The present disclosure relates to scalable network security functions and handling of packet flows between network security zones in a communications network. Packets that are part of a bidirectional packet flow between the network security zones are received, and a determination is made as to an instance of a security application to which to assign the bidirectional packet flow for security processing. The determination is made based on relative loading of a plurality of identical instances of the security application running on a host machine. All of the received packets that are part of the bidirectional packet flow are directed for processing on the host machine by the one of the security application instances.