An international group of researchers who have been examining the source code for an internet voting system Switzerland plans to roll out this year have found a critical flaw in the code that would allow someone to alter votes without detection.

The cryptographic backdoor exists in a part of the system that is supposed to verify that all of the ballots and votes counted in an election are the same ones that voters cast. But the flaw could allow someone to swap out all of the legitimate ballots and replace them with fraudulent ones, all without detection.

"The vulnerability is astonishing," said Matthew Green, who teaches cryptography at Johns Hopkins University and did not do the research but read the researchers' report. "In normal elections, there is no single person who could undetectably defraud the entire election. But in this system they built, there is a party who could do that."