Hundreds of German politicians, including Chancellor Angela Merkel, have had personal details stolen and published online, the BBC reports.
Contacts, private chats and financial details were put out on Twitter that belong to figures from every political party except the far-right AfD.
Data from celebrities and journalists were also leaked.
It is unclear who was behind the attack, which emerged on Twitter in the style of an advent calendar last month.
The leak appears to have originated on a Twitter account operated from Hamburg and the authorities in the north German city say they are now working with the Irish Data Protection Commissioner to stop the spread of German politicians' data.
The true extent of damage caused by the leak is not yet known although Justice Minister Katarina Barley said it was a "serious attack".
"The people behind this want to damage confidence in our democracy and institutions," she said.
A government spokeswoman said no sensitive data from the chancellor's office had been published. MPs, Euro MPs and MPs from state parliaments were affected, said Martina Fietz.