Linux kernel versions 4.9+ can be forced to make very expensive calls to tcp_collapse_ofo_queue and tcp_prune_ofo_queue for every incoming packet which can lead to a denial of service. An attacker can induce a denial of service condition by sending specially modified packets within ongoing TCP sessions. Maintaining the denial of service condition requires continuous two-way TCP sessions to a reachable open port. Thus, the attacks cannot be performed using spoofed IP addresses