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A great post by Kraken on how a mobile phone can be compromised, and how to avoid it. The contents of the article can easily be used to learn how to secure other online services and accounts, in particular online crypto wallets.
"In the past month, there’ve been at least 10 cases of people publicly involved in the cryptocurrency scene being victimized by mobile phone hijacking. The consequences have been expensive, embarrassing, enduring, and, in at least one case, life-threatening."
https://blog.kraken.com/post/219/security-advisory-mobile-phones/