FreeBSD: VID-c294c2e6-b309-11e9-a87f-a4badb2f4699 (CVE-2019-5599): FreeBSD -- Resource exhaustion in non-default RACK TCP stack

Problem Description: While processing acknowledgements, the RACK code uses several linked lists to maintain state entries. A malicious attacker can cause the lists to grow unbounded. This can cause an expensive list traversal on every packet being processed, leading to resource exhaustion and a denial of service. Impact: An attacker with the ability to send specially crafted TCP traffic to a victim system can degrade network performance and/or consume excessive CPU by exploiting the inefficiency of traversing the potentially very large RACK linked lists with relatively small bandwidth cost.