An attacker could try to obtain a valid access token on transport between the client and resource server. As access tokens are shared secrets between the authorization server and resource server, they should be treated with the same care as other credentials (e.g., end-user passwords).
OAuth 2.0 Threat Model and Security Considerations (RFC6819, section 4.6.1)
This threat is considered fully mitigated if all the test cases from the following test set succeed.
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