How a USB-connected speaker can infect a PC without ever being touched

After successfully replacing the firmware with a replacement image that did nothing more than display the word “patched” on the speaker’s LED display, the researcher got to wondering what else a hacker might do. So he turned his attention to FreeRTOS, the open source operating system that ran the Katana V2X. It contained a set of HID functions for allowing the speaker to act as a human interface device, a classification that includes keyboards, mice, and webcams. The speaker implemented a limited HID that allowed for things like changing the volume and playing or pausing sound, but little else.

The researcher discovered that he could change the speaker’s USB descriptor set, which is essentially a report that informs devices about the capabilities of a USB- or Bluetooth-connected peripheral. He was able to augment the existing descriptor set with a second one that reported the speaker being a keyboard. Then he used code already included in the firmware to streamline the process of sending keypresses.

All of this gave Moorats an idea: What if he used his device to send commands to the speaker that used the HID to pass them along to the connected PC? After some trial and error, he found that he could. In a blog post published on Wednesday, he wrote:

Chaining it all together, I was able to totally remotely, over the air, upload a custom firmware to my speaker which I hadn’t paired with, which would reboot, flash the custom firmware, and after rebooting type in the command echo pwned and execute it.

In a real attack scenario, I would execute the keystrokes for opening powershell.exe or similar and paste an actually malicious one-liner into that, but as a proof of concept, this was more than enough for me. A real attacker would also likely disable the routine for updating the firmware in both normal and recovery mode, making it impossible to wipe the malicious firmware from the device or patch it in the future.

This is worsened by the fact that Bluetooth is always on for the speaker, even in sleep mode, with no apparent way to disable it.

Before the speaker and USB-connected device can interact, they must successfully complete a challenge-and-response authentication procedure. Since the devices perform this handshake automatically each time the software boots, this isn’t usually a problem for the hacker. In certain cases, however, such as when the Katana V2X app isn’t open on the connected device, it’s a requirement.