An attacker can secretly eavesdrop on your private end-to-end encrypted group chats on WhatsApp, Threema and Signal messaging apps.

Even if the messaging services implement end-to-end encryption, an attacker or someone in the company that provides the service can decrypt your messages.

A Group of researchers from Ruhr-Universität Bochum (RUB) in Germany discovered that anyone who controls WhatsApp/Signal servers can covertly add new members to any private group without permission of the administrator, with this trick it is possible to spy on group conversations.

In case of multi-user chats, the servers manage the entire communication process.

“Contrary to classical multi-user chats, for example, to IRC in which all members are online, groups in IM protocols must work in asynchronous settings; Groups must be createable and messages must be deliverable even if some group members are offline” reads the paper published by the researchers, titled “More is Less: On the End-to-End Security of Group Chats in Signal, WhatsApp, and Threema,”

“We observed two shortcomings in the design of WhatsApp’s group protocol that allow to (1) burgle into a group and to (2) forge acknowledgments. The shortcomings have similar results as the attacks on Signal, although the underlying protocol and exploitation differ”

The experts discovered that both Signal and WhatsApp fail to properly authenticate an entity that is adding a new member to the group, this means that an unauthorized user that is not a group administrator or even a member of the group can add a member to the group conversations.

Experts also discovered that it is possible to add a new member without notifying the action to other members, this is possible because a rogue admin or employee with access to the server could manipulate (or block) the group management messages.

The abilities to burgle into a group and to forge acknowledgments could be chained to allow an attacker who controls the WhatsApp server or can break the transport layer security to fully control group activities.

“The described weaknesses enable attacker A, who controls the WhatsApp server or can break the transport layer security, to take full control over a group. Entering the group, however, leaves traces since this operation is listed in the graphical user interface. The WhatsApp server can therefore use the fact that it can stealthily reorder and drop messages in the group,” explained the researchers.

“Thereby it can cache sent messages to the group, read their content first and decide in which order they are delivered to the members. Additionally, the WhatsApp server can forward these messages to the members individually such that a subtly chosen combination of messages can help it to cover the traces.”

According to WhatsApp, the situation is quite different because if any new member is added to a group other group members will receive a notification.

“We’ve looked at this issue carefully. Existing members are notified when new people are added to a WhatsApp group. We built WhatsApp so group messages cannot be sent to a hidden user,” a WhatsApp spokesperson told Wired.

“The privacy and security of our users is incredibly important to WhatsApp. It’s why we collect very little information and all messages sent on WhatsApp are end-to-end encrypted.”

The RUB team also provide recommendations to the companies that are suggested to solve the issue by adding an authentication mechanism to group management messages, in this way only legitimate administrators can manage the activities of multi chats.

The Ruhr University researchers reported findings of their investigation to WhatsApp in July, in response to their report, WhatsApp fixed one problem with a feature of their encryption that made it harder to crack future messages even after an attacker obtained one decryption key.

“But they told the researchers the group invitation bug they’d found was merely “theoretical” and didn’t even qualify for the so-called bug bounty program run by Facebook, WhatsApp’s corporate owner, in which security researchers are paid for reporting hackable flaws in the company’s software.” continues Wired.

As said the experts also investigated Threema and Signal.

For Threema, the researchers found minor flaws, an attacker who controls the server can replay messages or add users to a group who have been removed. Once informed of the issues, Threema released a version to address the issues.

For Signal the attack is more difficult because the attacker would have to not only control the Signal server but also know an unguessable number called the Group ID. This means that to carry on the attack it is necessary the knowledge of the Group ID that can be obtained from one of the group member’s devices, in this case, the group is likely already compromised.

Pierluigi Paganini

(Security Affairs – hacking, instant messaging)

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