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The difference between user session management and OAuth is the level of trust between the communicating parties. In any app, the frontend is untrusted, whereas the backend is trusted. In OAuth flows, generally, the two communicating parties are both trusted. The frontend device could be compromised via social engineering techniques or malware, and there is nothing the app developers can do to mitigate that. OAuth 2.0 is used when your app (frontend or backend) needs to communicate with a third party’s.