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Hackers broke into email accounts belonging to members of Germany's far-right Alternative für Deutschland (AfD) party and stole large numbers of messages, the party said on Tuesday.
More than 421,000 emails were taken in the attack last month, including messages sent by party co-leader Alice Weidel and co-founder Alexander Gauland, the Bild daily reported.
A website published what it said were summaries of some of the emails but not the original messages.
Bild said the published messages included photos of ID cards of some party members, as well as bank statements, membership applications and membership lists from some local AfD branches.
The AfD confirmed to the AFP news agency that an email service provider used by the party had been targeted, and "a small number of email accounts, most of which had not been used for years, were apparently copied unlawfully".
"This attack was quickly detected and stopped, and all users were subsequently informed accordingly," said an AfD statement.
The party said it was reviewing what had been published online.
It remains unclear who was responsible for the attack and whether additional data has been compromised.
The anti-immigration, Russia-friendly AfD is topping national opinion polls and came first in two state elections last month, comfortably ahead of Chancellor Friedrich Merz's centre-right CDU.
The party scored its biggest win in Saxony-Anhalt, where it hopes to form Germany's first far-right state government since World War II.