OAuthLib: Bearer token can be retrieved as long as client knows a valid client ID for device workflow

Summary

A call to the token endpoint with a grant type of "urn:ietf:params:oauth:grant-type:device_code" and a bogus device_code is able to get a bearer token as long as they know a valid client ID enabled for the device workflow.

Details

Section 3.4 of the RFC 8628 spec says that the token endpoint should take in device_code as a required parameter.DeviceCodeGrant.validate_token_request does not perform any validation of device_code. DeviceCodeGrant.validate_token_request should be validating that device_code was passed in as a parameter and that it is valid, similar to how AuthorizationCodeGrant.validate_token_request does the checks here and here.

I would like to propose a new method be added to RequestValidator called validate_device_code that is similar to the validate_code method and is called inside of DeviceCodeGrant.validate_token_request.

PoC

Create an implementation of RequestValidation.authenticate_client that returns True if the client's client_type is "public". You would find yourself in this scenario because there's another issue with DeviceCodeGrant.validate_token_request where it doesn't properly handle public clients (see https://github.com/oauthlib/oauthlib/issues/919).

Create a client that has a client_type of "public" and set it's grant_type to "urn:ietf:params:oauth:grant-type:device_code".

With this implementation of RequestValidation.authenticate_client done, use that validator for your token endpoint. Call the token endpoint with a body of

{
  "client_id": <ID of the public client you made>,
  "grant_type": "urn:ietf:params:oauth:grant-type:device_code",
  "device_code": <anything you want since it's not validated>
}

You now have a bearer token for the client without a valid device_code.

Impact

Anyone using the RFC 8628 device workflow in oauthlib that doesn't realize the device_code parameter is not being validated.