Violation of Secure Design Principles, Hidden Functionality, Incorrect Provision of Specified Functionality vulnerability in ArcGIS (Authentication) allows Privilege Abuse, Manipulating Hidden Fields, Configuration/Environment Manipulation.
The ArcGIS client_credentials OAuth 2.0 API implementation does not adhere to the RFC/standards; This hidden (known and by-design, but undocumented) functionality enables a requestor (Referred to as client in RFC 6749) to request an, undocumented, custom token expiration from ArcGIS (Referred to as authorization server in RFC 6749).
The product violates well-established principles for secure design.