The Wiener Linien said the data, which included personal information such as names, addresses and email addresses but not payment data, was stolen by a former employee of its online ticket shop, which is run by a company based in Hamburg.
Wiener Linien spokesman Dominik Gries said that it is likely that only customers who created an online account before 2010 are affected. Around 600,000 people are now registered on the Wiener Linien's ticket site. All customers have been notified of the data theft by email, even if they are not personally affected.
The Wiener Linien has said it is unlikely that affected customers will be compensated as it will be hard to prove that they have suffered any financial damage from the theft.
However, lawyer and IT specialist Markus Dörfler said that details such as names, addresses and email addresses could be used by identity thieves to order things online, and then not pay for them.
Gries said that the Wiener Linien does not know why the former employee accessed and then published the data and that everything possible is being done to try and delete it from the web.
Concerned customers can contact the Wiener Linien by e-mail: email@example.com or by phone: 7909 100.