A Vonage VOIP 3-way call CID Spoofing Vulnerability

Original Advisory: Wednesday, August 13, 2003

Severity: Medium - High

Description: An attacker using the VOIP (Voice Over IP) carrier Vonage, has the ability to spoof the caller ID of a called party through the three-way calling feature. This trick essentially acts similar to a POTS-based diverter, as it allows the attacker to carry out illicit telephone activities while hiding his or her phone number.

Version: This was tested using Cisco Systems' ATA 186 VOIP hardware on the Vonage carrier.

Author:  N. Wosnack


Vonage Background:

"Using an existing high-speed Internet connection, Vonage technology enables anyone to make and receive phone calls - worldwide - with a touch-tone telephone. Offering quality phone service bundled with enhanced IP communications services, our interactive communications portal is a gateway to advanced features only available through digital telephone service. Utilizing our global network and advanced routing technologies, Vonage offers an innovative, feature-rich and cost effective alternative to traditional telephony services."

Description of the problem:

By using SIP-enabled voice over IP (VOIP) hardware such as the Cisco ATA 186 Analog Telephone Adaptor, it's possible to spoof the caller identification that shows up on a call. The attacker only needs to call up a regular phone line (POTS - plain old telephone service), place the caller on hold, flash over to a dial tone using the threeway call feature, and then call a second party for this to work. The caller ID information that tends to show up is the first called party's telephone number with either their name listed or "unknown name" showing on a conventional caller-id enabled telephone. The opportunity for abuse is high and could allow the determined attacker to social engineer your telephone, cable, or utility company into modifying your services.
Since many companies only require the person's name, address, and caller id for account authentication, this vulnerability helps the attacker. The other opportunities this vulnerability gives the attacker is the ability to spoof anyone's caller id information for phone hacking (often called "phreaking"); such as breaking into voice mail accounts and PBX exploitation for the purpose of proprietary information gathering and telephone fraud.

Solutions to the problem:

This issue is something that Vonage will need to investigate on their end. The proper routing of caller id information after a third-party call is initiated is the problem, and needs to be resolved by the Vonage IT staff figuring out why their VOIP switching equipment doesn't pass this data properly. I have contacted Vonage directly about this issue, so it can hopefully be resolved shortly.

For everyone else, your best defense is to be aware of who is calling you. If you happen to receive a phone call from an unknown party who wants to place you on hold, hang up immediately and then call them back.
If you hear a recording telling you the number is not in service, then you've likely reached a Vonage gateway number, which mean you were likely called by someone attempting to exploit this Vonage VOIP vulnerability.


In the past year, Voice over IP telephony has seen many security issues. The voip issues range from vendor implementations of the Session Initiation Protocol (SIP), problems with remote-accessible code which can be exploited to cause a denial of service, voip phones that are weak in ways that facilitate man-in-the-middle attacks directed at intercepting telephone traffic, and most recently 3-way caller ID spoofing on Vonage.

When the information security community works closely with vendors and carriers, these problems can be resolved quickly and efficiently enough to limit or even eliminate any abuse by phone phreaks and criminals.

Vendor Contact:

  • http://www.cisco.com/ - Cisco Systems, Inc. Manufacturer.
  • http://www.vonage.com/ - American Voip telecom carrier.

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