As others have said, Amazon's incipient plan to fill our skies with autonomous delivery drones might have a few flaws. To that list it can now add 'other drones'.
A new hack using a Raspberry Pi computer and a Parrot AR Drone has demonstrated a method by which drones can automatically hack other drones, using their WiFi networks.
Developed by Samy Kamkar, 'Skyjack' is able to sever drones' connections to WiFi networks and then effectively take them over:
"Using a Parrot AR.Drone 2, a Raspberry Pi, a USB battery, an Alfa AWUS036H wireless transmitter, aircrack-ng, node-ar-drone, node.js, and my SkyJack software, I developed a drone that flies around, seeks the wireless signal of any other drone in the area, forcefully disconnects the wireless connection of the true owner of the target drone, then authenticates with the target drone pretending to be its owner, then feeds commands to it and all other possessed zombie drones at my will."
We'd imagine that it would be possible for Amazon to defend against this - though the prospect of thieves hacking their way into your books and DVD packages probably can't be discounted.