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The second operating system hiding in every mobile phone
From: InfoSec News <alerts () infosecnews org>
Date: Fri, 15 Nov 2013 09:43:19 +0000 (UTC)

http://www.osnews.com/story/27416/The_second_operating_system_hiding_in_every_mobile_phone

By Thom Holwerda
osnews.com
12th Nov 2013

I've always known this, and I'm sure most of you do too, but we never really talk about it. Every smartphone or other device with mobile communications capability (e.g. 3G or LTE) actually runs not one, but two operating systems. Aside from the operating system that we as end-users see (Android, iOS, PalmOS), it also runs a small operating system that manages everything related to radio. Since this functionality is highly timing-dependent, a real-time operating system is required.

This operating system is stored in firmware, and runs on the baseband processor. As far as I know, this baseband RTOS is always entirely proprietary. For instance, the RTOS inside Qualcomm baseband processors (in this specific case, the MSM6280) is called AMSS, built upon their own proprietary REX kernel, and is made up of 69 concurrent tasks, handling everything from USB to GPS. It runs on an ARMv5 processor.

The problem here is clear: these baseband processors and the proprietary, closed software they run are poorly understood, as there's no proper peer review. This is actually kind of weird, considering just how important these little bits of software are to the functioning of a modern communication device. You may think these baseband RTOS' are safe and secure, but that's not exactly the case. You may have the most secure mobile operating system in the world, but you're still running a second operating system that is poorly understood, poorly documented, proprietary, and all you have to go on are Qualcomm's Infineon's, and others' blue eyes.

The insecurity of baseband software is not by error; it's by design. The standards that govern how these baseband processors and radios work were designed in the '80s, ending up with a complicated codebase written in the '90s - complete with a '90s attitude towards security. For instance, there is barely any exploit mitigation, so exploits are free to run amok. What makes it even worse, is that every baseband processor inherently trusts whatever data it receives from a base station (e.g. in a cell tower). Nothing is checked, everything is automatically trusted. Lastly, the baseband processor is usually the master processor, whereas the application processor (which runs the mobile operating system) is the slave.

So, we have a complete operating system, running on an ARM processor, without any exploit mitigation (or only very little of it), which automatically trusts every instruction, piece of code, or data it receives from the base station you're connected to. What could possibly go wrong?

[...]



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