It starts with a metaphorical open door. According to files posted on the text sharing website Pastebin by self-proclaimed hackers AnonSec on January 31st (which have since been removed), the hackers used Bitcoin to purchase access to a NASA computer network from another hacker outfit in China.

Upon gaining access to the network, AnonSec found NASA computers with passwords left on the default. From there, they searched for whatever they could find on NASA's use of drones, and eventually, they say, they tried to crash a drone into the sea.

Here's how the International Business Times describes AnonSec's claim:

The hackers then secretly programmed the NAS devices to quietly send a copy of all the flight logs out to the hackers' server outside Nasa's network, but when they looked at the flight logs, they realised that part of the data they were receiving consisted of pre-planned route files for Nasa's Global Hawk drones. Every time a drone mission took off, Nasa drone operators were uploading specific flight paths, so the hackers realised that they could simply replace the Global Hawk drone route file, and that would cause the drone to deviate from its set flight path and do whatever the hackers wanted it to do.