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

Workarounds

Upgrade existing applications created with a Dynamic Client registration to have confidential: true