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Trusting clients is probably a security flaw
If your service needs to trust the clients, hold my Big Mac
The result of such open-ness is that an entire series of services that need to trust the client (used in the oauth sense of the word) are not available to web apps. For example, some of the RootBeer checks will trigger on some unmodified Xiaomi, Asus, or Fairphone, or random cheap phones that happened to be in someone's nearest Tesco. Their devices by default do not have Google Play Services that can be tricked like this, and will require more work than I described to pass these checks.
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