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Comment by seba_dos1 | Hacker News Reader

"Evil USB" (or "bad USB") is possible thanks to the U part of USB - universal. If you connect a pendrive to your computer, it can easily say that it's a keyboard, because your computer cannot easily verify that you haven't just connected a keyboard. It would need to ask you in some trustworthy way to be sure, which sometimes can be problematic.

OTOH, on the device like Neo900 it is well-known what kind of device is connected to the internal bus and software stack (at least on Linux) can easily be advised to not accept anything that doesn't look and behave like the included modem should.

In a properly configured user OS, the modem would need to use some software vulnerability to exploit the USB stack, so the same principles apply there as with, say, OpenSSL, browser or the kernel. Secret zero-days aside, when some bug is found, it is patched and you upgrade the vulnerable component, just like on PC.

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This is correct. The problem in this example is not the pendrive, but the automatic selection of device drivers based on the PC OS 100% trusting physical access.

An easy way to see this is recompile the USB keyboard driver to ignore keyboard descriptors with a particular address or vendor ID. If you do this, the pendrive can't do anything because USB is host-controlled, as implemented by the OS. Without the OS initiating a conversation with the pendrive and saying "ok, I'll configure you as a keyboard and interpret responses from you as keystrokes", it can't happen.

As you point out, the main CPU on a phone does not implement HID autoconfig on the internal baseband bus.

I picked a terrible example which doesn't demonstrate what I intended to.

Any USB link requires the host to maintain some persistent state in data structures mirroring what it thinks the state of the device is. There's no "DMA" in the sense that the device has direct access to the host - but that doesn't preclude something as mundane as a buffer overflow, use-after-free and so on.

"DMA", with appropriate IOMMU, is just fancy shared memory communication. You're just as likely to mess that up as a serial link. It's happened. A lot.

Yes, parsing of untrusted data, race conditions, etc. are still a problem in general. My complaint was with the uninformed article, not your analysis.

I'm sure a udev rule would be sufficient to whitelist only the trusted hardware. However, nothing can stop a malicious one to spoof serial numbers or other characteristics of a trusted hardware.

If you connect a pendrive to your computer, it can easily say that it's a keyboard, because your computer cannot easily verify that you haven't just connected a keyboard.

Sure it can. Just pop up a dialogue on the screen displaying a random character string and ask the user to type the string. The dialogue could simply instruct the user to unplug and cast suspicion upon a device that is pretending to be a keyboard.

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