Amazon Linux 2 Security Advisory: ALAS-2018-1050
Advisory Release Date: 2018-08-06 19:11 Pacific
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. ( https://www.kb.cert.org/vuls/id/962459 )
Run yum update kernel and reboot your instance to update your system.