A team of German cryptographers say they have found a flaw in WhatsApp’s security that allowed them to bypass the chat app’s end-to-end encryption and eavesdrop on group chat messages.

The team, which hails from Ruhr University Bochum in Germany, presented their find in a research paper on Wednesday at the Real World Crypto security conference in Zurich, Switzerland.

In their paper, the researchers compared WhatsApp’s security practices with those of Signal and Threema, and they ultimately concluded that WhatsApp is the least secure of the three when it comes to group messages. They explained that the issue comes down to the way WhatsApp authorizes group members to decrypt messages.

WhatsApp keeps a guest list of approved users for each group chat, but the team said this method is vulnerable because the list is “manipulable from outside.” According to the researchers, anyone who has control over WhatsApp’s servers can insert uninvited users into a group chat by spoofing an invitation from the group’s administrator.

“The confidentiality of the group is broken as soon as the uninvited member can obtain all the new messages and read them,” Paul Rösler, on of the co-authors of the paper, told Wired. “If I hear there’s end-to-end encryption for both groups and two-party communications, that means adding of new members should be protected against. And if not, the value of encryption is very little.”

WhatsApp does notify members of a group chat when a new user joins, so they may realize that something suspicious is going on, but they may also assume that the new person was invited by the group administrator. Rösler noted that groups with several administrators could be even more vulnerable, as the invitations could be spoofed from multiple different accounts to make them harder to detect.

Alex Stamos, chief security officer for WhatsApp owner Facebook Inc., downplayed the vulnerability today in a series of tweets on Twitter, where he emphasized the app’s new chat member notifications as a key security feature. “On WhatsApp, existing members of a group are notified when new people are added,” said Stamos. “WhatsApp is built so group messages cannot be send to hidden users and provides multiple ways for users to confirm who receives a message prior to it being sent.”

Stamos added WhatsApp looked at the report carefully, but “following the researcher’s plan would necessitate a change to the way WhatsApp provides a popular feature called group invite links – which are used millions of times per day. There may be a way to provide this functionality with more protections, but it’s not clear cut.”

Photo: Yuri Yu. Samoilov via photopin cc