"The user is always a risk," said Constanze Kurz, spokeswoman for the Chaos Computer Club hacker group, noting that even the latest security software can't protect against human error.
It emerged in May that hackers had infiltrated a trojan – a virus designed to look like an innocuous document or link – into the Bundestag computer network via the email system and used it to scoop up large amounts of data.
"We have to assume that the Bundestag was an open book to the attackers for months," Constanze said, adding that "it won't be simple" to close all the security holes.
The computer system was originally supposed to be switched off last week, but was kept online for the extraordinary debate on the latest episode in the Greek bailout programme.
It is expected to be back online sometime on Monday.
But MPs will have no access to their files and email until then.
"This is a unique procedure, taking the Bundestag offline," Kurz said.
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