From that point on, the attacker can impersonate the hacked client to the server.Įnlarge / A figure showing the cross-protocol interaction of an E2E and a C2S session. One possible way for an attacker to do this is to send spam to a large number of users that tells them to send the character string to a specific account in order to be eligible for a prize. The attacker can exploit it by tricking a user into sending a set of characters (u9j6ߓ'jjखԻ^߃1כW:-́ ܡRA) to a special but innocuous account. A flaw in the way Threema's client-to-server (C2S) protocol interacts with its end-to-end (E2E) protocol that causes a user to create a special Threema value known as a vouch box and send it to the attacker.Sloppy key management also causes Threema to reuse ephemeral keys in places they should never be reused.
With Threema, leaking of an ephemeral key has the same effect as leaking a long-term key. This is a remarkable shortcoming because ephemeral keys should never be able to authenticate a user. In the event an ephemeral key is exposed even once, an attacker can permanently impersonate the client to the server and then obtain all metadata in all E2EE messages.The seven vulnerabilities the researchers uncovered include:
The remaining two can be exploited when an attacker gains access to an unlocked phone, such as at a border crossing. Three vulnerabilities require an attacker to gain access to a Threema server. Two of the vulnerabilities require no special access to a Threema server or app to cryptographically impersonate a user.
Researchers from the Zurich-based ETH research university reported on Monday that they found seven vulnerabilities in Threema that seriously call into question the true level of security the app has offered over the years. The app uses a custom-designed encryption protocol in contravention of established cryptographic norms. It’s among the top Android apps for a fee-based category in Switzerland, Germany, Austria, Canada, and Australia. Threema developers advertise it as a more secure alternative to Meta’s WhatsApp messenger. Threema has more than 10 million users, which include the Swiss government, the Swiss army, German Chancellor Olaf Scholz, and other politicians in that country. Despite the unusually strong claims and two independent security audits Threema has received, the researchers said the flaws completely undermine assurances of confidentiality and authentication that are the cornerstone of any program sold as providing end-to-end encryption, typically abbreviated as E2EE. Academic researchers have discovered serious vulnerabilities in the core of Threema, an instant messenger that its Switzerland-based developer says provides a level of security and privacy “no other chat service” can offer.
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