MicroSD Cards are tiny and hackable computers

Security researchers have found a way to hack SD Cards, the most common form of flash-memory cards used to store data mobile phones and digital cameras, and run software that intercepts data.

Andrew “bunnie” Huang and Sean “xobs” Cross have discovered a way to hack even the small microSD cards that go inside current smartphones and tablets to increase their storage, as well as other flash-based memory solutions, presenting their findings at the Chaos Computer Congress (30C3). In a detailed blog post on bunnie:studios, Huang explained how the hack works, and why many flash cards are susceptible to being hacked and used for malicious purposes by people who are aware of this particular potentially serious security vulnerability.

In a man-in-the-middle attack, someone intercepts data that's being transferred from one location to another, potentially scrutinizing or modifying it. Huang and Cross believe their attack could be used to secretly copy data, to modify sensitive data such as encryption keys, or to subvert authentication processes by substituting an unauthorized file for execution instead of the actual file that was authorized.

The approach works in principle not just with SD Cards, where the researchers demonstrated their approach, but also with other flash-memory devices such as SSDs (solid-state drives) used in place of traditional hard drives in personal computers and eMMC (Embedded Multimedia Controller) storage used in mobile phones.

To get the approach to work, the researchers had to reverse-engineer proprietary workings of the controller chips. First they figured out how to get a microcontroller to accept and then run new firmware -- a tiny operating system, in effect. Then they figured out the chips' actual proprietary commands.

The reason SD cards have microcontrollers in the first place is because it’s cheaper than producing reliable memory. Instead of testing each card to make sure it’s a flawless bit of hardware (it never is), SD card manufacturers just slap on a cheap microcontroller that can come up with workarounds for dead sectors and other hardware issues on the fly. This all gets set up at the factory, and average users never have to know a thing about it.

The details of exactly how you mess with this stuff are available over on bunnie’s blog, but the next time you plug in an SD card, just remember that it’s actually a tiny computer of its own. And though it’s probably not doing anything especially cool, or out to screw you over, it certainly has the potential to do either.