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People are the weakest link. A good messenger like Signal is nothing if you're talking to someone you don't know or trust.
Discussion
Even if you do know and trust the person, do you know and trust their device? Most of the people I talk to use Signal now. All but one use proprietary Google and Apple operating systems. I don't know what screen data is being sent from their device.
The only fix I can think of is purpose built device/software that only allows specific types of communication.
fatfinger
Not a fatfinger
I'm not an expert, i think the probability is 50/50
I use signal daily. It is not normal workflow to add people to a group chat.
Ok, now the claimnis that Waltz created the chat to begin with, implying he may have inadvertently added the Atlantic reporter when selecting members. Still, this is highly unlikely. Why would the reporter be in his signal contacts to begin with? I still don't believe it.