Threema is a Swiss encrypted messaging application. It has more than 10 million users and more than 7000 on-premise customers. Prominent users of Threema include the Swiss Government and the Swiss Army, as well as the current Chancellor of Germany, Olaf Scholz. Threema has been widely advertised as a secure alternative to other messengers.
In our work, we present seven attacks against the cryptographic protocols used by Threema, in three distinct threat models. All the attacks are accompanied by proof-of-concept implementations that demonstrate their feasibility in practice.
