The microcontroller used to control the charging of Apple's laptop batteries could be attacked by malware in a way that might damage the cells, a researcher has reported in advance of a presentation at the forthcoming Black Hat security conference.
In an interview given to Forbes magazine, Charlie Miller of consultancy Accuvant describes his discovery after analysing an Apple software update from 2009 that he could access the firmware and controller chip built into the battery used by all current Apple laptops using only one of two default passwords.
Armed with this information, he claims he was able to modify the firmware in a way that caused seven test batteries to overheat and malfunction, or 'brick', after which they couldn't be used again.
According to Miller - who will explain his reverse engineering feat in more detail at Black Hat - the attack points to a large issue with battery firmware, namely that it could also hide more significant malware capable of bypassing PC security that currently assumes malware will be resident on a hard drive.
"You could put a whole hard drive in, reinstall the software, flash the BIOS, and every time it would re-attack and screw you over. There would be no way to eradicate or detect it other than removing the battery." Miller told Forbes.
Could it be used to cause a battery to malfunction in order to cause an explosion or fire. In principle, yes, though sensational headlines about exploding laptops would perhaps be a dangerous exaggeration - as with all sensitive electronic equipment laptops also come with circuit-breakers designed to guard against such malfunctions when they occur by accident.
"I work out of my home, so I wasn't super inclined to cause an explosion there," Miller said.
Miller will release his own software tool, 'Caulkgun', designed to randomise the passwords used to access the battery firmware for anyone who wishes to use it although this would also block Apple from updating the firmware in future.
Not every security expert is convinced. If malware writers start attacking the firmware controlling laptop batteries why not any of the other types of firmware and controllers also used in computers?
"That includes the motherboard itself, your wireless card, your 3G modem, network card, graphics device, storage devices and much more," said Paul Ducklin, Asia Pacific head of technology for security company Sophos.
He might also have added the same interfaces built into set-top boxes, TV sets and any complex device that can connect to the Internet and can be updated. Malware writers have occasionally toyed with such attacks but no significant threat has ever emerged using this approach.
The main problem is that each device class and manufacturer will be using a different proprietary interface to get at the firmware, some of which might be easy to get around, some less so. The kind of attack that might consider this route would likely be targeted and aimed at a system used by one organisation.
It's also worth making clear that there is nothing especially vulnerable about Apple's battery firmware. All laptops monitor the state of batteries using such systems, but each will have its own proprietary interface.
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