According to the EYP, the group that carried out the attacks on the theme bank are Russians and even well-known. Their group is called “Killnet” and they usually carry out DDoS attacks. So we have solved the issue of cyber attack in the Subject Bank.
So we read on the internet various amazing things. The best thing was that the whole case was a "death contract" worth 200.000 euros.
Those in the know just laugh. Without words, see the traffic https://www.gr-ix.gr/traffic/
If you still want us to make scenarios and assumptions, we could also do:
Imagine a student who is just bored and wants to pass the time. Out there are the right tools to drop every page of the Greek state. They are not famous either for their safety... If someone doesn't know how to use the tools I mention and they're free, they can give a 50 bucks and drop whatever they want, just pasting the target URL.
But these scenarios are not circulating. It is in no one's interest to admit that system one threw us script kiddie.
It was probably the Russians, it might have been the Chinese too, because they didn't aim to hit anything but the subject bank, on exam day.... Did anyone actually mention the motive?
Have a good time and be careful with the information you consume.