Now there's a way for attackers to hijack a user's voice-over-IP service: "call-jacking" could let the bad guys launch sophisticated phishing attacks as well as perpetrate lucrative toll fraud.

Researchers from the hacker group GNUCitizen have released a proof-of-concept for call-jacking via a BT Home Hub user's router. The attack exploits cross-site request forgery (CSRF) and authentication bypass vulnerabilities in the router that were previously discovered by the researchers. The attack works even if the default password in the router has been changed.

"We believe that this technique, which we coined as 'call jacking,' is completely new. There is nothing like this in the public domain as far as we know," says Adrian Pastor, a senior IT security consultant for an unnamed penetrating testing firm in London. "The beauty of the attack is that the victim user thinks he/she is receiving a phone call, but in fact he/she is making the phone call and paying for it. We find this quite innovative and unique, hence the need for coining a new term."

The exploit could be used in a phishing attack, where the victim would get a phone call from his "bank" after clicking a link in a phishing email. Phishers typically don't know their victims' phone numbers, he says, so the phone call would help the attacker appear legitimate and gain the victim's trust.

Another attack scenario involves toll fraud, where the victim's router would be forced to dial a toll number. "Premium numbers are very expensive, and allow the identity who registered them -- in this case, the attackers -- make money every time someone dials them," Pastor says.

And the catch with call-jacking is that the victim thinks he or she is receiving a phone call, Pastor says. The victim's phone rings and shows an "incoming" call on the screen -- but his phone instead is actually dialing out to that number.

Pastor and his fellow researcher on the project, Petko D. Petkov, tested the POC on BT Home Hub 6.2.6.B, but expect other versions to be vulnerable as well. Only BT customers using the BT Broadband Talk service are at risk, they say. BT Home Hub is one of the more popular home routers in the U.K.