A ransomware attack is when hackers "lock" a system so that it cannot be used.
The only way to unlock it is to pay a large amount of money, often via bitcoin.
- The system is being held hostage, says Måns Jonasson.
Hard to know what's behind it
It is precisely ransomware that is believed to be behind Saturday's attack on Filmstaden.
Many expectant cinema-goers were turned away at the door as it was not possible to buy tickets, soft drinks or snacks.
Who or what is behind the attack is not known.
Måns Jonasson believes that it is either a "rogue" hacker group, which is mostly looking to make money - but that it could also be a state-supported group, which in that case wants to create disorder and mistrust.
How serious is this?
- It is always serious.
What is difficult is that those who manage the system must go back and see when and how the system was attacked and then try to restore it to an earlier version.
It can take a very long time.
Depending on how serious the attack is, and how many previous versions of the systems exist, it can take a different amount of time to get it working again.
- If you're lucky, it can go relatively quickly, a day or so.
If you are unlucky, it can take a very long time, as when the Swedish Church was exposed and it took several weeks.
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This is how a ransomware attack works – from breach to ransom demand.
Hear IT security expert Marcus Murray.
Photo: Fredrik Sandberg/TT