The hackers were able to carry out the so-called Ddos (Distributed Denial of Service) attack because of faulty settings in the electronic equipment, according to the incident report that Telia Sonera presented to the Swedish Post and Telecom Authority (PTS) on Tuesday.
A series of attacks were launched against Telia, starting on December 9. As a consequence, Telia's customers lost their internet connection, including IP phone services and tv. Customers with IP telephony were also unable to contact emergency services.
It is unclear who is behind the attacks, but the Russian group Lizard Squad has claimed responsibility on Twitter.