Network covert channels enable a policy-breaking network communication (e.g., within botnets). Within the last years, new covert channel techniques arose which are based on the capability of protocol switching. Such protocol switching covert channels operate within overlay networks and can (as a special case) contain their own internal control protocols. We present the first approach to effectively limit the bitrate of such covert channels by introducing a new active warden. We present a calculation method for the maximum usable bitrate of these channels in case the active warden is used. We discuss implementation details of the active warden and discuss results from experiments that indicate the usability in practice. Additionally, we present means to enhance the practical application of our active warden by applying a formal grammar-based whitelisting and by proposing the combination of a previously developed detection technique in combination with our presented approach.
