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.
TL;DR: keep your apps updated & don’t scan QR codes that you don’t trust.
I wonder if Simplex Chat is better in this regard
Safest is probably Briar, which is also among the hardest to use.