Parse Server is an open source backend that can be deployed to any infrastructure that can run Node.js. Internal fields (keys used internally by Parse Server, prefixed by _) and protected fields (user defined) can be used as query constraints. Internal and protected fields are removed by Parse Server and are only returned to the client using a valid master key.

However, using query constraints, these fields can be guessed by enumerating until Parse Server, prior to versions 4.10.14 or 5.2.5, returns a response object. The patch available in versions 4.10.14 and 5.2.5 requires the maser key to use internal and protected fields as query constraints. As a workaround, implement a Parse Cloud Trigger beforeFind and manually remove the query constraints.