Dubious address dealers and call centres have somehow gained access to names, addresses, contract details and bank information of about 1,000 thousand Telekom landline customers, the magazine reported on Wednesday.
Some customers have complained of illegal withdrawals from their bank accounts.
The magazine came into possession of some of these lists and showed them to company officials, who have said they don't know the source of the leak and want to file a report with authorities. Meanwhile the company said Stern's lists aren't original.
“First off, the format isn't right and some the bank and birth date details are different to our data sets,” head of security Volker Wagner told the magazine, suggesting that the lists have been put together from different sources.
Stern reported that Deutsche Telekom may have lost the most recent set of data in early 2007, when it farmed out work to outside firms in a push to gain new customers for its internet service. The other companies were provided with customer lists for marketing purposes.
It's not the first time Deutsche Telekom has been mired in a scandal regarding data protection. Last month the telecommunication firm admitted to losing the personal data belonging to its 17 million customers. The theft, which had only just been revealed, took place back in early 2006, and involved telephone numbers, dates of birth, addresses and email addresses.
Later that month further security lapses at Deutsche Telekom were exposed by Der Spiegel magazine, which reported that the data of more than 30 million mobile phone customers could be accessed and manipulated via the internet.
The company is also in the midst of a probe after revelations that it hired an outside firm to track hundreds of thousands of phone calls by senior executives and journalists to identify the sources of press leaks.