(translation):
The attack was noticed a few days ago and according to a report in the newspaper “Kathimerini”, the Ministry of National Defense and the Police have been involved in the investigation into the incident, while the EYP is monitoring the developments. Initially there was concern about the possibility that the company fell victim to cyber espionage from a foreign country.
Read more on LiFO.gr. This appears to be reported as a ransomware attack and it is thought that the attackers did not get to the production part of the system where more sensitive data would be. The type of malware was not named in this report.
h/t, @Chum1ng0