Pwning With Sewing Needles


If you don’t have root, you don’t own a device, despite what hundreds of Internet of Things manufacturers would tell you. Being able to access and write to that embedded Linux system in your new flashy gadget is what you need to truly own a device, and unfortunately this is a relatively uncommon feature. At this year’s DEF CON, [Brad Dixon] unveiled a technique that pwns a device using only a sewing needle, multimeter probe, or a paperclip. No, it won’t work on every device, and the devices this technique will work with are poorly designed. That doesn’t mean it doesn’t work, and that doesn’t mean the Pin2Pwn technique isn’t useful, though.

The attack relies on how an embedded Linux device boots. All the software needed to load Linux and the rest of the peripheral magic is usually stored on a bit of Flash somewhere on the board. By using a pin, probe, or paperclip to short two data pins, or two of the latch pins on this memory chip, the bootloader will fail, and when that happens, it may fall back to a uboot prompt. This pwns the device.

There are a few qualifications for this Pwn using a pin. If the device has JTAG, it doesn’t matter – you can already own the device. If, however, a device has a locked-down JTAG, unresponsive serial ports, or even their own secure boot solution, this technique might work.

Two data pins on a TSSOP Flash shorted by a multimeter probe
Two data pins on a TSSOP Flash shorted by a multimeter probe

This exploit works on the property of the bootloader. This bit of code first looks at a piece of Flash or other memory separate from the CPU and loads whatever is there. [Brad] found a few devices (mostly LTE routers) that would try to load Linux from the Flash, fail, try to load Linux again, fail, and finally drop to a uboot prompt.

As with any successful exploit, an equally effective mitigation strategy must be devised. There are two ways to go about this, and in this case, the software side is much better at getting rid of this attack than the hardware side.

Since this attack relies on the software falling back to uboot after an unsuccessful attempt at whatever it should be booting, the simplest and most effective mitigation technique is simply rebooting the device if the proper firmware can’t be found. Having a silent serial console is great, but if the attack relies on falling back to uboot, simply not doing that will effectively prevent this attack.

The hardware side is a little simpler than writing good firmware. Instead of using TSSOP and SOIC packages for storing the device firmware, use BGAs. Hide the pins and traces on an inner layer of the board. While this isn’t a foolproof way of preventing the attack – there will always be someone with a hot air gun, magnet wire, and a steadier hand than you – it’s hard to glitch a data line with a sewing needle if you can’t see the data line.



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