OS Exhaustion Flood

What the technique is

Adversaries may launch a denial of service (DoS) attack targeting an endpoint's operating system (OS). A system's OS is responsible for managing the finite resources as well as preventing the entire system from being overwhelmed by excessive demands on its capacity. These attacks do not need to exhaust the actual resources on a system; the attacks may simply exhaust the limits and available resources that an OS self-imposes. Different ways to achieve this exist, including TCP state-exhaustion attacks such as SYN floods and ACK floods. With SYN floods, excessive amounts of SYN packets are sent, but the 3-way TCP handshake is never completed. Because each OS has a maximum number of concurrent TCP connections that it will allow, this can quickly exhaust the ability of the system to receive new requests for TCP connections, thus preventing access to any TCP service provided by the server. ACK floods leverage the stateful nature of the TCP protocol. A flood of ACK packets are sent to the target. This forces the OS to search its state table for a related TCP connection that has already been established. Because the ACK packets are for connections that do not exist, the OS will have to search the entire state table to confirm that no match exists. […]

What MITRE says you would watch

• AN1012

  Burst of incomplete TCP handshakes (e.g., SYN floods) or uncorrelated ACK packets targeting the state table resulting in OS resource exhaustion.

• AN1013

  Flood of spoofed SYN or ACK packets causing exhaustion of OS TCP state table, potentially via user-space utilities or kernel-level DoS agents.

• AN1014

  Adversary tool/script issuing mass SYN/ACK floods that degrade OS responsiveness and interrupt service response on macOS endpoints.