Play the below sounds through your phone speaker and hold it next to a Gatekeeper Systems wheels to lock/unlock. Check me out on twitter @stoppingcart

How It Works

Most electronic shopping cart wheels listen for a 7.8 kHz signal from an underground wire to know when to lock and unlock. A management remote can send a different signal at 7.8 kHz to the wheel to unlock it. Since 7.8 kHz is in the audio range, you can use the parasitic EMF from your phone's speaker to "transmit" a similar code by playing a crafted audio file.
Link to my original DEFCON 29 Talk
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