The attack caused a breakdown which prevented train passengers across the country from buying tickets on Sunday.
The issue is resolved as of Monday morning, DSB confirmed.
Tickets purchases via the company’s app, ticket machines, website and in 7-Eleven stores were all out of action due to the issue on Sunday.
Passengers with Rejsekort travel cards were able to use that system, while others purchased tickets from ticket inspectors on board trains.
“Our technicians and IT contractors have analysed this closely during the night and have concluded this is an outside attack in which someone has attempted to bring our system down,” DSB vice-director Aske Wieth-Knudsen said.
Internal mail and telephone systems used by DSB staff were also affected by the cyber attack, rendering the company able to communicate via social media or provide staff or customers with further information.
A similar issue earlier this year was caused by an electronic failure rather than a cyber attack.
“They type of attack we saw yesterday is a new way of doing things that we [DSB, ed.] haven’t seen before. So a little more close analysis is required for us to see what exactly happened so we can prevent a reoccurrence,” Wieth-Knudsen told DR.
DSB would be in contact with Danish authorities over the issue in the coming days, the vice-director added.
“At this moment in time I have not yet been in contact with anyone. We are still clarifying some messages, since the attack was only resolved during the night,” he told Ritzau.
“Now the day has started we will naturally contact relevant bodies,” he added.
A DDoS attack is a type of cyber attack in which the perpetrator seeks to make a machine or network unavailable to its intended users by disrupting the functionality of a host connected to the internet.
The incident comes as Denmark’s police security service PET is due on Monday to publish its assessment of IT-related threats against Denmark.