Dan Black of Google’s Threat Intelligence Group writes:
Google Threat Intelligence Group (GTIG) has observed increasing efforts from several Russia state-aligned threat actors to compromise Signal Messenger accounts used by individuals of interest to Russia’s intelligence services. While this emerging operational interest has likely been sparked by wartime demands to gain access to sensitive government and military communications in the context of Russia’s re-invasion of Ukraine, we anticipate the tactics and methods used to target Signal will grow in prevalence in the near-term and proliferate to additional threat actors and regions outside the Ukrainian theater of war.
[…]
The most novel and widely used technique underpinning Russian-aligned attempts to compromise Signal accounts is the abuse of the app’s legitimate “linked devices” feature that enables Signal to be used on multiple devices concurrently. Because linking an additional device typically requires scanning a quick-response (QR) code, threat actors have resorted to crafting malicious QR codes that, when scanned, will link a victim’s account to an actor-controlled Signal instance. If successful, future messages will be delivered synchronously to both the victim and the threat actor in real-time, providing a persistent means to eavesdrop on the victim’s secure conversations without the need for full-device compromise.
Read more at Google.