The International Space Station is controlled by a number of computers. The computer systems control a lot of the hardware that keeps the living quarters in orbit, as well as keeping the craft in the proper trajectory as not to collide with other objects orbiting the planet.
On the other hand, all computer systems are vulnerable to some kind of hacking, and there have been a lot of strange instances where hardware on board the International Space Station, or even the Russian Soyuz spacecraft have malfunctioned in what would have looked like a computer hack.
As the past has proven, computer hacks are possible. There was once a time when a hacker located in China was able to take control of a satellite in space for up to 11 minutes, and the instance revealed to NASA that the threat of hacking exists for space equipment.
Because space is so critical, and because we rely on so much to keep astronauts safe from potential dangers, all a computer hacker would need to do is gain unsanctioned control and the International Space Station could come crashing down.
The threat is real, and NASA is constantly on the alert to keep its computer systems secure.