Stand-Alone EM4x RFID Harvester

Continuing off from our last RFID Cloning with Proxmark3 post, we wanted to build a small, portable, stand-alone EM4x RFID tag stealer. We needed an easy way of storing multiple tag IDs whilst “rubbing elbows” with company personnel. The proxmark3 seemed liked an overkill and not particularly fast at reading em4x tags so we figured we’d try hooking up our RoboticsConnection RFID reader to a Teensy and see if we could make them play nicely together.

Hooking up the RFID reader to the Teensy was mostly painless. We connected the serial RX/TX pins on the reader to serial pins 2 and 3 on a Teensy 2++ and after a bit of mucking around, we got the RFID reader to respond to the serial commands sent from the Teensy. We re-implemeted a couple of basic reader commands in the Teensy, namely, “clear all tags“, “save last seen tag“, and “list tags“. This particular reader is also able to store up to 48 tags internally, simplifying our “data storage” issues significantly.

Our Teensy continuously monitors it’s serial input for new tags read by the RFID reader, and when one is found, it is automatically saved into the reader and an indication for this, the built in LED turns blue.

Once our “harvesting” session is complete, we can either use the Teensy to list all discovered tags, or connect to the RFID reader directly over USB. The tags are stored in non volatile memory on the reader, so it can be powered off safely.

You can download our demo Teensy sketch via GitHub:

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git clone https://github.com/offensive-security/rfid-em4x-harvester
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