Adversaries may use a connection proxy to direct network traffic between systems or act as an intermediary for network communications. The definition of a proxy can also be expanded to encompass trust relationships between networks in peer-to-peer, mesh, or trusted connections between networks consisting of hosts or systems that regularly communicate with each other. The network may be within a single organization or across multiple organizations with trust relationships. Adversaries could use these types of relationships to manage command and control communications, to reduce the number of simultaneous outbound network connections, to provide resiliency in the face of connection loss, or to ride over existing trusted communications paths between victims to avoid suspicion. (Citation: Enterprise ATT&CK January 2018)
Break and inspect SSL/TLS sessions to look at encrypted web traffic for adversary activity.
M0931Network Intrusion PreventionUse intrusion detection signatures to block traffic at network boundaries. In industrial control environments, network intrusion prevention should be configured so it will not disrupt protocols and communications responsible for real-time functions related to control or safety.
M0937Filter Network TrafficUse network appliances to filter ingress or egress traffic and perform protocol-based filtering. Configure software on endpoints to filter network traffic. Perform inline allow/denylisting of network messages based on the application layer (OSI Layer 7) protocol, especially for automation protocols. Application allowlists are beneficial when there are well-defined communication sequences, types, rates, or patterns needed during expected system operations. Application denylists may be needed if all acceptable communication sequences cannot be defined, but instead a set of known malicious uses can be denied (e.g., excessive communication attempts, shutdown messages, invalid commands). Devices performing these functions are often referred to as deep-packet inspection (DPI) firewalls, context-aware firewalls, or firewalls blocking specific automation/SCADA protocol aware firewalls. (Citation: Centre for the Protection of National Infrastructure February 2005)
M0807Network AllowlistsNetwork allowlists can be implemented through either host-based files or system hosts files to specify what connections (e.g., IP address, MAC address, port, protocol) can be made from a device. Allowlist techniques that operate at the application layer (e.g., DNP3, Modbus, HTTP) are addressed in [Filter Network Traffic](https://attack.mitre.org/mitigations/M0937) mitigation.
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