Impact The "DynamicClientRegistrationController#register" action hard-codes "confidential: false" when creating applications (dynamic_client_registration_controller.rb:18-25), yet the response includes a client_secret and advertises "token_endpoint_auth_methods_supported: ["client_secret_basic", "client_secret_post"]". Because Doorkeeper's "Application.by_uid_and_secret" treats a blank/missing secret as valid for non-confidential (public) clients, an attacker who knows only the client_id (which is public information) can authenticate as the dynamically-registered client at the token endpoint. Note that Dynamic Client Registration is opt-in feature which is disabled by default so only projects that explicitly enabled it are affected. Steps to Reproduce 1. Enable dynamic client registration in the initializer 2. POST /oauth/registration with client_name, redirect_uris, and scope 3. Observe: response returns client_secret, but the created Doorkeeper::Application has confidential: false 4. Call "Doorkeeper::Application.by_uid_and_secret(client_id, nil)" — it returns the application (credentials bypass) 5. POST /oauth/token with grant_type=client_credentials and only client_id (no client_secret) — the token endpoint issues an access token without any secret verification Patches Patched in 1.10.0 Workarounds Upgrade existing applications created with a Dynamic Client registration to have "confidential: true"