How well do the big four networks protect your private voicemail?
We set up a VoIP handset to inject the necessary code to tell the network that our handset had the mobile number of the voicemail account we wanted to hack. We then dialled the voicemail service number to see if it would let us in. All networks have two voicemail numbers: a shortcode that you use from the mobile, and a long number when you call in from another phone or sometimes when you are abroad and the shortcode doesn’t work on the network you’ve roamed to.
We could only use the long number because we were not on the mobile network under test. The issue is: what does the voicemail system do when you dial the long number from a handset which identifies itself as being a subscriber?
We’ll get the secure ones out of the way first. We couldn’t hack either Vodafone or O2, so their systems must rely on more than simply checking the CLI sequence in a call. Vodafone handles the issue best. All calls to the long number ask you both for the number of the phone you are collecting the voicemail for and for a PIN. It ignores the caller display completely.
O2 got confused. The call wasn't placed, and we got instead a message generated by our VoIP system saying the number we were calling wasn’t available. O2 uses a system where you call your own mobile number and press star when you get the intro message to get to the voicemail menu. We suspect that calling from the mobile number on the VoIP network just confused everything, the voicemail system went round and round in circles until our VoIP timed it out.
But the results with Three and EE were shocking. I’d just bought a new pay-as-you-go SIM on Three, put credit on it, and set up the voicemail, which asks for a PIN right up front. I then switched off the mobile and called it using another phone to leave a voice message.
We programmed the VoIP system to present the Three mobile number to the network and dialled the long number for collecting voicemail. We got straight through as though we were using handset with the Three SIM.
It's enough to make your scr-EE-m
It was similar with EE. We obtained a phone on the EE network from a contact, so we programmed its mobe number into the VoIP phone and called the voicemail long number. We got straight in. Unfortunately our subscriber didn’t have any voicemail, so we called from another phone, left a message and then called back on the VoIP phone and listened to it.
Testing with an Orange number (Orange and T-Mobile UK are part of EE) was more interesting. For this, we turned to a contact who works closely with the kinds of people who legally carry concealed guns – anti-terrorist, organised crime, under-cover, witness protection and the like. They use a mix of SIMs from Vodafone and Orange. So we called him, explained what we were going to do first, and then spoofed his Orange number. We got in, but didn’t want to listen to his voicemail.
So we changed the greeting message.
Anyone calling him would learn that he had changed his name to “Mabel”. It makes fraping look tame.
We did however find that with Orange it would sometimes ask for a PIN and sometimes not. We put this down to routing. We suspect – and such things are the day-job for one of the people helping me – that when the call went through some routes its illegitimacy was spotted, and when it went through others the call went straight through as though it was kosher.
The urgent issue is for EE and Three to make their systems secure. We’ve deliberately not given blow by blow details of how to spoof the CLI, but we can’t be the only people to have figured this out.
It's not like the networks have not been warned. The majority of the information presented to the Leveson inquiry on how to hack voicemail was redacted, but in a brief public document [PDF] the danger of CLI spoofing is mentioned.
And the mobile networks' own industry body, the GSMA, also warned of the danger in its voicemail security guidelines published in February 2012. In this document the GSMA talks about fraud as well as security. It points to the danger that a crook could register a premium-rate number and then use that number to leave a message on the mark's voicemail. By spoofing the CLI, the miscreants can then pick up the message and return the call, raking in the profits from the premium-rate call.