“WHY stick a phone in the freezer?” sounds like the first line of a joke. But Tilo Müller and Michael Spreitzenbarth, from the Friedrich-Alexander University in Germany, believe the answer is not at all funny, as they have shown that chilling a mobile phone’s memory chip may permit mischief-makers to skim sensitive data from it.
Messrs Müller and Spreitzenbarth have built on research by a group at Princeton University. In 2008 Ed Felten and his team used nothing fancier than a can of compressed air to chill memory chip modules while a computer was running, remove them and pilfer critical data like passwords, which are temporarily stored in the modules’ chips. Previously, it had been thought that if this were even possible—given how rapidly bits in a computer’s “random access memory” (RAM) decay once power is removed—it would require complex and expensive kit.
The Princeton boffins found the freezing ruse bypassed the protections of full-disk encryption (FDE). An FDE-enabled drive exchanges data between disk and memory through an encryption layer, the key for which must be stored in dynamic memory. The data on disk are always encrypted. Chilling the memory chip allowed the team to retrieve this key, allowing the drive to be cracked.
The two German researchers thought mobiles may be vulnerable, too. In the past few years FDE has been available on Apple, Android and other devices. However, a mobile gizmo typically has RAM chips soldered to the motherboard, making removal impossible. So rather than spray the chips with cold air, Mr Müller and Mr Spreitzenbarth tried putting a phone in a plastic resealable bag in a standard fridge freezer. They found through repeated testing that if this were set to -15°C, a phone placed in the freezer for an hour would keep its hardware intact and retained all of the contents of its RAM during a restart.
As the researchers explain in a paper submitted to a security conference in June, timing is tricky. The battery must be removed and restored in under half a second; otherwise the data stored in RAM degrade. The second requirement is that a phone must include a so-called “open bootloader” that will let a device launch a different operating system on start-up than the one stored in the phone’s longer-term flash memory. Apple’s phones do not allow this kind of alternate booting, but those running Google’s Android 4 typically do. A user has to press and hold several buttons on the phone when restarting the device. This meant the researchers could substitute their own boot software, which they dub “forensic recovery of scrambled telephones”, or FROST.
When an Android 4 phone boots with the button combination, all user and cached data are deleted from flash memory. But the chilling coupled with FROST allows all volatile memory locations to be examined. Mr Müller and Mr Spreitzenbarth successfully retrieved full address books, some Wi-Fi information, e-mail passwords, photos and even web pages browsed a week earlier (since the device had not been turned off in that time).
Some information is reliably retrievable only if a phone is examined immediately after information is used or stored. The researchers discovered that photos which had been snapped moments before the phone went into a freezer could be reliably recovered in full as they had just been copied onto flash storage but not yet overwritten in RAM by newer temporary data. But many passwords, such as those for e-mail, remain cached in memory and retrievable while a phone is active as they may need to be sent to a remote server again and again.
Mr Müller says that FROST also recovers the device’s FDE encryption key. However, because the flash storage is erased when booting in this manner, having the key is of no use with the current software, as there are no data left to decrypt.
A plausible scenario for a ne’er-do-well making use of FROST would start with purloining a phone. He would need a freezer to store the device and an hour during which he cooled his heels. Then, after attaching a USB thumb drive to the phone, half a second to pop the battery in and out, and just minutes to analyse and retrieve passwords and other data.
Mr Müller says breaches can be easily prevented by an operating system being programmed to overwrite both RAM and flash memory when the open bootloader is triggered, or disabling the open-bootload option altogether. As things stand, however, many phones are vulnerable. A chilling thought.
Powered by Facebook Comments