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https://www.nytimes.com/2020/01/21/world/americas/glenn-greenwald-brazil-cybercrimes.html
"Prosecutors also say that Mr. Greenwald was communicating with the hackers while they were actively monitoring private chats on Telegram, a messaging app. "
Seems like they most likely hacked Greenwald's mobile phone, since private keys for telegram secret chats are stored only on the mobile device.
