Commercial quadcopters have been on the mainstream gadget scene for 15 years, proliferating across industries and among hobbyists. There's a swanky DJI store on New York City's Fifth Avenue, and you probably have a neighbor, not to mention a roofer, who owns a drone. So when researchers at the embedded-device security firm Red Balloon started seeing surprising quadcopter accessories on Chinese shopping platforms like Temu and AliExpress, they didn't think much of it at first. As with any popular...