The Chaos Computer Club, Europe's biggest hacker collective, said the system to count and transmit vote results lacked proper encryption and other security tools, labelling it a "write-off".
The privately developed "PC-Wahl" (PC Election) software -- used for years in several of Germany's 16 states -- "should never have been used," said a CCC spokesman, Linus Neumann.
"The number of possible attack targets and the severity of vulnerabilities exceeded our worst fears," he said in comments first published by news weekly Die Zeit.
The CCC warned that German parliamentary election results could potentially be manipulated remotely because the software failed to meet even "the basic principles of IT security".
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CCC hackers have in the past highlighted IT security flaws in high-profile cases, and their members often give expert testimony in German parliamentary hearings and court cases.
But the developer of the software, Volker Berninger, rejected the criticism, telling Die Zeit that "in the worst- case scenario, someone would create confusion".
But the CCC said any online attack would have "the potential to permanently undermine confidence in the democratic process".
"This is simply not the right millennium in which to turn a blind eye to IT security in elections," Neumann said.
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