Hackers have discovered that implementation quirks in Voice over IP make it easy to spoof Caller ID, and to unmask blocked numbers. They can make their phone calls appear to be from any number they want, and even pierce the veil of Caller ID blocking to unmask an anonymous phoner's unlisted number.
At root, the issue is one of what happens to a nugget of authentication data when it leaves the tightly-regulated realm of traditional telephony, and passes into the unregulated domain of the Internet.
On the old-fashioned phone network, Caller ID works this way: your local phone company or cell phone carrier sends your "Calling Party Number" (CPN) with every call, like a return address on an envelope. Transmitted along with your CPN is a privacy flag that tells the telephone switch at the receiving end of the call whether or not to share your number with the recipient: if you have blocking on your line, the phone company you're dialing into knows your number, but won't share it with the person you're calling.
This arrangement relies on telephone equipment at both ends of the call being trusted: the phone switch providing you with dial tone promises not to lie about your number to other switches, and the switch on the receiving end promises not to reveal your number if you've asked that it be blocked. In the U.S. that trust is backed by FCC regulations that dictate precisely how telephone carriers handle CPNs, Caller ID and blocking. Most subscribers have come to take Caller ID for granted, and some financial institutions even use Caller ID to authenticate customers over the phone.
Despite that, the system has long been open to manipulation. "A lot of times you can offer any number you want, and carriers won't validate that," says Lance James, chief security office of Secure Science Corporation. But in the past, the power to misrepresent your number came with a high price tag: you typically had to be a business able to pay the local phone company for a high-volume digital connection. On the other side of the equation, companies who pay for toll free numbers can often access an incoming caller's phone number even if it's blocked.
VoIP networks, currently outside FCC regulation, place those capabilities in the hands of ordinary netizens. In a telephone interview with SecurityFocus, 21-year-old phone hacker "Lucky 225" demonstrated how he could spoof his Caller ID to appear to be phoning from the reporter's office. In another demonstration, the reporter phoned Lucky's associate "Natas" from a residential phone with Caller ID blocked. Natas was able to rattle off the unlisted phone number.
As described by Lucky, who's scheduled to give a talk on the subject at the DefCon hacker convention later this month, much Caller ID chicanery can be accomplished by taking advantage of implementation quirks in Voice over IP networks that try, but fail, to implement Caller ID properly. "There are little exploits that you can do," says Lucky. But the most powerful tool for manipulating and accessing CPN data is the open-source Linux-based PBX software Asterisk, used in combination with a permissive VoIP provider. "It's fully configurable, you can pretty much do anything you want with it," says Lucky. "That's why Voice over IP is changing things."
Natas used Asterisk in conjunction with the NuFone Network for his demonstration of Caller ID unmasking. NuFone chief Jeremy McNamara didn't return phone calls for this story.
Privacy advocates, who had reservations about Caller ID when it was introduced in the 90s, aren't happy that it's becoming easier to subvert. "A worse case scenario is if you have a blocked number, and you're a victim of stalking, and you're duped into calling a number the stalker set up that was routed through a VoIP line," says Jordana Beebe of the San Diego-based Privacy Right's Clearinghouse. "It could put their life in danger."
Callers with life-or-death anonymity concerns might consider spoofing just to get a little privacy. For now, Lucky says pranks among friends are the most common use that he's seen of VoIP spoofing, but he believes that identity thieves and other swindlers could have a field day. "I've used it myself to activate my own credit cards, because I never give credit card companies my real number," he says. "One simple spoof, and it's like saying, if you have the guy's phone number, that piece of information is more important than his mother's maiden name and date of birth. If you have the phone number, you don't need anything else."