfully set up, videoconferencing initiation is implied. Below is how Q.931 connection is set up:
- Caller A sends a ‘SETUP’ message to Caller B
- Caller B sends a ‘ALERTING’ message to Caller A, indicating an incoming call
- Caller B answers the call and sends a ‘CONNECT’ message to Caller A.
Detecting 'Auto-answer' enabled with auto-detect.py tool
The ‘auto-detect.py’ tool needs two parameters, an IP address and a port number, and can be run as shown in the screen capture below. The tool also displays other information from the remote equipment like the Manufacturer ID and Product ID:
Under the hood
The tool establishes a TCP connection and creates a Q.931 ‘SETUP’ message. If you run Wireshark, you can see the ‘SETUP’ message being sent with other fields in the Q.931 protocol.
If the remote videoconferencing equipment answers with the ‘ALERTING’ message, it implies that the equipment is ringing to indicate an incoming call. It also implies that ‘auto-answer’ is turned OFF. But if the videoconferencing equipment answers with the ‘CONNECT’ message, that implies that we are connected to the videoconference. The remote equipment is accepting incoming calls automatically (‘auto-answer’ is ON).