Researcher Intercepts GSM Cell Phones During Defcon Demo

DEFCON18 -- Las Vegas -- A hardware hacking expert here at Defcon18 successfully faked several attendees' cell phones into connecting to his phony GSM base station during a live demonstration that had initially raised concerns at the Federal Communications Commission (FCC).

Security researcher Chris Paget's presentation here was aimed at demonstrating security weaknesses in the GSM protocol using a homegrown GSM base station running over ham-radio frequency. His so-called "IMSI Catcher" acted as a spoofed GSM tower and fake base station that could convince GSM handsets to connect to as the closest "tower" in proximity.

GSM technology is used in 80 percent of the world's mobile phone calls, and has been the subject of previous security research poking holes in it. Paget said his intention was to demonstrate how the protocol is basically broken: "The main problem is that GSM is broken. You have 3G and all of these later protocols with problems for GSM that have been known for decades. It's about time we move on," Paget said in a press briefing yesterday prior to today's demo.

Paget's demo almost didn't happen at all: it wasn't until the late last night that he decided to go forward with the live demo of the hack after conferring again with Electronic Frontier Foundation attorneys after the FCC voiced its concerns that the demo might involve the unlawful interception of phone calls: "The response we got from the FCC is that [they couldn't] advise whether this is a good thing or bad thing, but here's a long list of statutes you should read to make sure you're not in violation," Paget said yesterday. "It seemed almost a scare tactic to convince me not to go ahead."

Paget was careful to issue warnings about his demo to attendees during his presentation today and that his demo was in no way for malicious purposes nor would it retain any data gathered from "owned" phones. His use of ham-radio frequency to carry the GSM signal got around any spectrum violation issues, he said.

He built the IMSI (International Mobile Subscribe Identity) Catcher, a phony GSM tower/base station, for about $1,500 using open-source technology, which he said is "a thousand times" cheaper than a similar commercial device used by providers. Aside from the device, the setup also used two directional antennas, and a Debian laptop running OpenBTS and Asterisk, an open source tool that turns a computer into a voice communications server. He used the device only to intercept and handle outgoing voice calls -- which were sent via voice-over-IP -- and not incoming calls nor data. SMS messaging would require getting caller ID information, which is difficult to obtain, he said.

"When the phone is looking for a signal, it looks for the strongest tower. This offers the best signal," Paget said, even though it's only 25 milliwatts.

The system only intercepts outbound calls, and callers whose phones connected to Paget's phony tower would get a recorded message when trying to dial out. "When attached to my tower, your phone is [considered] off, so incoming calls go straight to your voicemail," he said.

Overall, Paget captured anywhere from 17 to 30 phones at a time during the demo, even after configuring the base station to appear as an AT&T tower. The phones automatically defaulted to 2G because Paget's base station is 2G. The base station could also be configured to disable encryption, he notes, as well as to target specific brands of phones to connect to it.

Paget destroyed the USB key that held any data gathered from the cellphones after the demo, so he didn't know for sure the total number of phones that connected to it.

In previous tests, Paget found that iPhones most commonly connect to his fake GSM station.

He also discussed methods of speeding up the capture of cellphones during his presentation. The solution, he said, is to move to 3G. "3G and later is the solution ... 3G authentication is much better," Paget said. But that's no small feat: the conversion would entail upgrading all phones, networks, and towers, he said.

Adding encryption could help protect phones from a malicious GSM interception attack, as well as using VoIP and noticing when the 3G icon is no longer present during a call, he said.

This isn't the first time Paget has been at the center of controversy over his research. At Black Hat DC in 2007, his talk on cloning HID's RFID-based proximity cards was pulled from the program at the eleventh hour after the RFID vendor threatened him with a patent lawsuit. "I had no choice," said Paget, who was a researcher with IOActive at the time. "We were a very small company and we had to pull the talk ... they threatened patent litigation, which is extremely expensive and can cost [millions]" even if it turns out the suit has no legs, he said.

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