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WhatsApp and Signal messages at risk of surveillance following EncroChat ruling, court hears | Computer Weekly
(www.computerweekly.com)
This is a most excellent place for technology news and articles.
You'd have enough control over the software that you can ensure nothing like this happens
The basic security stuff exists on Android and iOS as well, namely full disk encryption. When that is defeated through a missing or bad password nothing keeps them from installing their malware with device access.
If they got in through an external security vulnerabilities in some software package the situation is also the same on either OS.